arX

iv:0

807.

0532

v2 [

astr

o-ph

] 4

Jul

200

8

TheL1615/L1616 PM S population:April20,2013

T he star form ation in the L1615/L1616 C om etary C loud

DavideGandol�1;2;3,Juan M .Alcal�a2,Silvio Leccia2,Antonio Frasca1,Loredana Spezzi1;2,

Elvira Covino2,Leonardo Testi4,EttoreM arilli1,JouniKainulainen5

A B ST R A C T

The present work aim s at perform ing a com prehensive census and charac-

terisation ofthe pre-m ain sequence (PM S) population in the com etary cloud

L1615/L1616,in orderto assessthe signi�cance ofthe triggered starform ation

scenario and investigatetheim pactofm assivestarson itsstarform ation history

and m assspectrum .Ourstudy isbased on UB V R C IC and JH K s photom etry,

aswellasopticalm ulti-objectspectroscopy.W eperform ed aphysicalparam etri-

sation ofthe young stellar population in L1615/L1616. W e identi�ed 25 new

T Tauristarsm ainly projected on thedensehead ofthecom etary cloud,alm ost

doubling the currentnum berofknown m em bers. W e studied the spatialdistri-

bution ofthecloud m em bersasa function oftheageand H� em ission.Thestar

form ation e�ciency in thecloud is� 7{8% ,asexpected form olecularcloudsin

thevicinity ofOB associations.Theslopeoftheinitialm assfunction (IM F),in

the m assrange 0:1�M � 5:5 M � ,isconsistentwith thatofotherT and OB as-

sociations,providing furthersupportofan universalIM F down to thehydrogen

burninglim it,regardlessofenvironm entalconditions.Thecom etaryappearance,

aswellasthehigh starform atione�ciency,can beexplained in term softriggered

starform ation induced by thestrong UV radiation from OB starsorsupernovae

shockwaves. The age spread as wellas both the spatialand age distribution

ofthe PM S objects provide strong evidence ofsequential,m ultiple events and

possibly stillongoing starform ation activity in thecloud.

1INAF,O sservatorio Astro�sico diCatania,Via S.So�a 78,95123 Catania,Italy;dgandol�@ oact.inaf.it

2INAF, O sservatorio Astronom ico di Capodim onte, Salita M oiariello 16, 80131 Napoli, Italy;

jm ae@ na.astro.it

3Th�uringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, G erm any; davide@ tls-

tautenburg.de

4INAF,O sservatorio Astronom ico diArcetri,L.go E.Ferm i5,50125 Firenze,Italy;lt@ arcetri.astro.it

5O bservatory,PO Box 14,FIN-00014 University ofHelsinki,Finland;jouni.kainulainen@ helsinki.�

{ 2 {

Subject headings: com etary cloud: individual(L1615/L1616) | dust,extinc-

tion | stars: circum stellar m atter | stars: form ation | stars: fundam ental

param eters| stars:pre-m ain sequence

1. Introduction

TheLyndscloudsL1615andL1616(Lynds1962)arebothlocatedatanangulardistance

ofabout6� W estofthe Orion OB1 associations. These clouds actually form a com etary-

shaped singlecloud with a \head-tail" distribution subtending about400(5.2 pcassum ing a

distanceof450pc;Alcal�a etal.2004)roughly in theEast-W estdirection.Thedensehead of

thecloud com plex (i.e.L1616 only),pointing toward Eastand facing thebrightOrion-belt

stars,harbourstheIRAS Sm allScale StructureX0504-034 (Helou & W alker1988;Ram esh

1995)and abrightre ection nebula,NGC 1788(= DG 51,Ced40,vdB 33,RNO 35,LBN 916;

Lynds1965),which isillum inated by a sm allclusterofstars(Stankeetal.2002).The two

interm ediate-m assstarsHD 293815 and KisoA-0974 15 arethebrightestvisiblem em bersof

thiscluster.

L1615/L1616,likem any othercom etary cloudso�them ain Orion starform ingregions,

clearly showsevidenceofongoingstarform ation activity which m ighthavebeen triggered by

thestrongim pactoftheUV radiation from them assive,lum inousstarsin theOrion com plex

(M addalena etal.1986;Stankeetal.2002;Alcal�a etal.2004;Kun etal.2004;Leeetal.

2005,2007). The illum ination ofdense clum ps in m olecular clouds by OB stars could be

responsible for their collapse and subsequent star form ation. The UV radiation from the

OB starsm ay sweep them olecularm aterialofthecloud into a com etary shapewith adense

corelocated atthehead ofthecom etary cloud.

The radiation and wind ofOB starsm ay also have an im portantim pacton the m ass

accretion during the star form ation process. W hile in a T association a protostar m ay

accum ulate a signi�cant fraction ofm ass,the m ass accretion ofa low-m ass protostarin a

region exposed to the wind ofOB stars can be term inated earlier because ofthe photo-

evaporation ofthe circum stellar m atter (Kroupa 2001,2002). Therefore,m any low-m ass

protostarsm ay notcom plete theiraccretion and hence can resultasbrown dwarfs(BDs).

Thism echanism m ighta�ectthe low-m assend ofthe initialm assfunction (IM F).Recent

studies have provided som e inform ation aboutthe shape ofthe IM F in the very low-m ass

and sub-stellarregim es. W hile the IM F in the Orion Nebula Clusterappearsto rise below

0.1 M � (Hillenbrand & Carpenter 2000),in T association like Taurus-Auriga and ChaII

thereissom eindication ofade�citofsub-stellarobjects(Luhm an etal.2000;Brice~no etal.

2002;Spezzietal.2008). Other studies ofthe young cluster IC 348,which is devoid of

{ 3 {

very m assive stars,have also revealed a de�citofBDsrelative to theOrion Nebula Cluster

(Preibisch etal.2003;M uench etal.2003;Lada etal.2006).

Because ofitsvicinity to the Orion OB associationsthe L1615/L1616 com etary cloud

constitutesan ideallaboratorytoinvestigatethetriggered starform ation scenario.Them ost

recentcensusofthepre-m ain sequence (PM S)starsin L1616 wasprovided by Alcal�a etal.

(2004),who presented a m ulti-wavelength study ofthe region,from X-ray to near-infrared

wavelengths. They found 22 new low-m ass PM S stars distributed m ainly to the East of

L1616,in about1 square degree �eld. By adding the 22 new PM S starsto the previously

con�rm ed m em bers of the cloud (Cohen & Kuhi1979;Sterzik etal.1995;Nakano etal.

1995;Stankeetal.2002) and counting the m illim eter radio-source,i.e. the Class-0 pro-

tostarM M S1A found by Stankeetal.(2002),Alcal�a etal.(2004)ended up with a sam ple

of33 young stellarobjectsassociated with L1616.However,thelatterwork could notinves-

tigateim portantaspectsofthestarform ation in thiscloud,liketheIM F and theim pactof

environm entalconditionson them assspectrum ,becausetheirsam plewasratherincom plete.

Therefore, the aim of the present study is to perform a com prehensive census and

characterisation ofthe PM S population in L1615/L1616,in order to investigate the star

form ation history,the relevance ofthe triggered scenario,and to study the IM F.To this

aim ,we reportboth opticaland near-infrared observations,aswellasm ulti-objectoptical

spectroscopy in L1615/L1616. An im portant goalofour study is to com pare the m ass

function in L1615/L1616 with thatofotherT and OB associations.

Theoutlineofthepaperisasfollows:wedescribeourobservationsand data reduction

in Section 2 and 3;the results are reported in Section 4,while the data analysis and the

physicalproprieties ofthe new PM S starsare presented in Section 5. Ourdiscussion and

conclusions are developed in Section 6. Som e detailson the spectral-type classi�cation as

wellasreddening,radius,and lum inosity determ ination aregiven in theAppendix A and B,

respectively.

2. Photom etry

2.1. O ptical

M ost ofthe opticalphotom etry com es from previous B ,V ,R C and IC broad-band

im aging observationsperform ed by Alcal�a etal.(2004),who obtained CCD m osaic im ages

with the W ide Field Im ager (W FI) cam era at the ESO/M PIA 2.2m telescope (La Silla

Observatory,Chile,program No. 64.I-0355),covering a sky-area ofabout360� 340 around

NGC 1788.Thepre-reduction oftheraw im ages,aswellastheastrom etricand photom etric

{ 4 {

calibration have been already discussed in Alcal�a etal.(2004). In order to im prove the

photom etry oftheblended sources,wecarried outpointspread function �tting photom etry

on theim agesusing theDAOPHOT package(Stetson 1987)undertheIRAF1 environm ent.

Further photom etric observations ofsix previously known m em bers ofL1615/L1616

(Alcal�a etal.2004),stilllacking opticalphotom etry and falling outsidethesky-area covered

by W FI,were carried out in Novem ber 17th and 18th,2006 with the 91cm telescope at

Catania AstrophysicalObservatory (OAC).The observationswere perform ed in the John-

son UB V standard system underphotom etric sky conditions,by using a photon-counting

single-head photom eter equipped with an EM I9893QA/350 photom ultiplier. In order to

determ inethetransform ation coe�cientsto theJohnson standard system ,severalstandard

stars selected from the GeneralCatalogue ofPhotom etric Data (GCPD,M erm illiod etal.

1997)and from Landolt(1992)werealso observed in between thetargetsobservations.The

observing set-up and the data reduction were the standard ones already adopted during

previousobservationalcam paignsoflow-m assstars(see,e.g.,M arillietal.2007).

2.2. N ear-infrared

Near-infrared J,H and K s photom etry ofa � 300� 300 region centered on NGC 1788

wasobtained using the SOFIcam era atthe ESO NTT telescope (program No.70.C-0629)

on Decem ber 17th,2002 and January 16th,2003. 36 pointingsofthe 50� 50 �eld ofview

were required to coverthe�eld in each band.Severaldithered exposureswere obtained for

a totalintegration tim eperpointing of80 secondsin theJ band and 180 secondsin theH

and K sbands.Thedatawerecorrected forelectroniccrosstalk,di�erential at�elding,and

illum ination following standard SOFIproceduresunderIRAF.

Photom etriccalibration wasobtained observing a setofstandard starsfrom thelistof

Persson etal.(1998)(LasCam panasObservatory standards). Source extraction and aper-

ture photom etry were perform ed using the SExtractorsoftware package (Bertin & Arnouts

1996); aperture corrections were estim ated from sources away from crowded regions and

applied to allextracted sources.Thelim iting m agnitudesat3� levelachieved in oursurvey

are19,18 and 17.5 m ag atJ,H ,and K s,respectively.

1IRAF is distributed by the NationalO pticalAstronom y O bservatory,which is operated by the Asso-

ciation ofthe UniversitiesforResearch in Astronom y,inc. (AURA)undercooperative agreem entwith the

NationalScience Foundation.

{ 5 {

3. Spectroscopy

Based on IC versus (R C � IC ) colour-m agnitude diagram and theoreticalisochrones

from Bara�eetal.(1998),Alcal�a etal.(2004)selected a sam pleofabout200 PM S starand

BD candidates in L1615/L1616,by using the spectroscopically con�rm ed young low-m ass

stars to de�ne the PM S locus,as shown in their Figure12. In addition to this sam ple,

8 X-ray em itting stars detected by the ROSAT All-Sky Survey (RASS),as wellas m id-

infrared sources(Stankeetal.2002)and H� em ission-lineobjects(Nakano etal.1995),were

also selected as further PM S star candidates by Alcal�a etal.(2004). W e perform ed the

spectroscopicfollow-up ofabout70% ofthecandidatesin thissam ple,down toIC � 19m ag,

in orderto de�nitively assess theirnature and single outthe young objects. Additionally,

about30% of�eld starswith IC . 20:0 m ag wasalso observed in orderto detectpossible

\veiled" PM S starswhich m ighthaveescaped theselection criterion (seelaterFigure8).

Interm ediate- and low-resolution spectra were acquired during di�erent runs at the

ESO Very LargeTelescope(VLT;ParanalObservatory,Chile),by using theFOcalReducer

and low-dispersion Spectrograph 2 (FORS2;Appenzelleretal.1998)and theVIsibleM ulti-

Object Spectrograph (VIM OS;LeFevreetal.2003). M ulti-object interm ediate-resolution

spectroscopy was also perform ed with the Fibre Large Area M ulti-Elem ent Spectrograph

(FLAM ES;Pasquinietal.2002),but these observations willbe described in m ore detail

in a forthcom ing paper. The journalofthe FORS2 and VIM OS observations is given in

Table 1. The distribution on the sky ofthe area covered by the FORS2,VIM OS,and

FLAM ES spectroscopicfollow-upsisshown in Figure1.In thefollowing sub-sectionsa brief

description ofthespectroscopic observationsand data reduction ispresented.

{ 6 {

Fig.1.| Sky areassurveyed by theFORS2 (continuouslines),VIM OS (dashed lines),and

FLAM ES (dotted lines)spectroscopicfollow-upsoverlaid on theIRAS 100�m dustem ission

m ap covering a sky-area ofabout1�� 1� around L1615/L1616.Notethatthegapsofabout

20between thefourVIM OS quadrantsarenotreproduced in the�gure.

{ 7 {

3.1. FO R S2 observations

Interm ediate-resolution (�/��=2140) FORS2 spectra ofthe PM S candidates m ainly

projected on theNGC 1788 re ection nebula were collected in service m ode (ESO program

No.70.C-0536)on thenights7,22,25 February and 8 M arch 2003,underm ostly clearand

stable weatherconditions,with seeing typically in the range 0:008� 1:000. These observations

were perform ed setting the spectrograph in the m ulti-object spectroscopy m ode. A total

of5 m ask con�gurations,each including 18{19 objects,were observed. Three consecutive

spectra of960 secwereobtained foreach m ask con�guration in orderto rem ove cosm ic-ray

hitsand im prove the signalto noise (S/N)ratio. Bias, at-spectrum lam p,and Ar/He/Ne

lam p exposures for each m ask were taken in daytim e according to the FORS2 standard

calibration plan.ThetypicalS/N ratio fora IC � 16 m ag starwason theorderof90.

The standard data reduction process was carried out with a sem i-autom atic pipeline

that we have developed on the basis ofboth M IDAS2 (W arm els 1991) and IRAF pack-

age.Thereduction includesbiassubtraction, at-�eld division,wavelength calibration,sky

subtraction and one-dim ensionalspectra extraction. Relative ux calibration wasachieved

observing �vespectrophotom etricstandard starsfrom Ham uy etal.(1992,1994).

3.2. V IM O S observations

The VIM OS observationswere perform ed in service m ode (ESO program No. 074.C-

0111)during the period from 9 Decem ber2004 to 12 M arch 2005.The weatherconditions

were photom etric with seeing varying between 0:006 and 1:002. The larger sky-area covered

with VIM OS allowed us to extend the spectroscopic survey in a wider region around the

NGC 1788re ection nebula3.Furtherm ore,wecarried outboth interm ediate-(�/��=2500)

and low-resolution (�/��=580)spectroscopy,in orderto detecttheLii�6708 �A absorption

line and m easure its equivalent width,as wellas to obtain a wider wavelength range for

spectraltypeclassi�cation purpose(seeSection 5.2).

A totalof3 interm ediate-and 6 low-resolution m ask con�gurations,each including an

average num berofabout100 targets,were observed. To e�ciently pursue ourprogram m e

and m axim ise the num berofobserved �elds,only one exposure of900 sec and one of2000

secwereobtained foreach low-and interm ediate-resolution m ask con�guration,respectively.

2M IDAS isdeveloped and m aintained by ESO .

3Note,however,that the gaps ofabout 20 between the four VIM O S quadrantsprevented us from per-

form ing a uniform coverage.

{ 8 {

ThetypicalS/N ratio atIC � 16 m ag wasabout80 and 180 fortheinterm ediate-and low-

resolution spectra,respectively.

Fivespectrophotom etricstandard starsfrom Oke(1990)and Ham uy etal.(1992,1994)

wereobserved to perform theabsolute ux calibrations.Biasand at�eld fram esaswellas

wavelength calibration exposureswere acquired during daytim e,following theservice m ode

VIM OS calibration plan.

TheVIM OSspectrawereautom aticallyreduced byusingtheESO pipeline,fulldetailsof

which aregiven by Izzo etal.(2004).In orderto check thereliability ofthedata reduction,

the spectra were also independently processed by using the VIM OS Interactive Pipeline

GraphicalInterface(VIPGI;Scodeggio etal.2005),obtaining consistentresults.

3.3. FLA M ES-G IR A FFE observations

The FLAM ES-GIRAFFE observationswere perform ed in visitorm ode (ESO program

No. 076.C-0385) during the nights 28 February 2006 and 1,2 M arch 2006,under good

seeing conditions(0:007� 1:000),setting thespectrograph in theM EDUSA con�guration.The

low-resolution grating LR06 was used in conjunction with an order separating �lter and

a slit width of100. The adopted con�guration yielded a spectralcoverage ofabout 750 �A

(6438{7184 �A)with a m ean resolving power�/��=8600.

Sincetheseobservationsarepartofaseparatestudyon radialvelocitiesin L1615/L1616,

these data willbe described in m ore detailin a future paper. Here we have used the

FLAM ES spectra ofonly three objects,nam ely TTS050644.4�032913,RX J0507.3�0326,

and TTS050741.0�032253 (see Table 2),forwhich both FORS2 and VIM OS observations

arem issing.

4. PM S objects in L1615/L1616

Sincelithium israpidly destroyed in theconvectivelayersoflow-m assstarsin theearly

phases oftheir stellar evolution (Bodenheim er 1965),the presence ofstrong Lii�6708 �A

absorption line has been considered as the prim ary criterion for de�nitely assessing the

PM S nature ofthe observed stars. The H� em ission hasbeen used asfurtherindicatorof

youth,though itspresencealonedoesnotguaranteethePM S natureoftheobserved object.

Therefore,alltheK and M starswith H� in em ission butlackingtheLiiabsorption linehave

been rejected and classi�ed asolderdKeand dM e�eld starsprojected onto L1615/L1616.

{ 9 {

In addition to the above criteria,the presence offorbidden em ission lines,as wellas

non-photosphericUV and infrared continuum excesses(asevidenceofbipolarjets,accretion

colum ns,and circum stellardisk,respectively) have yielded furtherargum entson the PM S

natureoftheyoung starsidenti�ed in L1615/L1616.

Based on these criteria,we have identi�ed 25 new PM S starswith spectraltype later

thanaboutK1.ByaddingthesesourcestothepreviouslyreportedPM Sobjects(Alcal�a etal.

2004),the num berofoptically detected young starsknown so farin L1615/L1616 risesto

56 objects4. The fullsam ple is listed in Table 2;coordinatesare those from the 2M icron

AllSky Survey (2M ASS;Cutrietal.2003)pointsourcecatalogue,which haveaprecision of

about0:001. Allthe PM S starsrevealed by ourspectroscopic follow-up,butnotdetected in

previousX-rayand H� surveys,havebeen designated with \TTS"followed by theirposition,

according to theIAU convention.By adding them illim eterClass-0 protostardiscovered by

Stankeetal.(2002),which isnotreported in Table2,theyoung population ofL1615/L1616

increasesto57.Thus,ourspectroscopicsurvey alm osthasdoubled thenum berofcon�rm ed

PM S objectsassociated to L1615/L1616.Som eexam plesofFORS2and VIM OS spectra are

shown in Figure2.

In total,4 out ofthe 8 X-ray em itting sources listed by Alcal�a etal.(2004),nam ely

RX J0506.8�0305,RX J0507.4�0317,RX J0507.6�0318,and RX J0507.3�0326,have been

spectroscopically con�rm ed asPM S stars;theotherX-ray sources(RX J0506.5�0320,

RX J0506.4�0323,RX J0506.4�0328W ,and RX J0506.4�0328E)turned outto be �eld ac-

tive stars. Note that lithium has been detected for the �rst tim e in the spectrum ofthe

m id-infrared sourceL1616M IR4 (Stankeetal.2002).

TheS/N ratio ofthespectra ofthefaintsourcesTTS050654.5�032046 and

TTS050730.9�031846 turned outto be insu�cientfora reliable m easurem entoftheirLii

equivalent width. Nevertheless,the presence ofa broad and strong H� em ission in both

spectra (Table 4) can not be explained by chrom ospheric activity only and is typicalof

classicalT Tauristars. Indeed,according to the de�nition by W hite& Basri(2003),the

upper lim it on the H� equivalent width for chrom ospheric activity,in the spectralrange

M 3{M 5.5,is40 �A.TTS050654.5�032046 and TTS050730.9�031846 havespectraltypeM 4

and M 5.5andequivalentwidth 60�A and 290�A,respectively;thus,theyareclearlyaccretors.

ThestarKisoA-097417was�rstclassi�ed asaH� em ission-lineobjectby Nakano etal.

(1995). Although Alcal�a etal.(2004)did notobtain any spectrum ofthissource,they in-

cluded itin theirlistofPM S stars. W e observed thisobjectduring the FLAM ES spectro-

4Note thatthe starK isoA-0974 17 (Nakano etal.1995;Alcal�a etal.2004)hasbeen excluded from our

listofPM S objects(seebelow).

{ 10 {

scopicrun.NeitherH� em ission norLiiabsorption linewereobserved in itsspectrum .This

could be due to a m ism atch between the H� em ission source and the opticalcounterpart

detected by Nakano etal.(1995).Thusweexcluded thisobjectfrom ourlistofPM S stars.

The optical(UB V R C IC ) and near-infrared (JH K s) m agnitudes ofthe 56 optically

con�rm ed PM S stars in L1615/L1616 is presented in Table 3. Som e ofthe objects lack

W FI,OAC,and/orSOFIphotom etry becausethey falloutsidethesky-area covered by our

survey,are saturated in our im ages or are too faint to be observed with the 91cm tele-

scope at the OAC observatory. Nevertheless,for these stars we retrieved IC m agnitudes

from theTASS M ark IV photom etricsurvey (Droegeetal.2006),IC J m agnitudesfrom the

DEep Near-Infrared Survey (DENIS;Epchtein etal.1997),and JH K s photom etry from

the 2M ASS pointsource catalogue. Both the SOFIand DENIS NIR m agnitudeswere ho-

m ogenised to the 2M ASS photom etric system using the transform ation equationsprovided

by Carpenter(2001).Fora few sources,weused theopticalphotom etry from theliterature

(Lee 1968;M undt& Bastian 1980;Tayloretal.1989;Alcal�a etal.1996;Cieslinskietal.

1997;Frasca etal.2003;Vieira etal.2003)asexplained in thefootnotesofTable3.

{ 11 {

Fig. 2.| VIM OS and FORS2 spectra of three PM S objects in L1615/L1616, nam ely

TTS050650.7-032008,TTS050733.6-032517,and RX J0507.6-0318. The spectra have been

arbitrarily norm alised to the ux at6760 �A.Foreach objectthe whole observed spectrum

isshown in the leftpanel.The spectralrangeencom passing the H� and Lii�6708 �A lines

isplotted in m oredetailin therightpanel.

{ 12 {

5. D ata analysis

5.1. H � and Liiequivalent w idths

H� and Lii �6708 �A line equivalent widths, W (H�) and W (Li), respectively, were

determ ined by directintegration ofthelinepro�les,asdescribed in Alcal�a etal.(2004)and

Spezzietal.(2008),adopting the convention that a negative equivalent width m eans an

em ission line.Them ain sourceoferroron thesem easurem entscom esfrom theuncertainty

in the placem entofthe photospheric continuum ,especially forstarslaterthan M 1,whose

spectra are strongly a�ected by m olecular absorption bands. W (H�) and W (Li) for the

L1615/L1616m em bersarereported in thethird and fourth colum nsofTable4,respectively.

According totheintensity oftheH� em ission lineand thede�nitionsby W hite& Basri

(2003),weassigned theclassical(CTTS)orweak (W TTS)T Tauristarclassi�cation to the

PM S objectsin L1615/L1616 (Table4).

Theadditionalem ission linesobserved in thespectra,including thelinesofHei�5876,

�6678.7 and �7065.2�A,and Nai�5889.9 and �5895.9�A,aswellasthe forbidden linesof

[N ii]�6583.5 �A,[O i]�6300.3 and �6363.8 �A,and [Sii]�6716.4 and �6730.8 �A,are also

reported in Table 4. Note that alm ost only those PM S stars classi�ed as CTTSs show

forbidden em ission linesin theirspectra.

5.2. Spectraltypes and e� ective tem peratures

Spectraltypeswere determ ined using a m odi�ed version ofROTFIT,a code forspec-

traltype and vsinideterm ination developed by Frasca etal.(2003,2006)underthe IDL5

environm ent. The generalidea ofthe m ethod,already adopted in Alcal�a etal.(2006)and

Spezzietal.(2008),isto recoverthespectraltypeofa starby com paring itsspectrum with

a suitable grid oftem plates,taking into accountthe am ountofextinction along the line of

sight.Since ourspectra are ux calibrated,the m ethod allowed usto geta rough estim ate

oftheextinction (A V )along thelineofsightto thestar.

Tothisaim ,alibraryofrelative uxcalibrated and extinction-corrected tem plates,with

awavelength coverageencom passing thatoftheobserved spectra,isneeded.A suitablegrid

ofinterm ediate-resolution (R � 1000� 5000)tem plateshasbeen obtained by collectingspec-

traofdwarfand giantstarsfrom thelibrariesprovided by M art��n etal.(1999),Hawley etal.

5IDL isdistributed by ITT VisualInform ation Solutions,Boulder,Colorado.

{ 13 {

(2002),LeBorgneetal.(2003),Valdesetal.(2004),and Bochanskietal.(2007).

In perform ingthespectraltypeclassi�cation,we�rsttook intoaccountthePM S nature

ofthestarsweweredealingwith.DuetothepresenceofTiO andVO absorptionbandswhich

arestronglysensitivetothesurfacegravity (Torres-Dodgen & W eaver1993),m ostofM -type

PM S objectsshow opticalspectroscopic featuresthatcan be closely reproduced averaging

dwarfand giant spectra with the sam e spectraltype (Luhm an 1999;Luhm an etal.2003;

Guieu etal.2006).Though thespectra ofG-typeand K-typePM S starshavebeen usually

com pared with tem plateofnorm aldwarfstarsin severalworks(e.g.Basri& Batalha 1990),

we found that also the late K-type objects in our sam ple are best represented by using

an average ofdwarfand giant tem plates ofthe sam e spectraltype. Indeed TiO and VO

absorption m olecular bands are stillpresent in late K-type stars,although they are very

weak.

Thus,both late-type dwarfand gianttem plateswere included in ourgrid ofreference

spectra;m oreover,forspectraltypeslaterorequalthan K5,wealsobuiltup an adhocgrid of

tem platesby averagingdwarfand giantspectraofthesam espectralsub-class.Furtherm ore,

starting from spectraltypeslaterthan K7,weconstructed tem platesforthehalfsub-classes

(e.g.K8.5,M 0.5,M 1.5,etc.) by averaging thetwo contiguousspectra.Indeed,theintensity

ofthe absorption m olecular bands characterising late-type stars displays a rather strong

variation from one spectralsub-classto theother,allowing to appreciate di�erencesofhalf

a sub-class.Som edetailson thespectra �tting procedureisprovided in Appendix A.

ThespectraltypesofthePM S objectsin oursam plearereported in thesecond colum n

ofTable 4. From an inspection ofthe residualsofthe �tting procedure we have estim ated

an accuracy ofabout� 1 sub-classforstarsearlierthan K7 and about� 0:5 sub-classfor

cooler objects. A typicaluncertainty on A V of� 0:2 m ag was found. Forallthe objects

observed atleasttwice,with di�erentspectrographsand/orresolving powers,we obtained

consistent results,regardless ofspectralresolution and wavelength coverage. Forthe sake

ofhom ogeneity, we re-determ ined the spectraltypes also for the previously known low-

m assm em bers,by using thespectra from Alcal�a etal.(2004).A generalagreem entwithin

one/two spectralsub-classes wasfound between ourclassi�cation and the one reported in

theliterature(Cohen & Kuhi1979;Alcal�a etal.2004).

Forthetwoearly-typestarsHD 293815andKisoA-097415,wehaveadoptedthespectral

typesfrom Sharpless(1952)and Vieira etal.(2003),respectively.Both starswere spectro-

scopically observed byus,butm any photosphericfeaturessuitableforan accurateearly-type

classi�cation were notcovered in ourspectra. Nevertheless,we attem pted a spectraltype

estim ateforboth starsby applyingtheHern�andezetal.(2004)’scriteria.Them ain features

weused aretheHei�5876 and �7065.2�A absorption lines.Theresultsareconsistentwith

{ 14 {

thosereported by Sharpless(1952)and Vieira etal.(2003).

In linewith ourspectraltypeclassi�cation,thee�ectivetem perature(Te�)ofeach PM S

starwas assigned on the basis ofthe following criteria: the dwarftem perature scale from

thecom pilation ofKenyon & Hartm ann (1995)wasadopted foralltheobjectswith spectral

type earlierorequalto K4;forlaterspectraltypesan interm ediate tem perature scale was

derived byaveragingthegiantcom pilation provided byvan Belleetal.(1999)with thedwarf

onefrom Kenyon & Hartm ann (1995)forK0-M 0 and from Leggettetal.(1996)forM 1-M 7

spectraltypes,respectively. The Te� values corresponding to the halfspectralsub-classes

were com puted by linear interpolation. The e�ective tem peratures assigned to each PM S

object are reported in the second colum n ofTable 5. In assessing the errors on Te� the

uncertainty on thespectraltypeclassi�cation wastaken into account.

5.3. Extinction,stellar radii,and lum inosities

The m ethod we used for com puting extinction,stellar radii,and lum inosities follows

the generalguidelines described in Rom aniello etal.(2002,2006),although we m odi�ed

their schem e to m atch our purposes. The eight broad-band m agnitudes available for the

L1615/L1616PM S populationallowed ustoconstructthespectralenergydistribution (SED)

ofeach PM S starcovering a wide spectralrange,from opticalto near-infrared wavelengths

(seeTable6).Byusingsim ultaneously allthephotosphericcoloursencom passed bytheSED,

wecom puted foreach objectboth theinterstellarextinction (A V )and theratio oftotal-to-

selective extinction (R V = A V =E B�V ),as wellas the stellar radius (R ?) and lum inosity

(L?).

Apart for eventualultraviolet and infrared excesses caused by the presence ofan in-

teracting accretion disk around the PM S star and by the related boundary layer,or hot

accretion spots,the shape ofthe observed SED ism ostly determ ined by the e�ective tem -

peratureoftheobject,theam ountofdustalong thelineofsight,and thedust-grain m ean

size in the interstellar/cicum stellarenvironm ent. Therefore,once the e�ective tem perature

is known,both the value ofA V and R V m ay be recovered sim ultaneously by �tting the

observed SED with a grid oftheoreticalones. Indeed,both the star radius and distance

causejusta rigid shiftofthe ux on a logarithm icscale,withouta�ecting theshapeofthe

SED.TheoreticalSEDsm ay beobtained by using stellaratm ospherem odelswith thesam e

e�ective tem peratureasthestarand reddened by variousam ountofA V and R V .

A detailed description oftheSED �tting procedureisreported in Appendix B.Herewe

wantto stressthat,ourtwo-param eter�tting procedurecan beapplied only to theobjects

{ 15 {

which aresigni�cantly a�ected by extinction,i.etothestarswhich arestilldeeply em bedded

in theirparentcloud.Indeed,forslightly reddened starsthevalueofA V isrelatively insensi-

tiveto changesin theslopeofthereddening law (R V ),asshown in Appendix B.Therefore,

foralltheobjectsforwhich A V turned outto belessthan about0.5 m ag wereiterated the

�tting procedure�xing R V to thestandard valueof3.1.Thesam eassum ption wasadopted

foralltheobjectslacking opticalphotom etry and fortheveiled starTTS050649.8�032104.

ThevaluesofA V ,R V ,R ?,and L? arelisted in Table5.Foralltheobjectsspectroscop-

ically observed in this work,we found thatthe value ofA V derived from the SED �tting

procedureisin agreem ent,within � 0:2 m ag,with theoneobtained from thespectraltype

classi�cation.

R V wasderived for15 outofthe 56 PM S starsin L1615/L1616 (Table 5). W e found

a weighted m ean value of3:32� 0:04,closely m atching the standard one of3.1,typicalof

the di�use interstellarm edium . Forthe Herbig starKisoA-0974 15 we obtained a value of

R V = 5:5 (see Table 5). This m ay be related to the presence ofdust grains around the

interm ediate-m ass starwhich are largerin size than those characteristic ofthe interstellar

m edium ,asalready found by di�erentauthorsin large sam plesofHAeBe stars(Th�eetal.

1981;Herbstetal.1982;W aters& W aelkenset 1998;W hittetetal.2001;Hern�andezetal.

2004).

5.4. M asses and A ges

The m ass and age of each m em ber of L1615/L1616 have been derived by com par-

ing the location ofthe object on the H-R diagram with the theoreticalPM S evolutionary

tracksby Bara�eetal.(1998)& Chabrieretal.(2000),D’Antona & M azzitelli(1997),and

Palla & Stahler(1999)(Figure3).

Since stellar evolutionary m odels are stillrather uncertain, particularly in the low-

m ass star and sub-stellar regim es (M < 0:5M � ,Bara�eetal. 2002),the use ofdi�erent

evolutionary tracks allowed us to estim ate the m odel-dependent uncertainties associated

with thederived stellarparam eters.

Table 7 lists m asses and ages ofthe L1615/L1616 m em bers as inferred by the three

setsofm odels.Notethattheevolutionary tracksby Bara�eetal.(1998)& Chabrieretal.

(2000),D’Antona & M azzitelli(1997),and Palla & Stahler(1999)areavailablein them ass

range0:003� M � 1:40 M � ,0:017� M � 3 M � ,and 0:1� M � 6 M � ,respectively.Note

also thatisochronesby Bara�eetal.(1998)& Chabrieretal.(2000)are available only for

agesgreaterorequalthan 1 M yr.

{ 16 {

Fig. 3.| H-R diagram for the young population in L1615/L1616;the PM S evolutionary

tracksbyBara�eetal.(1998)& Chabrieretal.(2000)(upperpanel),D’Antona & M azzitelli

(1997)(centralpanel),and Palla & Stahler(1999)(lowerpanel)areover-plotted.The H-R

diagram fortheinterm ediate-m assm em bersofL1615/L1616,nam elyHD 293815and KisoA-

0974 15,isshown in theinsetofthelowerpanel.

{ 17 {

6. D iscussion

Once the physicalparam eters ofthe PM S objects are known,we can study the star

form ationin L1615/L1616.In thenextsub-sectionswediscussthehistory,rateand e�ciency

ofstarform ation,and som eissuesrelated to them ass-spectrum .

6.1. T he star form ation history

{ 18 {

Fig.4.| IRAS 100�m dustem ission m ap coveringasky-areaof5�� 5� around L1615/L1616

(left panel). The Classicaland W eak T Tauristars are represented with �lled dots and

squares,respectively. The upperarrow showsthe direction to the Orion OB1 associations,

located atabout7.5� (� 60 pcata distance of450 pc)to the North-EastofL1615/L1616.

The lowerarrow pointstoward the Orion A GiantM olecularCloud,also located atabout

7.5� to the South-EastofL1615/L1616. The dashed square de�nesthe area surveyed with

W FI.Thisregion iszoom ed in the rightpanel,where the two interm ediate-m assm em bers

HD 293815 and KisoA-0974 15 arem arked with �ve-pointed starsym bols.

{ 19 {

Based on them ostcom pletecensusoftheL1615/L1616’spopulation perform ed in this

work,we further investigated the triggered star form ation scenario suggested by Ram esh

(1995),Stankeetal.(2002),and Alcal�a etal.(2004).

In left panelofFigure 4 the IRAS 100 �m dust em ission m ap ofa 5� � 5� sky-area

around L1615/L1616 isshown.About64% oftheCTTSsareclustered,in thedensestpart

ofL1615/L1616,i.e.within the boundaries ofthe NGC 1788 re ection nebula and to the

W estofthebrightrim ofthecloud (Figure4,rightpanel).Such rim islocated totheEastof

NGC 1788,at� 6:50from thehead ofthecom etary cloud (about0.85pcatadistanceof450

pc)and itisdirectly exposed to the UV radiation from the Orion OB stars. On the other

hand,the W TTSsare m ore scattered (only � 22% isprojected on NGC 1788)and m ainly

occupy thesideofthecloud facing theOB1 associationsand to theEastofthebrightrim .

By applying the Kolm ogorov-Sm irnov testto the W TTS and CTTS age distributions

wefound thatthetwopopulationsaresim ilaratacon�dencelevelof� 80% .Thus,itseem s

there are no age di�erences between CTTSs and W TTSsin ourPM S sam ple. Thisresult

isconsistentwith previousPM S populationsstudies(see Feigelson & M ontm erle 1999,and

referencetherein);CTTSsarepredicted tohaveagesbetween about0.5and 3M yr,although

som e starsretain CTTS characteristicseven atagesasold as20 M yr. On the otherhand,

m any W TTSsoccupy the sam e region on the H-R diagram asCTTSsdo,whereassom e of

them areapproaching thezero-agem ain sequence.

The age distribution ofthe L1615/L1616 population peaksbetween 1 and 3 M yr,de-

pending on the adopted evolutionary tracks(Figure 5,rightpanels). Thisisin agreem ent

with the �ndings by Alcal�a etal.(2004),but the age spread found by us is signi�cantly

higherthan whatthese authorsclaim ,exceeding thevalueexpected on thebasisoftheun-

certaintieson lum inosity and tem perature. The L1615/L1616 m em bersspan a wide range

in age,from lessthan 0.1 M yrup to about30 M yr. Thism ightsuggestm ultiple eventsof

star form ation in the cloud,which would further support the hypothesis oftriggered star

form ation.

In ordertoinvestigateapossibleagedi�erencebetween \on-cloud"and \o�-cloud"PM S

objects,we divided the sam ple in two groups,�xing asdividing line the brightrim ofthe

cloud.Twenty of56 objectsin oursam ple arelocated within theboundariesofNGC 1788,

i.e.to theW estofthebrightrim ,while36 areo�-cloud m em bers.Theagedistributionsof

thetwogroupsareshown in Figure6.Theon-cloud PM S starsarestatistically youngerthan

those located to the Eastofthe brightrim ,regardlessofthe adopted evolutionary tracks.

Applying a Kolm ogorov-Sm irnov test we found that the probability thatthe two sets are

sub-sam pleofthesam estatisticalpopulation isvery low;in particular,wefound con�dence

levels of1.29,0.18,and 0.19% when using the PM S evolutionary tracks by Bara�eetal.

{ 20 {

(1998)& Chabrieretal.(2000),D’Antona & M azzitelli(1997),and Palla & Stahler(1999),

respectively.W ethusconcluded thatthereisa clearagedi�erencebetween thetwo groups

ofstars.

Theabove�ndingsfurthersupportthescenariooftriggeredstarform ationinL1615/L1616,

as proposed by Stankeetal.(2002). In this context,the spatialdispersion and older age

ofthe \o�-cloud" m em berscan be explained asa consequence ofthe \rocketacceleration"

e�ect. As recently pointed out by Kun etal.(2004),this acceleration continues after the

onset ofstar form ation and the parentalcloud is further accelerated with respect to the

newly form ed objects. As a consequence, the cloud is soon swept o� the newly form ed

stars. This hypothesis,together with the rapid dispersion typicalofsm allclouds,causes

the spatialdisplacem ent ofthe oldest cloud m em bers. The conspicuous num ber ofPM S

starsfound apparently isolated from classicalstarform ing regions(e.g.Alcal�a etal.1995;

Covino etal.1997;Guilloutetal.1998a,b;Frasca etal.2003;Zickgrafetal.2005)m ightbe

a consequence ofthism echanism aswell.

{ 21 {

Fig. 5.| M ass (left panel) and age (right panel) distributions ofthe L1615/L1616 PM S

population derived byusingtheevolutionarytracksbyBara�eetal.(1998)& Chabrieretal.

(2000) (upper panels),D’Antona & M azzitelli(1997) (centralpanels) and Palla & Stahler

(1999)(lowerpanels).

Fig. 6.| Age distributions ofthe on-cloud and o�-cloud PM S stars in L1615/L1616 de-

rived by using theevolutionary tracksby Bara�eetal.(1998)& Chabrieretal.(2000)(up-

perpanel),D’Antona & M azzitelli(1997)(centralpanel)and Palla & Stahler(1999)(lower

panel).

{ 23 {

6.2. T he star form ation e� ciency

Based on 12CO and 13CO colum n density m aps,Ram esh (1995)estim ated thatthem ass

ofL1616 aloneisin therange169� 193 M � ,depending on theused tracer.Yonekura etal.

(1999)m apped both L1615 and L1616 in theCO J=1-0 transition line.They inferred that

the m assofthe cloud system asa whole is� 530 M � ,� 350 M � and � 440 M � based on12CO,13CO and 18CO observationsrespectively.They alsoderived them assofthe13CO and18CO cores(i.e.L1616only)�nding avalueof� 146M � and � 161M � ,in good agreem ent

with theRam esh (1995)’sdeterm ination.

Alcal�a etal.(2004) estim ated the SFE in L1616 to be � 14% ,i.e. higher than the

average value m easured in otherlow-m assstarform ing region (. 3% ). The SFE reported

by Alcal�a etal.(2004) for L1616 is based on the cloud m ass reported by Ram esh (1995)

and the totalstellarm assofthe 32 m em bersofthe cloud investigated by the authors(i.e.

� 30 M � ).

Since the starform ation history ofL1615 and L1616 isintim ately connected,we have

re-calculated the SFE considering the system L1615 plus L1616 asa whole. By using the

NICER colorexcess m ethod by Lom bardi& Alves (2001),we m apped the dustextinction

ofthecloud com plex and derived itsm ass.In theNICER techniquetheJ � H and H � K s

coloursofthe�eld starsarecom pared tothecoloursofstarsin anearby reference�eld.The

colour-excessesofthe�eld starsarethen com bined and transform ed to thevisualextinction

A V ,�xing the form ofthe reddening curve. The norm alinterstellarextinction law derived

by (Cardellietal.1989)hasbeen adopted by usforthispurpose.W eretrieved the2M ASS

colours ofallthe point-like sources located in a 500� 500 region centered on NGC 1788

and in a reference �eld close to the L1615/L1616 cloud6. The latter�eld was selected by

using the nearby low-intensity regions ofthe IRAS 100�m dust em ission m ap,according

to prescription by Kainulainen etal.(2006). Finally,the visualextinction m ap resulting

from theNICER m ethod wasconverted tocloud m assby assum ing thestandard gas-to-dust

ratio (Bohlin etal.1978).W efound a valueof� 550 M � ,in good agreem entwith the one

inferred by Yonekura etal.(1999)on the basisof12CO observations. Based on thisvalue

and on thetotalm assofthe56 present-day known m em bersofthecom plex (i.e.41{46M � ,

depending on theadopted evolutionary tracks),wederived a SFE in L1615/L1616of7{8% ,

i.e. signi�cantly lowerthan the previousestim ate by Alcal�a etal.(2004),butstillin good

agreem entwith the one generally found in giantm olecularcloudshosting OB associations

(5{10% )and predicted by theoreticalcalculationson theform ation ofOB associations(see

6TheSO FIsurvey isonly centered on thedensestpartofthecloud and can notbeused to m ap thecloud

asa whole.

{ 24 {

Clark etal.2005,and referencetherein).

6.3. D ensity ofstar form ation and star form ation rate

An interesting question is whether L1615/L1615 can be considered as a cluster. Ac-

cording to the de�nition suggested by Lada & Lada (2003),a cluster is a group ofsom e

35 m em bers with a totalm ass density larger than 1.0 M � pc�3 . To estim ate the density

ofPM S objectsin L1615/L1615,we considered the region spectroscopically surveyed by us

(Figure 1). This region covers an area ofapproxim ately 0.25 square degrees and includes

about40 PM S objects. Assum ing a distance of450 pc,the resulting area is about4 pc2,

which m eans a surface density ofabout10-11 PM S objects perpc2 and a volum e density

on the orderof7 PM S objectsperpc3. In the lattercalculation we estim ated the volum e

asV = 0:752�Area1:5 (see J�rgensen etal.2007),assum ing a locally sphericaldistribution

ofsources. The average m ass ofthe 40 PM S objects,as determ ined from the results in

Section 5.4,ison theorderof0.7M � ,which im pliesa volum edensity ofabout4-5M � pc�3 .

Therefore,thegroup of40 PM S objectscon�ned in thespectroscopically surveyed area can

beconsidered asa clusteraccording to thecriterion by Lada & Lada (2003).

Now,itisinteresting to estim ate the rate atwhich the starsin thissm allcluster are

form ed. According to the resultsofSection 5.4,we estim ated thatthe totalm assin PM S

objectsin thatregion ison the orderof41{46 M � . Therefore,considering the average age

of2 M yrfortheseobjects(seeFigure5),wefound thatthecom etary cloud isturning som e

20-23 M � into PM S objectsevery M yr,which islowerthan thestarform ation ratein other

clusterslike those in Serpens(Harvey etal.2007),buthigherthan in otherT associations

like Cham aeleon II(Alcal�a etal.2007)and Lupus(M er��n etal.2007). Thuswe concluded

thatL1615/L1616 isa sm allclusterwith a m oderatestarform ation rate.

6.4. T he InitialM ass Function

Though based on a low-num ber statistics,the �rst clue on the InitialM ass Function

in L1615/L1616 was given by Alcal�a etal.(2004). Based on this study,the IM F in this

region appearsroughly consistent with thatofthe solarneighbourhood in the m assrange

0:3 < M < 2:5 M � (M iller& Scalo 1979). The authors also found severalcandidates for

young BDsand estim ated theexpected num berofBDsrelativeto m orem assivePM S stars

in L1615/L1616 to be interm ediate between Taurus(� 13% ,Brice~no etal.2002)and the

Trapezium cluster(� 26% ,Luhm an etal.2000).

{ 25 {

The depth of the spectroscopic survey conducted in this work goes down to IC �

19:0 m ag (i.e. M � 0:03� 0:05 M � ,see Section 6.5),leading to the discovery ofyoung

objects with a m ass close to the Hydrogen burning lim it. W e have thus attem pted a fair

IM F determ ination down to thelow-m assstellarregim e(i.e.M & 0:1 M � ).

Allthepresent-day known m em bersofL1615/L1616 havem assesin therangebetween

�0.1and 5.5M � ;theirm ean m assvariesbetween 0.6and 0.8M � depending on theadopted

evolutionary tracks (see Table 7 and Fig 5,left panels). However, although our survey

recovers m ost ofthe previously known young objects in L1615/L1616,the com pleteness

ofour PM S sam ple is di�erent from the one of Alcal�a etal.(2004) sam ple. The latter

includesbrightand solar-likeobjectslocated in areasnotcovered by thedeeperW FIsurvey.

Therefore,we restriced ourIM F analysisto the sam ple ofobjectsthatlie on the sky-area

covered with W FI(Figure4,rightpanel).

The shape ofthe IM F in the cloud m ay then be constrained using the usualapproxi-

m ation forthe low-m ass function (dNdM

/ M �� ),asreported by (M oraux etal.2003). W e

divided them assrangeintom assbinsofabout0:2M � ;thisvalueislargerthan theaccuracy

on m assestim atesasderived from theuncertaintieson tem peraturesand lum inositiesand,

atthesam etim e,allowsusto havea statistically signi�cantnum berofobjectsin each bin.

{ 26 {

Fig.7.| TheL1615/L1616IM F between 0.1and5.5M � asderived byusingtheevolutionary

tracksbyBara�eetal.(1998)& Chabrieretal.(2000)(upperpanel),D’Antona & M azzitelli

(1997)(centralpanel)and Palla & Stahler(1999)(lowerpanel).

{ 27 {

Figure7showstheIM F in L1615/L1616asobtained byusingthethreesetsofevolution-

ary tracksby Bara�eetal.(1998)& Chabrieretal.(2000),D’Antona & M azzitelli(1997)

and Palla & Stahler(1999). Valuesof� in the three casesagree within the errorsand we

can derivea weighted m ean valueof:

� = 0:84� 0:07

asthebestapproxim ationoftheIM F slopeinL1615/L1616inthem assrange0:1�M � 5:5M � .

W enotice,however,thattheevolutionary m odelsby D’Antona & M azzitelli(1997)tend to

produceasteeperIM F with respecttothosebyBara�eetal.(1998)& Chabrieretal.(2000)

and Palla & Stahler(1999),asalready found by Spezzietal.(2008).

In Table 8 the � slope obtained forthe IM F in L1615/L1616 iscom pared with those

obtained in other star form ing regions with di�erent ages and environm entalconditions.

Thesevaluesindicateacom m on shapeofIM F in them assrangefrom � 0:1to� 1M � .This

providesfurtherevidenceofauniversalIM F alsoin thelow-m assregim eand isin agreem ent

with theresultsalready established fortheinterm ediateand m assivestardom ains(Kroupa

2002).

6.5. N o sub-stellar objects in L1615/L1616?

Priorto oursurvey,no con�rm ed BDsin L1615/L1616 werereported in theliterature.

However,Alcal�a etal.(2004) photom etrically selected som e 30{40 candidates for BDs in

this region and estim ated a fraction ofsub-stellar objects relative to PM S stars (R SS) in

the range from 18% to 25% ,i.e. sim ilar to the value m easured in the Trapezium cluster

(Hillenbrand & Carpenter 2000;Brice~no etal.2002;M uench etal.2002)and in otherOB

associations.

Our spectroscopic observations,which investigated about 50% ofthe BD candidates

with IC . 19:0 m ag reported by Alcal�a etal.(2004),revealed 3{4 young objectswith m ass

M � 0:1 M � (seeTable7),closeto theHydrogen burning lim it,and no objectswith lower

m ass.However,takingintoaccounttheaccuracy with which weestim ated them asses,these

objectscould be BDswith a m assjustbelow the Hydrogen burning lim it. The R SS in the

cloud would be then around 5{7% . Though the very low-m ass BDs(M . 0:03 M � )and

som e deeply em bedded low-m ass objects m ight have escaped detection in oursurvey,this

value should be slightly lower than the one estim ated for other T associations (12{15% ;

Brice~no etal.2002;L�opezM art�� etal.2004).

The photom etric survey conducted by usin L1615/L1616 isspatially and photom etri-

cally com plete down to IC � 21:5 m ag at10� level. However,only objects brighterthan

{ 28 {

IC � 19:0 m ag could be observed spectroscopically with a S/N ratio su�cientforspectral

type classi�cation purpose. A 1{3 M yrold object(i.e. the typicalage ofthe L1615/L1616

population) at the distance of450 pc (i.e. a distance m odulus of8.27 m ag) with a m ass

M & 0:03 M � would have IC . 19:0 m ag in theabsence ofextinction (Bara�eetal.1998).

Thus,in the o�-cloud regionsofL1615/L1616,where A V � 0 m ag,oursurvey issensitive

down to � 0:03 M � . The visualextinction occurring in the dense head ofthe cloud varies

in the range 1{5 m ag which displacesthe lim iting m assofthe objectswith IC . 19:0 m ag

to 0.05{0.50 M � (Bara�eetal.1998). W e conclude thatwe are probably m issing the very

low-m assobjectsonly in thesm alldensecoreofthecloud.Given thepeculiarstarform ation

history in thiscloud (Section 6.1),theknowledgeofitssub-stellarm assspectrum could give

crucialinsightsinto theprocessform ing BDsunderparticularenvironm entalconditions.

{ 29 {

Fig.8.| IC versus(R C � IC )diagram forthepoint-likeobjectsin L1615/L1616detected in

ourW FIsurvey above3� level(grey littledots).ThetheoreticalisochronesbyBara�eetal.

(1998) & Chabrieretal.(2000) forthe Cousins photom etric system ofBessel(continuous

line)and fortheW FI-Cousinssystem (dashed lines,Spezzietal.2007)areoverplotted.The

PM S stars with both R C -and IC -band m easurem ents (see Table 3) are overplotted with

black big dots.TheA V = 2 m agreddening vectorisalso shown.Thecon�rm ed PM S object

falling wellbelow the10 M yrisochroneistheveiled starTTS050649.8�032104

.

{ 30 {

The BD candidates sam ple by Alcal�a etal.(2004) was selected by inspection ofIC

versus (R C � IC )colour-m agnitude diagram in com parison with the theoreticalisochrones

by Bara�eetal.(1998)& Chabrieretal.(2000).Thisapproach to search forPM S starand

young BD candidatesisthem ostdirectway totackletheproblem using opticalphotom etric

data;however,inordertoselectclusterm em bersonthebasisoftheirm agnitudesandcolours,

isochronesthatadequately m odelthe data m ustbe used,since the shape ofthe isochrones

m ay vary signi�cantly from onephotom etric system to another.The theoreticalisochrones

forlow-m ass starsand BDsare provided by Bara�eetal.(1998)& Chabrieretal.(2000)

in the Cousinsphotom etric system (Cousins1976),while the observationsby Alcal�a etal.

(2004)wereperform ed byusingtheW FIcam eraattheESO 2.2m telescope.TheW FI�lters

transm ission curvesaresom ewhatdi�erentfrom theoriginalCousinsones,in particularfor

the I-band. Spezzietal.(2007) have com puted ad hoc isochrones for the W FI-Cousins

system . In Figure 8 these isochrones are com pared with those by Bara�eetal.(1998) &

Chabrieretal.(2000).Thetwosetsofisochronesappearsigni�cantly di�erent,in particular

forthecoolestobjects,i.e.(R C � IC )> 1:7,wheretheuseoftheisochronesin theCousins

system would produced m any spurious m em ber candidates. This consideration,together

with thecontam ination bybackground/foreground starsinthe�eld taken intoaccountbythe

authors,could explain why Alcal�a etal.(2004)overestim ated thenum berofBD candidates.

TheuseoftheadequateisochronesfortheW FI-Cousinssystem would producem uch lessBD

candidates,in agreem entwith thenum berofPM S objectswith m asscloseto theHydrogen

burning lim itcon�rm ed by ourspectroscopicfollow-up (seeTable 7).

Though theR SS estim ated by usforL1615/L1616needsfurtherre�nem ents,theregion

seem stobepoorofBDs,withaR SS ratioclosetothatobserved inT associationsor,perhaps,

even lower.Though in L1615/L1616 theradiation from them assivestarsin theOrion OB1

associationsislikely responsibleforthestarform ation (Section 6.1),thisprim ordialprocess

does not seem s to favour the form ation ofBDs. As reported in Alcal�a etal.(2004),the

radialvelocity (RV)distribution ofthe starsin L1615/L1616 showsa wellde�ned peak at

22:3Km /swith thestandard deviation of4:6Km /swhich isconsistentwith theaverageRV

error(i.e. � 5 Km /s). Thism ay indicate thatthe velocity dispersion ofthe starsm ustbe

lessthan 5 Km /s.Considering a velocity dispersion ofa few Km /s,2 M yrold objectswould

disperseoveradistanceofabout2pc,which istheapproxim ateprojected sizeofthehead of

thecom etary cloud and hasbeen com pletely covered by ourphotom etricand spectroscopic

observations.

Thus,the results ofour study in L1615/L1616 does not play in favour ofdynam ical

ejection orphotoevaporation byionisingradiation from m assivestarsbeingtriggeringfactors

oftheBDsform ation m echanism .

{ 31 {

7. Sum m ary

W ehaveundertaken astudy ofthestarform ation processin theL1615/L1616com etary

cloud,a sm allstarform ing region located in theoutskirtoftheOrion OB associations.W e

aim ed atcharacterising itsPM S population and studying both itsIM F and starform ation

history,in orderto assesstheroleofthetriggered starform ation scenario.

Our study is based on opticaland near-infrared photom etric observations,as wellas

m ulti-objectfollow-up spectroscopy,both carried outusing ESO and OAC facilities. Com -

plem entary JH K sphotom etricdata from the2M ASS and DENIS surveyshasbeen used as

well.

Oneofthem ajorgoalofthiswork hasbeen thephysicalparam etrisation oftheyoung

stellarpopulation in L1615/L1616. The spectraltype classi�cation hasbeen perform ed by

using a grid ofreference spectra ofgiants,dwarfs,and interm ediate tem plates,constructed

by averaging spectra ofgiantand dwarfstarsofthesam espectraltype.W ehavecom puted

stellar lum inosities by m eans ofa SED �tting procedure properly developed by us,which

also evaluatesthe interstellar/circum stellar reddening. Thisallowed usto derive the m ass

and ageofeach m em berofL1615/L1616by com paringthelocation oftheobjecton theH-R

diagram with di�erentsetsoftheoreticalPM S evolutionary tracks.

Ouranalysisoftheyoung population in L1615/L1616 yielded thefollowing results:

� By using theH� em ission lineintensity,aswellasthestrength ofthelithium �6708�A

absorption lineasm ain diagnosticsofthePM S natureoftheobjects,weidenti�ed 25

new m em bersofL1615/L1616,alm ostdoublingthenum berofpreviously known young

objectsin thiscloud.

� The age distribution ofthe L1615/L1616 population peaks between 1 and 3 M yr;

however,the m em bers ofthe cloud span a wide range in age,from �0.1 M yr up to

30 M yr,suggesting m ultipleeventsofstarform ation.

� Theevidenceofm ultiplestarform ation events,thespatialdistribution oftheclassical

and weak T Tauripopulations,aswellasthe cloud shape,supportthe hypothesisof

thestarform ation in L1615/L1616 being triggered by them assivestarsin thenearby

Orion OB associations.

� TheslopeoftheIM F in L1615/L1616in them assrange0:1�M � 5:5M� is0:84� 0:07,

i.e. sim ilarto thatobtained in otherstarform ing regionsby di�erentauthors. This

provides further support ofa universalIM F in the stellar dom ain,regardless ofthe

environm entalconditions.

{ 32 {

� The star form ation e�ciency is about 7{8% as expected for m olecular clouds in

the vicinity ofOB associations. According to the criterion by Lada & Lada (2003),

L1615/L1616 isa sm allclusterwith m oderatestarform ation.

� A very low-fraction ofpossiblesub-stellarobjects(� 5{7% )isfound in L1615/L1616;

thus,thedynam icalinteraction with theclosem assive starsin theOrion OB associa-

tionsand/orthe photoevaporation induced by theirionising radiation do notappear

ase�cienttriggering factorsforBD form ation.

W e thank the anonym ous referee for his/her carefulreading, usefulcom m ents, and

suggestionswhich helped to im prove the m anuscript. Thispaperisbased on observations

carried out at the European Southern Observatory,La Silla and Paranal(Chile), under

observing program snum bers64.I-0355,70.C-0629,70.C-0536,074.C-0111 and 076.C-0385.

The authors are gratefulto M enadora Barcellona and Salvatore Spezzifor their supports

during the preparation ofthispaper. W e thank J.Hernandez forproviding the calibration

curves for spectralclassi�cation,and for discussions concerning the spectralclassi�cation

ofearly-type stars. W e are also gratefulto M atula forherassiduous and warm assistance

during the writing ofthis m anuscript. This work was partially �nanced by the Istituto

Nazionale diAstro�sica (INAF) through PRIN-INAF-2005. D.G.acknowledges �nancial

supportfrom PRIN-INAF-2005(\Stellarclusters:abenchm arkforstarform ation and stellar

evolution"). This publication m akes use ofdata products from the Two-M icron All-Sky

Survey,which isajointprojectoftheUniversityofM assachusettsandtheInfraredProcessing

and AnalysisCenter/California InstituteofTechnology,funded by NASA and theNational

Science Foundation. This research has m ade use ofthe SIM BAD database,operated at

CDS,Strasbourg,France. This paper includes data from the DENIS project,which has

been partly funded by the SCIENCE and the HCM plans ofthe European Com m ission

undergrantsCT920791 and CT940627.

A . SpectralType C lassi� cation

In order to �nd the tem plate spectrum which better reproduces the targetspectrum ,

weproceed asfollows:

� W ehave�rstperform ed a splineinterpolation ofeach standard spectra onto thewave-

length pointsofthetargetspectrum .Thisallowed ustogetahom ogeneouswavelength

{ 33 {

grid with thesam estep astheobserved spectrum .M oreover,thespectralregionsen-

com passing both the O 3 and H 2O telluric bands near 6900,7250,and 7600 �A,have

been excluded in the�tting procedure.

� The standard spectra have been shifted in wavelength to the radialvelocity ofthe

target;a cross-correlation algorithm was used to �nd the velocity shift between the

tem platesand targetspectrum and superim pose each other.

� The extinction law derived byCardellietal.(1989)have been adopted to redden the

wavelength-shifted standard spectra before com paring them to the observed one. W e

assum ed a norm alslope ofthe reddening law,i.e. we adopted a value ofthe ratio of

total-to-selectiveextinction R V = A V =E (B � V )equalto3.1.Each standard spectrum

hasbeen progressively reddened assum ing an increasing valueoftheextinction in the

V -band (A V );weletitvary from 0 to 15 m ag,with a step of0.05 m ag.

� Aftereach reddening step,thestandard spectrum wassubtracted to theobserved one.

The sum ofthe residuals was taken as indicator for the goodness ofthe \�t". The

reddened standard spectrum which gavethelowestvalueofthisindicatorprovided the

spectraltypeofthetargetaswellasa �rstestim ation oftheextinction valueA V .

As an exam ple,the FORS2 spectra oftwo PM S stars,nam ely RX J0507.6�0318 and

RX J0507.4�0320,as wellas the low-and the interm ediate-resolution VIM OS spectra of

TTS050752.0�032003 are shown in Figure 9 and Figure 10,respectively;the best-�tting

tem plates(thick lines)aresuperim posed on theobserved spectra (thin lines).Notethatthe

two VIM OS spectra yielded thesam espectraltypeand aconsistentvalueofA V ,wellwithin

theerrors,regardlessofthedi�erentresolving powerand wavelength coverage.

{ 34 {

Fig. 9.| FORS2 spectra (thin lines) of RX J0507.6�0318 (upper panel) and

RX J0507.4�0320 (lower panel). Overplotted with thick lines are the best-�tting K7 and

M 4 tem plates,reddened with A V = 0:1 and 0:2 m ag,respectively,asderived by the spec-

trum �ttingprocedure.Thespectra arearbitrarily norm alised tothe ux at6500 �A.TheO 2

absorption telluricbandsnear�6900 �A arem arked.Thisspectralregion hasbeen excluded

in the �tting procedure.The Lii�6708 �A absorption and the H� em ission linesare shown

in m ore detailin the insetofeach panel. Note the di�erences in H� and Liibetween the

tem plate and the observed spectra,con�rm ing thePM S nature ofthetwo stars.Also note

the good agreem ent in the two independent determ inations ofA V ,as obtained from the

spectraltypeclassi�cation and theSED �tting procedure(Table 5).

{ 35 {

Fig.10.| Low-(upperpanel)and interm ediate-resolution (lowerpanel)VIM OS spectra of

TTS050752.0�032003. The observed spectra are displayed with thin lines,while the best-

�ttingM 5.5tem plate,reddened with A V = 0:6and 0:5m ag,respectively,isoverplotted with

a thick line. The spectra have been arbitrarily norm alised to the ux at6760 �A.The O 2

absorption telluricbandsnear�6900and �7600�A,aswellastheH 2O onenear�7250�A are

m arked.Notethatthese spectralregionshave been excluded in the �tting procedure.The

m ain TiO an VO bandsarem arked also.TheLii�6708�A absorption and theH� em ission

lines are shown in m ore detailin the inset ofthe lower panel. Note the di�erences in H�

and Liibetween thetem plate and theobserved spectra,con�rm ing thePM S natureofthe

star.Also notethegood agreem entbetween thetwo valuesofA V and theonederived from

theSED �tting procedure(Table 5).

{ 36 {

B . SED � tting procedure

Them ethod weem ployed todeterm ineboth thereddeningparam eters(i.e.A V and R V )

and thestellarintrinsicproperties(i.e.R ? and L?)ofeach PM S star,isbased on theuseof

appropriatestellaratm ospherem odels.Taking into accountthee�ectivetem peraturerange

oftheL1615/L1616m em bers(seeTable5),aswellasthewidewavelength coveragerequired

to encom pass their SEDs (see Table 6), we m erged three distinct sets of low-resolution

synthetic spectra,adopting thefollowing criteria:

� forthetwo interm ediate-m assstars,nam ely theB9V-typeHD 293815 and theHerbig

B3e-typestarKisoA-097415,wechosethegrid ofstellaratm ospherem odelscom puted

by Kurucz(1979);

� forsourcescovering the e�ective tem perature intervalbetween 4000 and 10000 K we

used the low-resolution synthetic spectra calculated by Hauschildtetal.(1999)with

theirNextGen m odel-atm ospherecode;

� forobjectscoolerthan 4000 K weused thesyntheticlow-resolution StarDusty spectra

for low-m ass stars and BDs by Allard etal.(2000). The StarDusty spectra,which

take into account dust grain form ation in the m odelatm osphere due to an e�cient

gravitationalsettling process,arethe m ostsuitable forsim ulating coolobjects,down

to the very low-m assend. Indeed,Allard etal.(2001)found thatsilicate dustgrains

can form abundantly in theouteratm osphericlayersofthelatestM dwarfsand BDs.

Each set of atm osphere m odels contains ux density spectra at the stellar surface,

com puted fordi�erent value ofgravity and m etallicity. W e kept the gravity ofthe atm o-

spherem odelsto a �xed valueoflog(g)= 4:0 (cgsunits),which isappropriateforlow-m ass

starsand BDsyoungerthan 10 M yr(Chabrieretal.2000).Furtherm ore,thisvalueclosely

m atches the one expected for HAeBe and early type m ain sequence stars (Kurucz 1979;

Schm idt-Kaler1982). W e also adopted a solarm etallicity. However,we rem ind the reader

thatRom aniello etal.(2002)have analysed the in uence ofgravity and m etallicity on the

�tting-procedureand found thatneitherofthem a�ectsthevaluesofthederived param eters

signi�cantly.

A grid ofm odels m atching the adopted e�ective tem perature scale (see Section 5.2)

was�rstderived by perform ing a linearinterpolation ofthem erged setofsyntheticspectra.

In orderto com pare the observed SEDswith theoreticalones,synthetic m agnitudesin the

sam e photom etric system s in which the observations were perform ed are required. Thus,

for each adopted tem perature we derived a look-up table ofsynthetic UB V R C IC JH K s

{ 37 {

absolutem agnitudesbyreddeningthecorrespondingtheoreticalspectrum with an increasing

value ofA V (from 0 to 15 m ag with a step of0.05 m ag)and R V (from 1 to 9 with a step

of0.1),and subsequently integrating the reddened spectrum over each photom etric pass-

band. The extinction law by Cardellietal.(1989)wasadopted to perform thisreddening

procedure. Since the m odelspectra are given as ux density at the star’s surface, the

syntheticm agnitudeswerecom puted forastar7 with onesolarradius,asseen from adistance

of10pc.TheJohnson-CousinsUB V R C IC and 2M ASS JH K stransm ission curvesfrom The

Asiago Database on Photom etric System s(M oro & M unari2000;Fiorucci& M unari2002),

aswellastheabsolute ux calibration constantsreported in Table 6 wereused to integrate

each progressively reddened theoreticalspectra.

Finally,thevaluesofA V and R V ofeach PM S star(seeTable5)werederived sim ultane-

ously by �tting thecorresponding observed SED to thelook-up tableofreddened synthetic

m agnitudeshaving thesam ee�ectivetem peratureasthegiven object.In orderto elim inate

the m agnitude shift introduced by the unknown star’s radius and distance,the observed

and the synthetic m agnitudes were previously norm alised to the IC -band. ForPM S stars

showing clearevidence ofnear-infrared excessassociated with thepresence ofcircum stellar

m aterial,the�twasperform ed to theshort-wavelength portion (� � �J)oftheSED,which

islesscontam inated by near-infrared excess. The erroron A V and R V hasbeen evaluated

taking into accountboth the accuracy on the spectraltype classi�cation (Section 5.2)and

thephotom etricerrorson theobserved m agnitudes.

An exam ple ofthe outputofthe �tting procedure isshown in the upperpanelofFig-

ure 11 ,where a countourplotofthe �2 value in the R V -A V plane isreported. Asalready

stated in Section 5.3,ourtwo-param eter�ttingprocedurecan beapplied only tostarswhich

su�era visualextinction largerthan about0.5 m ag.Forlow-reddened starsthevalueofA V

isfairly independentofchangesin R V .Asclearly evidentin thelowerpanelofFigure11,in

thiscase a unique solution could notbe found sim ultaneously in the R V -A V plane. There-

fore,the value ofR V = 3:1,typicalofthe di�use interstellarm edium ,wasassum ed forall

theobjectwith A V . 0:5 m ag.

Oncetheextinction param eterswere determ ined,thevalueofR ? foreach PM S object

wasobtained from the m agnitude shiftsrequired to m atch the observed m agnitudesto the

theoreticalones,reddened with the best-�tting A V and R V . Only the photom etric bands

actually used in the �tting procedure for each given object were em ployed to determ ine

its stellar radius. As suggested by Alcal�a etal.(2004),we m ade the assum ption that all

7This choice is only conventional,since at this step we were only interested in the shape ofthe ux

distribution.

{ 38 {

the m em bers of L1615/L1616 are located at 450 � 20 pc. Finally the value ofL? was

derived assum ing a black body em ission atthestar’se�ectivetem peratureand radius.The

associated uncertainties were derived taking into account the errors on the distance and

e�ective tem perature.

{ 39 {

Fig. 11.| �2 contour plots produced by the SED �tting procedure for the two stars

RX J0506.9�0320E and RX J0506.8�0327.

{ 40 {

R EFER EN C ES

Alcal�a J.M .,KrautterJ.,Schm ittJ.H.M .M .etal.1995,A&AS 114,109

Alcal�a J.M .,Terranegra,L.,W ichm ann,R.etal.1996,A&A 119,7

Alcal�a J.M .,W achterS.,Covino E.etal.2004,A&A 416,677

Alcal�a J.M .,SpezziL.,Frasca A.etal.2006,A&A 453,L1

Alcal�a J.M .,SpezziL.,Chapm an,N.etal.2006,ApJ 676,427

Allard F.,HauschildtP.H.& Schwenke D.2000,ApJ 540,1005

Allard F.,HauschildtP.H.,AlexanderD.R.etal.2001,ApJ 556,357

AppenzellerI.,FrickeK.,Furtig,W .etal.1998,The M essenger 94,1

Bara�eI.,ChabrierG.,Allard F.etal.1998,A&A 337,403

Bara�eI.,ChabrierG.,Allard F.& HauschildtP.H.2002,A&A 382,563

BarradoY Navascu�esD.,Stau�erJ.R.,BouvierJ.& M art��netE.L.1998,Ap&SS 263,1-4,303

Barrado Y Navascu�esD.,BouvierJ.,Stau�erJ.R.etal.2002,A&A 395,813

Barrado Y Navascu�esD.,Stau�erJ.R.,BouvierJ.etal.2004,ApJ 610,1064

BasriG.& Batalha C.1990,ApJ 363,654

B�ejarV.J.S.,M art��n E.L.,Zapatero Osorio M .R.etal.2001,ApJ 556,830

Bertin E.& ArnoutsS.1996,A&AS 117,393

Blecha A.,CayatteV.,North P.etal.2000,SPIE 4008,467

BochanskiJ.J.,W estA.A.,Hawley S.L.etal.2007,AJ 133,531

Bodenheim erP.1965,ApJ 142,451

Bohlin R.C.,SavageB.D.& DrakeJ.F.1978,ApJ 224,132

Brice~no C.,Luhm an K.L.,Hartm ann L.etal.2002,ApJ 580,317

CardelliJ.A.,Clayton G.C.& M athisJ.S.1989,ApJ 345,245

CarpenterJ.M .2001,AJ 121,2851

{ 41 {

ChabrierG.,Bara�eI.,Allard F.& HauschildtP.H.2000,ApJ 542,464

CieslinskiD.,JablonskiF.J.& SteinerJ.E.1997,A&AS 124,55

Clark P.C.,BonnellI.A.,ZinneckerH.& BateM .R.2005,M NRAS 359,809

Cohen M .& KuhiL.V.1979,ApJS 41,743

Cohen M .,W heaton W .A.& M egeath S.T.2003,AJ 126,1096

Covino E.,Alcal�a J.M .,Allain S.etal.1997,A&A 328,187

Com eron F.,Neuh�auserR.& KaasA.A.2000,A&A 359,269

CousinsA.W .J.1976,M m RAS 81,25

CutriR.M .,SkrutskieM .F.,van DykS.etal.2003,2M ASS All-SkyCatalogofPointSources,

NASA/IPAC Infrared ScienceArchive

D’Antona F.& M azzitelliI.1997,M em .S.A.It.68,807

DroegeT.F.,Richm ond M .W .,Sallm an M .P.,CreagerR.P.2006,PASP 118,1666

Epchtein,N.,etal.1997,M essenger,87,27

Feigelson E.D.& M ontm erleT.1999,ARA&A 37,363

FiorucciM .& M unariU.2002,Ap&SS 280,77

Frasca A.,Alcal�a J.M .,Covino E.etal.2003,A&A 405,149

Frasca A.,GuilloutP.,M arilliE.etal.2006,A&A 454,301

GuilloutP.,Sterzik M .F.,Schm ittJ.H.M .M .etal.1998,A&A 334,540

GuilloutP.,Sterzik M .F.,Schm ittJ.H.M .M .etal.1998,A&A 337,113

Guieu S.,DougadosC.,M onin J.-L.etal.2006,A&A 446,485

Ham uy M .,W alkerA.R.,Suntze� N.B.etal.1992,PASP 104,533

Ham uy M .,Suntze� N.B.,HeathcoteS.R.etal.1994,PASP 106,566

Harvey,P.,etal.2007,ApJ,663,1139 (H07)

HauschildtP.H.,Allard F.& Baron E.1999,ApJ 512,377

{ 42 {

Hawley S.L.,Covey K.R.,Knapp G.R.etal.2002,AJ 123,3409

Helou G.& W alkerD.W .1988,Infrared Astronom icalSatellite(IRAS)Catalogsand Atlases,

Vol.7:The Sm allScale Structure Catalog (NASA RP-1190;W ashington:GPO)

HerbstW .,W arnerJ.W .,M illerD.P.etal.1982,AJ 87,98

Hern�andezJ.,CalvetN.,Brice~no C.etal.2004,AJ 127,1682

Hillenbrand L.A.& CarpenterJ.2000,ApJ 540,236

Izzo C.,KornweibelN.,M cKay D.etal.2004,M sngr177,33

Johnson H.L.1965,ApJ 141,923

J�rgensen,J.K.,etal.,2007,ApJ,subm itted

Kainulainen J.,Lehtinen K.& Harju J.2006,A&A 447,597

Kenyon S.J.& Hartm ann L.1995,ApJS 101,117

Kroupa P.2001,M NRAS 322,231

Kroupa P.2002,ScienceVol.295,Issue5552,pag.82

Kun M .,PrustiT.,NikolicS.,Johansson L.E.B.& W alton N.A.2004,A&A 418,89

KuruczR.L.1979,ApJS 40,191

Lada,C.J.,Lada,E.A.,A.R.A&A,41,57 (LL03)

Lada,C.J.,M uench,A.A.,Luhm an,K.L.,etal.2006,AJ 131,1574

LandoltA.U.1992,AJ 104,340

LeeT.A.1968,ApJ 152,913

LeeH.-T.,Chen W .P.,Zhang Z.-W .& Hu J.-Y.2005,ApJ 624,808

LeeH.-T.& Chen W .P.2007,ApJ 657,884

LeBorgneJ.F.,BruzualG.,Pell�o R.etal.2003,A&A 402,433

LeFevreO.,SaisseM .,M anciniD.etal.2003,SPIE 4841,1670

LeggettS.K.,Allard F.,Berrim an G.etal.1996,ApJS 104,117

{ 43 {

Lom bardi,M .,& Alves,J.2001,A&A,377,1023

L�opezM art�� B.,Eisl�o�elJ.,ScholzA.& M undtR.2004,A&A 416,555

Luhm an K.L.1999,ApJ 525,466

Luhm an K.L.& RiekeG.H.1999,ApJ 525,440

Luhm an K.L.,RiekeG.H.,Young E.T.etal.2000,ApJ 540,1016

Luhm an K.L.,Stau�erJ.R.,M uench A.A.etal.2003,ApJ 593,1093

LyndsB.T.1962,ApJS 7,1L

LyndsB.T.1965,ApJS 12,163

M addalena R.J.,M orrisM .,M oscowitz J.& ThaddeusP.1986,ApJ 303,375

M arilliE.,Frasca A.,Covino E.etal.2007,A&A 463,1081

M art��n E.L.,DelfosseX.,BasriG.etal.1999,AJ 118,2466

M er��n etal.,2007,ApJ,subm itted

M erm illiod J.C.,M erm illiod M .,& Hauck B.1997,A&AS,124,349

M illerG.E.& Scalo J.M .1979,ApJS 41,513

M oraux E.,BouvierJ.,Stau�erJ.R.etal.2003,A&A 400,891

M oro D.& M unariU.2000,A&AS 147,361

M uench A.A.,Lada E.A.,Lada C.J.etal.2002,ApJ 573,366

M uench A.A.,Lada E.A.,Lada C.J.etal.2003,AJ 125,2029

M undtR.& Bastian U.1980,A&AS 39,245

Nakano M .,W iram ihardja S.D.& KogureT.1995,PASJ 47,889

OkeJ.B.1990,AJ 99,1621

Palla F.& StahlerS.W .1999,ApJ 525,772

PasquiniL.,Avila G.,Blecha A.etal.2002,The M essenger 110,1

{ 44 {

Persson,S.E.,M urphy,D.C.,Krzem inski,W .,Roth,M .,& Rieke,M .J.1998,AJ,116,

2475

Preibisch T.,StankeT.& ZinneckerH.2003,A&A 409,147

Ram esh B.1995,M NRAS 276,923

Rom aniello R.,Panagia N.,ScuderiS.etal.2002,AJ 123,915

Rom aniello R.,ScuderiS.,Panagia N.etal.2006,A&A 446,955

Schm idt-Kaler Th.1982,in Landoldt-B�ornstein,New Ser.,ed.K.H.Hellwege,Vol.VI/2b

(Springer-Verlag,Berlin,Heidelberg,New York),12

Scodeggio M .,FranzettiP.,GarilliB.etal.2005,PASP 117,1284

SharplessS.1952,ApJ 116,251

SpezziL.,Alcal�a J.M .,Fasca A.,Covino E.& Gandol� D.2007,A&A,470,281

SpezziL.,Alcal�a J.M .,Covino E.,etal.2008,ApJ,in press(arXiv:0802.4351)

StankeT.,Sm ith M .D.,GredelR.etal.2002,A&A 393,251

Sterzik M .F.,Alcal�a J.M .,Neuh�auserR.etal.1995,A&A 297,418

Stetson P.B.1987,PASP 99,191

TaylorB.J.,JonerM .D.& Johnson S.B.1989,AJ 97,1798

TejA.,Sahu K.C.,ChandrasekharT.& Ashok N.M .2002,ApJ 578,523

Th�eP.S.,Tjin-A-DjieH.R.E.,BakkerR.etal.1981,A&AS 44,451

Torres-Dodgen A.V.& W eaverW .B.1993,PASP 105,693

ValdesF.,Gupta R.,RoseJ.A.etal.2004,ApJS 152,251

van BelleG.T.,LaneB.F.,Thom pson R.R.etal.1999,AJ 117,521

Vieira S.L.A.,CorradiW .J.B.,AlencarS.H.P.etal.2003,AJ 126,2971

VogesW .,Aschenbach B.,BollerTh.etal.1999,A&A 349,389

W arm elsR.H.1992,in Astronom icalData AnalysisSoftware and System sI,D.M .W orrall,

C.Biem esderfer& J.Barneseds.,ASP Conf.SeriesVol.25,115

{ 45 {

W aters,L.B.F.M .& W aelkensC.1998,ARA&A 36,233

W hite,R.J.& Basri,G.2003,ApJ 582,1109

W hittetD.C.B.,GerakinesP.A.,Hough J.H.etal.2001,ApJ 547,872

Yonekura Y.,Hayakawa T.,M izuno N etal.1999,PASJ 51,837

ZickgrafF.J.,KrautterJ.,Re�ertS.etal.2005,A&A 433,151

Thispreprintwasprepared with the AAS LATEX m acrosv5.2.

{46{

Table1. JournaloftheFORS2 and VIM OS spectroscopicobservations.

Spectrograph Period Grism Ordersorting �=�� � �range Texp N m ask N object

�lter (�A) (sec) perm ask

FORS2@VLT Feb{M ar2003 GRIS-1200R GG435 2140 5700{7300y 960 5 18{19

VIM OS@VLT Dec2004{M ar2005 HR red GG475 2500 6300{8700y 2000 3 � 80

VIM OS@VLT Dec2004{M ar2005 M R GG475 580 5000{8000 900 6 � 120

�Achieved at� = 6500�A with a 100slitwidth.

yCorresponding to a slitprojected onto thecentreofthedetector.

{ 47 {

Table2. PM S starsin L1615/L1616.Coordinatesarefrom the2M ASS catalogue.The

PM S objectsidenti�ed in thiswork and thepreviously known onesarelabelled with

\NEW " and \P.K.",respectively.Foreach star,additionaldesignationsfound in literature

arelisted in thelastcolum n.

Star �(2000) �(2000) Note O therDesignations

(hh:m m :ss) (dd:m m :ss)

1RXS J045912.4� 033711 04:59:14.59 � 03:37:06.3 P.K .

1RXS J050416.9� 021426 05:04:15.93 � 02:14:50.5 P.K .

TTS050513.5� 034248 05:05:13.47 � 03:42:47.8 NEW

TTS050538.9� 032626 05:05:38.85 � 03:26:26.4 NEW

RX J0506.6� 0337 05:06:34.95 � 03:37:15.9 P.K .

TTS050644.4� 032913 05:06:44.42 � 03:29:12.8 NEW

TTS050646.1� 031922 05:06:46.05 � 03:19:22.4 P.K .

RX J0506.8� 0318 05:06:46.64 � 03:18:05.6 P.K .

TTS050647.5� 031910 05:06:47.45 � 03:19:09.7 NEW

RX J0506.8� 0327 05:06:48.32 � 03:27:38.2 P.K .

RX J0506.8� 0305a 05:06:48.98 � 03:05:42.9 NEW

TTS050649.8� 031933 05:06:49.77 � 03:19:33.1 NEW

TTS050649.8� 032104 05:06:49.78 � 03:21:03.6 NEW 1RXSJ050651.9� 032031

TTS050650.5� 032014 05:06:50.50 � 03:20:14.3 NEW

TTS050650.7� 032008 05:06:50.74 � 03:20:08.0 NEW

RX J0506.9� 0319NW 05:06:50.83 � 03:19:35.2 P.K .

RX J0506.9� 0319SE 05:06:50.99 � 03:19:38.0 P.K . L1616 M IR5

HD 293815 05:06:51.05 � 03:19:59.9 P.K .

RX J0506.9� 0320W 05:06:52.86 � 03:20:53.2 P.K . L1616 M IR2

RX J0506.9� 0320E 05:06:53.32 � 03:20:52.6 P.K . L1616 M IR1

TTS050654.5� 032046 05:06:54.53 � 03:20:46.0 NEW

LkH� 333 05:06:54.65 � 03:20:04.8 P.K . HBC 82,K isoA-0974 14,CSI-03-05045

L1616 M IR4b 05:06:54.93 � 03:21:12.7 P.K .

K isoA-0974 15 05:06:55.52 � 03:21:13.2 P.K . NSV 1832,IRAS 05044� 0325,L1616 M IR3

RX J0507.0� 0318 05:06:56.94 � 03:18:35.5 P.K .

TTS050657.0� 031640 05:06:56.97 � 03:16:40.4 NEW

TTS050704.7� 030241 05:07:04.71 � 03:02:41.0 NEW

TTS050705.3� 030006 05:07:05.32 � 03:00:06.2 NEW

RX J0507.1� 0321 05:07:06.10 � 03:21:28.2 P.K . K isoA-0974 16

TTS050706.2� 031703 05:07:06.22 � 03:17:02.9 NEW

RX J0507.2� 0323 05:07:10.95 � 03:23:53.4 P.K . K isoA-0974 18

TTS050713.5� 031722 05:07:13.52 � 03:17:22.1 NEW

RX J0507.3� 0326a 05:07:14.99 � 03:26:47.3 NEW

TTS050717.9� 032433 05:07:17.85 � 03:24:33.1 P.K .

RX J0507.4� 0320 05:07:22.28 � 03:20:18.5 P.K .

RX J0507.4� 0317a 05:07:25.93 � 03:17:12.3 NEW

{ 48 {

Table2| Continued

Star �(2000) �(2000) Note O therDesignations

(hh:m m :ss) (dd:m m :ss)

TTS050729.8� 031705 05:07:29.80 � 03:17:05.1 NEW

TTS050730.9� 031846 05:07:30.85 � 03:18:45.6 NEW

TTS050733.6� 032517 05:07:33.58 � 03:25:16.7 NEW

TTS050734.8� 031521 05:07:34.83 � 03:15:20.7 NEW

RX J0507.6� 0318a 05:07:37.67 � 03:18:15.6 NEW

TTS050741.0� 032253 05:07:41.00 � 03:22:53.0 NEW

TTS050741.4� 031507 05:07:41.35 � 03:15:06.7 NEW

TTS050752.0� 032003 05:07:51.95 � 03:20:02.8 NEW

TTS050801.4� 032255 05:08:01.43 � 03:22:54.5 P.K .

TTS050801.9� 031732 05:08:01.94 � 03:17:31.6 P.K .

TTS050804.0� 034052 05:08:04.00 � 03:40:51.7 P.K .

TTS050836.6� 030341 05:08:36.55 � 03:03:41.4 P.K . K isoA-0974 21

TTS050845.1� 031653 05:08:45.10 � 03:16:52.5 P.K .

RX J0509.0� 0315 05:09:00.66 � 03:15:06.6 P.K . 1RXSJ050859.6� 031503

RX J0510.1� 0427 05:10:04.60 � 04:28:03.7 P.K . 1RXSJ051004.9� 042757

1RXSJ051011.5� 025355 05:10:10.86 � 02:54:04.9 P.K . V1011 O ri,IRAS 05076� 0257

RX J0510.3� 0330 05:10:14.78 � 03:30:07.4 P.K . 1RXSJ051015.7� 033001

1RXSJ051043.2� 031627 05:10:40.50 � 03:16:41.6 P.K . RXJ0510.7� 0316

RX J0511.7� 0348 05:11:38.93 � 03:48:47.1 P.K .

RX J0512.3� 0255 05:12:20.53 � 02:55:52.3 P.K . V531 O ri,1RXSJ051219.9� 025547

aX-ray sourceby Alcal�a etal.(2004)

bM id-infrared sourceby Stankeetal.(2002)

{49{

Table3. Optical(UB V R C IC )and near-infrared (JH K s)photom etry ofthePM S starsin L1615/L1616.Labelsare

asexplained in thefootnotes.

PM S Star U B V R C IC J H K s N otes

1R X S J045912.4�033711 12.74�0.05 12.45�0.04 11.73�0.03 N O 10.72�0.06 10.07�0.02 9.62�0.02 9.47�0.02 a,b,c

1R X S J050416.9�021426 14.84�0.09 13.99�0.08 12.96�0.02 N O 11.59�0.03 10.66�0.02 10.10�0.02 9.98�0.03 a,c,d

TTS050513.5�034248 N O N D N D 18.97�0.02 17.03�0.02 15.12�0.04 14.60�0.05 14.20�0.06 c

TTS050538.9�032626 N O 18.88�0.02 17.30�0.02 16.21�0.02 14.67�0.01 13.29�0.02 12.60�0.02 12.39�0.02 c

R X J0506.6�0337 13.12�0.04 12.93�0.02 12.23�0.01 N O 11.24�0.05 10.66�0.02 10.30�0.03 10.20�0.02 a,b,c

TTS050644.4�032913 N O 19.33�0.01 17.69�0.02 16.68�0.02 15.08�0.01 13.64�0.03 13.04�0.03 12.78�0.03 c

TTS050646.1�031922 N O 18.88�0.01 17.34�0.02 16.09�0.01 14.81�0.01 12.60�0.01 11.09�0.01 10.22�0.01

R X J0506.8�0318 N O 16.26�0.01 14.86�0.02 13.91�0.01 13.01�0.01 11.95�0.01 11.22�0.01 10.97�0.01

TTS050647.5�031910 N O 22.55�0.12 20.51�0.03 19.01�0.03 16.75�0.02 14.30�0.01 13.38�0.01 13.04�0.01

R X J0506.8�0327 N O 17.59�0.01 15.96�0.02 14.90�0.02 13.30�0.01 11.74�0.01 10.92�0.01 10.59�0.01

R X J0506.8�0305 N O 18.99�0.01 17.33�0.02 16.23�0.02 14.59�0.01 13.02�0.01 12.39�0.01 12.13�0.01

TTS050649.8�031933 N O 19.42�0.03 17.63�0.02 16.32�0.02 14.48�0.01 12.44�0.01 11.52�0.01 11.18�0.01

TTS050649.8�032104 N O 19.91�0.02 18.59�0.02 17.30�0.02 15.56�0.01 12.69�0.01 11.43�0.01 10.51�0.01

TTS050650.5�032014 N O 23.58�0.77 21.11�0.05 19.18�0.03 16.51�0.02 13.78�0.01 13.09�0.01 12.60�0.01

TTS050650.7�032008 N O 21.50�0.13 19.54�0.03 18.14�0.02 16.03�0.02 13.78�0.01 13.07�0.01 12.59�0.01

R X J0506.9�0319N W N O 18.18�0.01 16.73�0.02 15.70�0.02 14.31�0.01 12.46�0.01 11.60�0.01 11.33�0.01

R X J0506.9�0319SE N O 16.34�0.01 14.82�0.02 13.80�0.02 12.67�0.01 11.09�0.01 10.16�0.01 9.97�0.01

H D 293815 10.33�0.01 10.28�0.01 10.08�0.01 9.94�0.01 9.77�0.01 9.50�0.02 9.48�0.03 9.37�0.03 c,e

R X J0506.9�0320W N O 17.36�0.01 15.33�0.02 13.94�0.02 12.39�0.01 10.66�0.01 9.51�0.01 8.94�0.01

R X J0506.9�0320E N O 17.66�0.01 15.74�0.02 14.36�0.02 12.87�0.01 10.75�0.01 9.51�0.01 8.76�0.01

TTS050654.5�032046 N O N D N D 20.98�0.08 18.92�0.03 15.92�0.02 14.92�0.01 14.23�0.01

LkH � 333 16.55�0.02 15.74�0.01 14.19�0.01 S S 10.34�0.02 9.24�0.02 8.44�0.03 c,f

L1616 M IR 4 N O 21.12�0.13 18.99�0.04 17.59�0.02 15.94�0.01 12.78�0.02 10.83�0.01 9.66�0.01

K isoA -0974 15 13.54�0.01 13.65�0.01 12.84�0.01 12.18�0.01 11.42�0.01 9.89�0.03 9.13�0.03 8.49�0.03 c,g

R X J0507.0�0318 N O 16.89�0.01 15.21�0.02 14.08�0.02 12.91�0.01 11.48�0.01 10.48�0.01 10.13�0.01

TTS050657.0�031640 N O 18.80�0.01 17.69�0.02 16.48�0.02 14.88�0.01 13.12�0.01 12.48�0.01 12.11�0.01

TTS050704.7�030241 N O 21.87�0.04 20.04�0.02 18.81�0.02 16.75�0.02 14.94�0.01 14.32�0.01 14.00�0.01

TTS050705.3�030006 N O 16.95�0.01 15.50�0.02 14.55�0.02 13.60�0.01 12.63�0.01 11.75�0.01 11.63�0.01

R X J0507.1�0321 N O 17.56�0.01 16.13�0.02 15.08�0.02 13.87�0.01 12.40�0.01 11.53�0.01 11.20�0.01

TTS050706.2�031703 N O 21.83�0.04 19.99�0.02 18.71�0.02 16.66�0.02 14.69�0.01 14.18�0.01 13.84�0.01

R X J0507.2�0323 N O 15.09�0.01 13.95�0.01 S 12.61�0.02 11.79�0.01 11.01�0.01 10.95�0.01 d

TTS050713.5�031722 N O 19.09�0.01 17.57�0.02 16.55�0.01 15.61�0.01 14.05�0.01 13.02�0.01 12.61�0.01

R X J0507.3�0326 N O 15.72�0.01 14.31�0.01 13.42�0.02 12.58�0.02 11.45�0.02 10.78�0.02 10.63�0.02 c,d

TTS050717.9�032433 N O 18.24�0.01 16.67�0.02 15.64�0.01 14.40�0.01 13.17�0.01 12.48�0.01 12.23�0.01

R X J0507.4�0320 N O 18.37�0.01 16.71�0.02 15.58�0.01 13.98�0.01 12.42�0.01 11.73�0.01 11.49�0.01 h

{50{

Table3| Continued

PM S Star U B V R C IC J H K s N otes

R X J0507.4�0317 N O 18.68�0.01 17.19�0.02 16.17�0.02 14.70�0.01 13.25�0.01 12.61�0.01 12.34�0.01

TTS050729.8�031705 N O 22.77�0.08 20.83�0.03 19.39�0.02 17.16�0.02 14.94�0.04 14.23�0.04 13.96�0.05 c

TTS050730.9�031846 N O N D 22.12�0.08 21.09�0.04 18.93�0.02 16.49�0.03 15.94�0.03 15.40�0.02

TTS050733.6�032517 N O 21.36�0.03 19.62�0.02 18.39�0.02 16.49�0.02 14.79�0.01 14.21�0.01 13.90�0.01

TTS050734.8�031521 N O 20.59�0.02 19.05�0.02 17.85�0.02 16.10�0.01 14.51�0.01 13.87�0.01 13.60�0.01

R X J0507.6�0318 N O 16.52�0.01 15.16�0.02 14.27�0.01 13.42�0.01 12.44�0.01 11.71�0.01 11.50�0.01

TTS050741.0�032253 N O 19.16�0.01 17.55�0.02 16.51�0.02 14.96�0.01 13.52�0.03 12.81�0.03 12.57�0.03 c

TTS050741.4�031507 N O 19.43�0.01 17.92�0.02 16.86�0.02 15.13�0.01 13.60�0.01 12.90�0.01 12.66�0.01

TTS050752.0�032003 N O 21.30�0.03 19.55�0.02 18.27�0.02 16.32�0.02 14.52�0.03 13.98�0.04 13.51�0.04 c

TTS050801.4�032255 N O N O N O N O 13.82�0.04 12.51�0.02 11.63�0.02 11.23�0.02 c,d

TTS050801.9�031732 N O N O N O N O 13.84�0.04 12.39�0.02 11.66�0.02 11.33�0.03 c,d

TTS050804.0�034052 N O N O N O N O 14.28�0.04 13.00�0.03 12.33�0.03 12.07�0.02 c,d

TTS050836.6�030341 N O N O N O N O 13.46�0.03 11.93�0.02 11.16�0.03 10.73�0.02 c,d

TTS050845.1�031653 N O N O N O N O 13.88�0.04 12.37�0.04 11.82�0.04 11.46�0.03 c,d

R X J0509.0�0315 12.26�0.04 12.09�0.03 11.39�0.02 N O 10.51�0.04 9.91�0.02 9.53�0.02 9.41�0.02 a,b,c

R X J0510.1�0427 13.36�0.01 12.65�0.01 11.73�0.01 N O 10.41�0.05 9.68�0.02 9.14�0.03 8.99�0.02 a,b,c

1R X S J051011.5�025355 14.27�0.24 13.43�0.23 12.42�0.23 11.81�0.23 11.24�0.23 10.45�0.02 9.95�0.02 9.73�0.02 c,i

R X J0510.3�0330 N O 12.53�0.04 11.74�0.03 11.29�0.02 10.87�0.02 10.04�0.15 9.81�0.06 9.75�0.05 l,m ,n

1R X S J051043.2�031627 12.11�0.04 12.01�0.03 11.38�0.03 N O 10.57�0.04 10.08�0.03 9.73�0.02 9.65�0.03 b,c,p

R X J0511.7�0348 13.13�0.07 12.77�0.06 12.02�0.04 N O 11.01�0.04 10.34�0.03 9.87�0.02 9.81�0.02 a,b,c

R X J0512.3�0255 14.10�0.06 13.56�0.04 12.61�0.03 12.05�0.04 11.49�0.04 10.43�0.02 9.69�0.02 9.14�0.02 c,i

S Saturated object.

N D O bjectnot detected.

N O O bjectnotobserved.

a U B V from observations carried outatthe Catania A strophysicalO bservatory (Section 2.1).

b IC from TA SS M ark IV catalogue (D roege etal.2006).

c JH K s from 2M A SS pointsource catalogue (Cutriet al.2003).

d IC from D EN IS catalogue (Epchtein etal.1997).

e U and B from Lee (1968);V ,R C ,and IC from Tayloret al.(1989).

f U from M undt& Bastian (1980).

{51{

g U B V R C IC from V ieira etal.(2003).

h Source blended with another starofsim ilarbrightness:the W FIB V R C IC photom etry corresponds to the com bined lightofboth stars.

i W eighted m ean opticalphotom etry from Cieslinskiet al.(1997).

l B V R C IC from A lcal�a etal.(1996).

m J from D EN IS catalogue (Epchtein et al. 1997, converted to the 2M A SS photom etric system using the transform ation equations provided by

Carpenter (2001)).

n H K s from 2M A SS pointsource catalogue (Cutrietal.2003).

p U B V from Frasca et al.(2003).

{ 52 {

Table4. Spectraltype,H� and LiiequivalentwidthsofthePM S starsin L1615/L1616.

Additionalem ission linesobserved in thespectra arealso tabulated.

Star SpT W (H�) W (Li) Type O therem ission lines Notes

(�A) (m �A)

1RXS J045912.4� 033711 G 8 1:80� 0:20 355� 10 W TTS a

1RXS J050416.9� 021426 K 3 0:03� 0:20 475� 10 W TTS a

TTS050513.5� 034248 M 5.5 � 9:80� 0:40 800� 100 W TTS

TTS050538.9� 032626 M 3.5 � 4:90� 0:20 350� 20 W TTS Hei

RX J0506.6� 0337 G 9 1:00� 0:20 299� 10 W TTS a

TTS050644.4� 032913 M 4.5 � 4:70� 0:30 560� 40 W TTS

TTS050646.1� 031922 K 4 � 41:00� 2:00 560� 20 CTTS Hei,[N ii],[Sii]

RX J0506.8� 0318 K 8.5 � 3:80� 0:20 570� 30 W TTS Hei

TTS050647.5� 031910 M 5.5 � 10:20� 0:80 560� 50 W TTS

RX J0506.8� 0327 M 3.5 � 6:75� 0:20 470� 10 W TTS

RX J0506.8� 0305 M 4.5 � 4:80� 0:30 590� 50 W TTS Hei

TTS050649.8� 031933 M 3.5 � 15:50� 1:00 540� 50 W TTS Hei,[O i]

TTS050649.8� 032104 M 1: � 195:00� 5:00 190� 20 CTTS Hei,Nai,[O i] b

TTS050650.5� 032014 M 6.5 � 14:00� 1:00 680� 40 W TTS

TTS050650.7� 032008 M 4.5 � 26:00� 0:50 620� 50 CTTS Hei,[O i],[N ii],[Sii]

RX J0506.9� 0319NW M 2.5 � 3:50� 0:30 590� 50 W TTS

RX J0506.9� 0319SE K 5 � 4:80� 0:50 470� 10 CTTS [O i],[N ii],[Sii]

HD 293815 B9V 8:60� 0:50 c

RX J0506.9� 0320W K 8.5 � 2:90� 0:50 500� 20 W TTS

RX J0506.9� 0320E K 0 � 1:50� 0:50 380� 10 W TTS

TTS050654.5� 032046 M 4 � 60:00� 5:00 CTTS

LkH� 333 K 4 � 60:00� 2:00 430� 10 CTTS Hei,[O i],[Sii]

L1616 M IR4 K 1 � 60:00� 6:00 370� 50 CTTS [O i],[N ii],[Sii]

K isoA-0974 15 B3e � 67:50� 1:00 HAeBe e

RX J0507.0� 0318 M 0 � 2:00� 0:50 570� 30 W TTS

TTS050657.0� 031640 M 4.5 � 94:00� 2:00 430� 40 CTTS Hei,[O i]

TTS050704.7� 030241 M 6 � 17:50� 1:00 740� 50 W TTS

TTS050705.3� 030006 M 0 � 2:00� 0:40 400� 20 W TTS

RX J0507.1� 0321 M 1 � 38:50� 3:00 590� 20 CTTS Hei,[O i]

TTS050706.2� 031703 M 6 � 13:50� 0:50 730� 50 W TTS

RX J0507.2� 0323 K 4 � 1:10� 0:10 480� 20 W TTS

TTS050713.5� 031722 K 8.5 � 10:50� 0:50 520� 30 CTTS

RX J0507.3� 0326 M 0 � 1:90� 0:20 550� 10 W TTS

TTS050717.9� 032433 M 2.5 � 3:50� 0:50 470� 10 W TTS

RX J0507.4� 0320 M 4 � 5:00� 0:50 530� 50 W TTS Hei

RX J0507.4� 0317 M 3 � 14:50� 0:50 510� 50 W TTS Hei

{ 53 {

Table4| Continued

Star SpT W (H�) W (Li) Type O therem ission lines Notes

(�A) (m �A)

TTS050729.8� 031705 M 6.5 � 12:00� 0:50 450� 20 W TTS

TTS050730.9� 031846 M 5.5 � 290:00� 30:00 CTTS Hei

TTS050733.6� 032517 M 5.5 � 2:30� 0:20 610� 20 W TTS

TTS050734.8� 031521 M 5 � 6:80� 0:40 610� 50 W TTS Hei

RX J0507.6� 0318 K 7 � 1:60� 0:20 520� 50 W TTS

TTS050741.0� 032253 M 4 � 4:30� 0:10 540� 20 W TTS

TTS050741.4� 031507 M 4.5 � 6:80� 0:40 550� 20 W TTS

TTS050752.0� 032003 M 5.5 � 9:50� 0:20 530� 30 W TTS

TTS050801.4� 032255 M 0.5 � 21:50� 0:50 510� 25 CTTS Hei,[Sii] a

TTS050801.9� 031732 M 1 � 15:50� 0:50 410� 25 CTTS Hei,[Sii] a

TTS050804.0� 034052 M 2.5 � 2:20� 0:20 410� 20 W TTS a

TTS050836.6� 030341 M 1.5 � 68:50� 1:00 560� 30 CTTS Hei a

TTS050845.1� 031653 M 3.5 � 6:70� 1:00 530� 50 W TTS

RX J0509.0� 0315 G 8 1:10� 0:20 320� 10 W TTS a

RX J0510.1� 0427 K 4 � 0:20� 0:20 230� 10 W TTS a

1RXS J051011.5� 025355 K 0 � 0:10� 0:20 415� 10 W TTS a

RX J0510.3� 0330 G 8 1:50� 0:20 320� 10 W TTS a

1RXS J051043.2� 031627 G 2 2:40� 0:20 235� 10 W TTS a

RX J0511.7� 0348 K 1 1:80� 0:20 370� 15 W TTS a

RX J0512.3� 0255 K 2 � 6:50� 0:50 440� 10 CTTS a

aW (H�)and W (Li)from Alcal�a etal.(2004).

bVeiled CTTS.

cSpectraltype from Sharpless(1952).

eSpectraltype from Vieira etal.(2003).

{ 54 {

Table5. Stellarparam etersofthePM S starsin L1615/L1616.

PM S Star logTeff R V A V R ? logL=L�

(m ag) (R � )

1RXS J045912.4� 033711 3:746� 0:009 3:1� 0:1? 0:30� 0:12 2:63� 0:12 0:78� 0:05

1RXS J050416.9� 021426 3:675� 0:014 3:1� 0:1? 0:35� 0:22 2:35� 0:11 0:40� 0:07

TTS050513.5� 034248 3:494� 0:010 3:1� 0:1? 0:49� 0:20 0:55� 0:03 � 1:60� 0:06

TTS050538.9� 032626 3:531� 0:009 3:1� 0:1? 0:05� 0:10 1:08� 0:05 � 0:85� 0:05

RX J0506.6� 0337 3:733� 0:013 3:1� 0:1? 0:05� 0:10 1:92� 0:09 0:45� 0:06

TTS050644.4� 032913 3:513� 0:009 3:1� 0:1? 0:00+ 0:05 1:00� 0:05 � 0:99� 0:05

TTS050646.1� 031922 3:662� 0:026 4:2� 0:4 4:33� 0:35 1:91� 0:11 0:17� 0:11

RX J0506.8� 0318 3:597� 0:008 3:1� 0:1? 0:00+ 0:05 1:62� 0:07 � 0:24� 0:05

TTS050647.5� 031910 3:494� 0:010 3:0� 0:2 2:30� 0:17 1:03� 0:05 � 1:04� 0:05

RX J0506.8� 0327 3:531� 0:009 3:1� 0:1? 0:44� 0:10 2:43� 0:11 � 0:15� 0:05

RX J0506.8� 0305 3:513� 0:009 3:1� 0:1? 0:05� 0:10 1:28� 0:06 � 0:78� 0:05

TTS050649.8� 031933 3:531� 0:009 4:1� 0:4 1:67� 0:10 2:04� 0:09 � 0:30� 0:05

TTS050649.8� 032104 3:573� 0:008 3:1� 0:1? 4:72� 0:12 2:29� 0:10 � 0:03� 0:05

TTS050650.5� 032014 3:476� 0:008 1:7� 0:1 2:93� 0:19 1:30� 0:06 � 0:91� 0:05

TTS050650.7� 032008 3:513� 0:009 3:4� 0:2 2:20� 0:17 1:28� 0:06 � 0:78� 0:05

RX J0506.9� 0319NW 3:549� 0:008 3:1� 0:1? 0:00+ 0:05 1:22� 0:05 � 0:68� 0:05

RX J0506.9� 0319SE 3:622� 0:025 4:1� 0:4 1:62� 0:49 2:78� 0:14 0:33� 0:11

HD 293815 4:021� 0:049 3:5� 0:5 0:76� 0:05 2:03� 0:09 1:65� 0:20

RX J0506.9� 0320W 3:597� 0:008 2:4� 0:2 2:24� 0:12 3:98� 0:18 0:54� 0:05

RX J0506.9� 0320E 3:720� 0:014 3:3� 0:1 4:72� 0:15 3:65� 0:17 0:96� 0:07

TTS050654.5� 032046 3:522� 0:009 3:1� 0:1? 4:33� 0:10 0:58� 0:03 � 1:42� 0:05

LkH� 333 3:662� 0:026 3:1� 0:1? 2:60� 0:44 3:92� 0:29 0:79� 0:12

L1616 M IR4 3:706� 0:015 4:1� 0:1 7:09� 0:15 2:45� 0:12 0:56� 0:07

K isoA-0974 15 4:272� 0:058 5:5� 0:1 5:20� 0:20 2:59� 0:15 2:87� 0:24

RX J0507.0� 0318 3:589� 0:008 4:2� 0:5 1:08� 0:15 2:56� 0:12 0:13� 0:05

TTS050657.0� 031640 3:513� 0:009 3:1� 0:1? 0:44� 0:12 1:30� 0:06 � 0:76� 0:05

TTS050704.7� 030241 3:484� 0:009 2:5� 0:4 0:74� 0:47 0:71� 0:08 � 1:41� 0:10

TTS050705.3� 030006 3:589� 0:008 3:1� 0:1? 0:00+ 0:05 1:25� 0:06 � 0:49� 0:05

RX J0507.1� 0321 3:573� 0:008 3:1� 0:1? 0:49� 0:15 1:55� 0:07 � 0:37� 0:05

TTS050706.2� 031703 3:484� 0:009 3:0� 0:1 0:88� 0:22 0:78� 0:04 � 1:33� 0:06

RX J0507.2� 0323 3:662� 0:026 3:1� 0:1? 0:25� 0:22 1:53� 0:07 � 0:03� 0:11

TTS050713.5� 031722 3:597� 0:008 3:1� 0:1? 1:18� 0:15 0:79� 0:04 � 0:86� 0:05

RX J0507.3� 0326 3:589� 0:008 3:1� 0:1? 0:00+ 0:05 2:23� 0:10 0:01� 0:05

TTS050717.9� 032433 3:549� 0:008 3:1� 0:1? 0:00+ 0:05 1:09� 0:05 � 0:77� 0:05

RX J0507.4� 0320 3:522� 0:009 3:1� 0:1? 0:20� 0:10 1:68� 0:08 � 0:51� 0:05

RX J0507.4� 0317 3:540� 0:009 3:1� 0:1? 0:00+ 0:05 1:04� 0:05 � 0:85� 0:05

{ 55 {

Table5| Continued

PM S Star logTeff R V A V R ? logL=L�

(m ag) (R � )

TTS050729.8� 031705 3:476� 0:008 2:5� 0:1 1:32� 0:19 0:73� 0:03 � 1:42� 0:05

TTS050730.9� 031846 3:494� 0:010 3:1� 0:1? 1:81� 0:15 0:33� 0:01 � 2:02� 0:05

TTS050733.6� 032517 3:494� 0:010 3:1� 0:1? 0:54� 0:20 0:69� 0:03 � 1:39� 0:06

TTS050734.8� 031521 3:504� 0:009 3:1� 0:1? 0:05� 0:10 0:67� 0:03 � 1:37� 0:05

RX J0507.6� 0318 3:604� 0:008 3:1� 0:1? 0:00+ 0:05 1:26� 0:06 � 0:43� 0:05

TTS050741.0� 032253 3:522� 0:009 3:1� 0:1? 0:05� 0:10 1:02� 0:05 � 0:94� 0:05

TTS050741.4� 031507 3:513� 0:009 3:1� 0:1? 0:05� 0:07 1:00� 0:05 � 0:99� 0:05

TTS050752.0� 032003 3:494� 0:010 3:1� 0:1? 0:64� 0:20 0:74� 0:03 � 1:33� 0:06

TTS050801.4� 032255 3:581� 0:008 3:1� 0:1? 0:54� 0:12 1:47� 0:07 � 0:39� 0:05

TTS050801.9� 031732 3:573� 0:008 3:1� 0:1? 0:39� 0:12 1:46� 0:07 � 0:42� 0:05

TTS050804.0� 034052 3:549� 0:008 3:1� 0:1? 0:00+ 0:05 1:15� 0:05 � 0:73� 0:05

TTS050836.6� 030341 3:565� 0:008 3:1� 0:1? 0:64� 0:10 1:97� 0:09 � 0:20� 0:05

TTS050845.1� 031653 3:531� 0:009 3:1� 0:1? 0:00+ 0:05 1:61� 0:07 � 0:51� 0:05

RX J0509.0� 0315 3:746� 0:009 3:1� 0:1? 0:00+ 0:05 2:62� 0:12 0:78� 0:05

RX J0510.1� 0427 3:662� 0:026 3:1� 0:1? 0:00+ 0:05 3:74� 0:17 0:75� 0:11

1RXS J051011.5� 025355 3:720� 0:014 3:1� 0:1? 0:60� 0:22 2:37� 0:11 0:59� 0:07

RX J0510.3� 0330 3:746� 0:009 3:1� 0:1? 0:10� 0:08 2:16� 0:10 0:61� 0:05

1RXS J051043.2� 031627 3:768� 0:004 3:1� 0:1? 0:00+ 0:05 2:25� 0:10 0:73� 0:04

RX J0511.7� 0348 3:706� 0:015 3:1� 0:1? 0:00+ 0:10 2:40� 0:11 0:54� 0:07

RX J0512.3� 0255 3:690� 0:016 3:1� 0:1? 0:00+ 0:10 1:99� 0:10 0:31� 0:08

?Assum ed (seeSection 5.3).

{ 56 {

Table 6:E�ective wavelengths and absolute ux calibration constants for the Johnson-

Cousinsand 2M ASS passbands.Them ain referencepapersarelisted in thelastcolum n.

Filter �eff F 0� Reference

(�m ) (erg=s2=cm 2=�A)

UJ 0.36 4.350�10�9 Johnson (1965)

B J 0.44 7.200�10�9 Johnson (1965)

VJ 0.55 3.920�10�9 Cousins(1976)

R C 0.64 2.254�10�9 Cousins(1976)

IC 0.79 1.196�10�9 Cousins(1976)

J 1.24 3.129�10�10 Cohen etal.(2003)

H 1.66 1.133�10�10 Cohen etal.(2003)

K s 2.16 4.283�10�11 Cohen etal.(2003)

{ 57 {

Table7. M assesand agesforthePM S starsin L1615/L1616 asinferred by the

evolutionary m odelsby Bara�eetal.(1998)& Chabrieretal.(2000),

D’Antona & M azzitelli(1997)and Palla & Stahler(1999).

PM S Star M B a98+ C h00 AgeB a98+ C h00 M D M 97 AgeD M 97 M P S99 AgeP S99

(M � ) (M yr) (M � ) (M yr) (M � ) (M yr)

1RXS J045912.4� 033711 > 1:40 � 2.00 5.00 1.70 3.50

1RXS J050416.9� 021426 > 1:40 � 0.95 0.70 1.60 3.00

TTS050513.5� 034248 0.12 6.00 0.17 10.00 0.14 10.00

TTS050538.9� 032626 0.35 4.04 0.25 2.20 0.26 2.80

RX J0506.6� 0337 1.35 14.00 1.65 5.50 1.40 8.00

TTS050644.4� 032913 0.25 3.17 0.20 2.50 0.19 2.70

TTS050646.1� 031922 > 1:40 � 0.75 1.00 1.35 4.00

RX J0506.8� 0318 1.05 5.00 0.50 1.00 0.68 3.00

TTS050647.5� 031910 0.18 2.00 0.16 2.00 0.13 2.00

RX J0506.8� 0327 0.55 < 1:00 0.20 0.10 0.25 0.40

RX J0506.8� 0305 0.26 1.78 0.19 1.50 0.18 1.50

TTS050649.8� 031933 0.47 1.00 0.22 0.20 0.25 0.70

TTS050649.8� 032104 0.87 1.30 0.35 0.40 0.50 0.80

TTS050650.5� 032014 0.09 < 1:00 0.13 1.00 < 0:10 0.70

TTS050650.7� 032008 0.26 1.60 0.19 1.50 0.18 1.50

RX J0506.9� 0319NW 0.50 4.50 0.33 2.00 0.35 2.50

RX J0506.9� 0319SE 1.40 1.30 0.45 0.30 0.93 0.80

HD 293815 > 1:40 � 2.50 3.00 2.50 3.00

RX J0506.9� 0320W 1.25 < 1:00 0.32 0.07 0.68 0.40

RX J0506.9� 0320E > 1:40 � 2.30 0.90 2.30 1.50

TTS050654.5� 032046 0.26 14.20 0.25 14.00 0.22 10.00

LkH� 333 > 1:40 � 0.75 0.20 1.70 0.50

L1616 M IR4 > 1:40 � 1.70 1.70 1.75 3.00

K isoA-0974 15 > 1:40 � > 3:00 � 5.50 0.30

RX J0507.0� 0318 1.10 1.45 0.35 0.30 0.60 0.80

TTS050657.0� 031640 0.27 1.60 0.19 1.50 0.18 1.50

TTS050704.7� 030241 0.11 3.00 0.15 4.50 0.10 3.00

TTS050705.3� 030006 0.90 10.05 0.55 3.00 0.63 4.50

RX J0507.1� 0321 0.80 4.30 0.40 1.00 0.50 2.50

TTS050706.2� 031703 0.12 2.55 0.15 4.00 0.10 2.50

RX J0507.2� 0323 1.25 11.30 0.85 2.00 1.20 7.50

TTS050713.5� 031722 0.73 35.80 0.70 20.00 0.60 20.00

RX J0507.3� 0326 1.05 2.00 0.37 0.40 0.60 0.80

TTS050717.9� 032433 0.50 6.32 0.34 2.80 0.35 3.00

RX J0507.4� 0320 0.36 1.13 0.20 0.70 0.22 0.80

RX J0507.4� 0317 0.42 5.50 0.28 2.80 0.31 3.00

{ 58 {

Table7| Continued

PM S Star M B a98+ C h00 AgeB a98+ C h00 M D M 97 AgeD M 97 M P S99 AgeP S99

(M � ) (M yr) (M � ) (M yr) (M � ) (M yr)

TTS050729.8� 031705 0.09 2.53 0.13 4.00 < 0:10 3.00

TTS050730.9� 031846 0.11 20.11 0.15 30.00 0.13 30.00

TTS050733.6� 032517 0.14 4.00 0.17 5.50 0.13 4.00

TTS050734.8� 031521 0.17 5.65 0.20 6.50 0.16 5.00

RX J0507.6� 0318 1.00 11.27 0.62 3.00 0.75 7.00

TTS050741.0� 032253 0.30 3.60 0.23 2.50 0.22 2.50

TTS050741.4� 031507 0.25 3.17 0.20 2.50 0.18 2.50

TTS050752.0� 032003 0.15 3.56 0.17 4.50 0.13 3.00

TTS050801.4� 032255 0.88 5.67 0.47 1.60 0.56 3.00

TTS050801.9� 031732 0.80 5.02 0.44 1.50 0.50 2.70

TTS050804.0� 034052 0.50 5.67 0.33 2.50 0.35 2.80

TTS050836.6� 030341 0.75 2.01 0.32 0.60 0.44 1.00

TTS050845.1� 031653 0.40 1.61 0.23 0.90 0.26 1.00

RX J0509.0� 0315 > 1:40 � 2.00 4.50 1.75 3.50

RX J0510.1� 0427 > 1:40 � 0.70 0.20 1.70 0.50

1RXS J051011.5� 025355 > 1:40 � 1.85 2.50 1.65 4.00

RX J0510.3� 0330 > 1:40 � 1.70 7.00 1.50 6.00

1RXS J051043.2� 031627 > 1:40 � 1.65 8.00 1.50 6.50

RX J0511.7� 0348 > 1:40 � 1.70 2.00 1.70 4.00

RX J0512.3� 0255 > 1:40 � 1.30 2.00 1.50 5.00

{ 59 {

Table8. � slopesoftheIM F in di�erentclustersand associations.

Cluster Age M assrange � Ref.

(M yr) (M � )

M 35 150 0.1-0.4 0.58 a

� Per 80 0.06-0.2 0.56 b

Pleiades 120 0.03-0.45 0.60�0.11 c

� Orionis 5 0.02-1.2 0.60�0.06 d

� Orionis 5 0.013-0.2 0.8�0.4 e

Trapezium 0.4 0.035-0.56 �0.3 f

0.25-3 �0.7

� Oph 0.1-1 0.02-0.4 �0.5 g

IC 348 3 0.035-0.5 0.7�0.2 h

Taurus-Auriga 1-2 0.08-1 1.35 i

Cha I � 3 0.08-1.2 0.6-1.1 j,k

Cha II 2-5 0.1-1 0.5-0.6 l

L1615/L1616 1-3 0.1-5.5 0.84�0.07 Thiswork

References. | a) Barrado Y Navascu�esetal. (1998); b)

Barrado Y Navascu�esetal.(2002); c) M oraux etal.(2003); d)

Barrado Y Navascu�esetal. (2004b); e) B�ejaretal. (2001); f)

Luhm an etal.(2000); g) Luhm an & Rieke (1999); h) Tejetal.

(2002);i) Brice~no etal.(2002);j) L�opezM art�� etal.(2004);k)

Com er�on etal.(2000);l)Spezzietal.(2008).