multiband photometry and spectroscopy of an all-sky sample

warwickacuklib-publications

Original citation Raddi R Gentile Fusillo N P Pala A Hermes J J Gaensicke B T (Boris T) Chote P Hollands M Henden A Catalaacuten S Geier S Koester D Munari U Napiwotzki R and Tremblay Pier-Emmanuel (2017) Multiband photometry and spectroscopy of an all-sky sample of bright white dwarfs Monthly Notices of the Royal Astronomical Society 472 (4) pp 4173-4192 Permanent WRAP URL httpwrapwarwickacuk95038 Copyright and reuse The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions Copyright copy and all moral rights to the version of the paper presented here belong to the individual author(s) andor other copyright owners To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available Copies of full items can be used for personal research or study educational or not-for-profit purposes without prior permission or charge Provided that the authors title and full bibliographic details are credited a hyperlink andor URL is given for the original metadata page and the content is not changed in any way Publisherrsquos statement This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society copy 2017 The Authors published by Oxford University Press on behalf of the Royal Astronomical Society All rights reserved A note on versions The version presented here may differ from the published version or version of record if you wish to cite this item you are advised to consult the publisherrsquos version Please see the lsquopermanent WRAP URLrsquo above for details on accessing the published version and note that access may require a subscription For more information please contact the WRAP Team at wrapwarwickacuk

MNRAS 472 4173ndash4192 (2017) doi101093mnrasstx2243Advance Access publication 2017 September 4

Multiband photometry and spectroscopy of an all-skysample of bright white dwarfs

R Raddi1lsaquo N P Gentile Fusillo1 A F Pala1 J J Hermes2dagger B T Gansicke1

P Chote1 M A Hollands1 A Henden3 S Catalan4 S Geier5 D Koester6

U Munari7 R Napiwotzki8 and P-E Tremblay1

1Department of Physics University of Warwick Gibbet Hill Road Coventry CV4 7AL UK2Department of Physics and Astronomy University of North Carolina Chapel Hill NC 27599 USA3AAVSO 49 Bay State Rd Cambridge MA 02138 USA4Centre for Astronomy National University of Ireland Galway Galway Ireland5Institute for Astronomy and Astrophysics Eberhard Karls University Sand 1 D-72076 Tubingen Germany6Institut fur Theoretische Physik und Astrophysik Universitat Kiel D-24098 Kiel Germany7INAFndashAstronomical Observatory of Padova I-36012 Asiago (VI) Italy8Centre for Astrophysics Research STRI University of Hertfordshire College Lane Hatfield AL10 9AB UK

Accepted 2017 August 28 Received 2017 August 28 in original form 2017 February 27

ABSTRACTThe upcoming NASA Transiting Exoplanet Survey Satellite (TESS) will obtain space-baseduninterrupted light curves for a large sample of bright white dwarfs distributed across theentire sky providing a very rich resource for asteroseismological studies and the search fortransits from planetary debris We have compiled an all-sky catalogue of ultraviolet opticaland infrared photometry as well as proper motions which we propose as an essential toolfor the preliminary identification and characterization of potential targets We present datafor 1864 known white dwarfs and 305 high-probability white dwarf candidates brighter than17 mag We describe the spectroscopic follow-up of 135 stars of which 82 are white dwarfs and25 are hot subdwarfs The new confirmed stars include six pulsating white dwarf candidates(ZZ Cetis) and nine white dwarf binaries with a cool main-sequence companion We identifyone star with a spectroscopic distance of only 25 pc from the Sun Around the time TESS islaunched we foresee that all white dwarfs in this sample will have trigonometric parallaxesmeasured by the ESA Gaia mission next year

Key words catalogues ndash binaries spectroscopic ndash binaries visual ndash subdwarfs ndash whitedwarfs

1 IN T RO D U C T I O N

White dwarfs are the final evolutionary stage for the vast majorityof single stars Characterized by long steady cooling rates whitedwarfs can be used to infer properties of the Milky Way like the starformation history (Tremblay et al 2014) the age of the disc (Wingetet al 1987 Oswalt et al 1996) or their contribution to baryonic darkmatter (Reid Sahu amp Hawley 2001 Pauli et al 2003)

Asteroseismology is a powerful tool for probing the internal struc-ture of white dwarfs through the study of multiperiodic variabil-ity that occurs during short-lived instability phases on the whitedwarf cooling sequence (Fontaine amp Brassard 2008 Winget ampKepler 2008 Althaus et al 2010) Over the last few years the

E-mail rraddiwarwickacukdaggerHubble Fellow

NASA Kepler mission (Borucki et al 2010) and its re-purposedtwo-wheeled K2 mission (Howell et al 2014) have enabled high-precision asteroseismology of white dwarfs (Hermes et al 2011Oslashstensen et al 2011 Greiss et al 2016) Its successor the NASATransiting Exoplanet Survey Satellite (TESS Ricker et al 2015)will acquire all-sky times-series photometry for at least 2 yr search-ing for exoplanet transits and enabling asteroseismology for manyclasses of stars (Campante et al 2016) Roughly 200 000 selectedtargets including bright white dwarfs may be observed with 2-mincadence Full-frame images will be taken every 30 min The missionwill survey areas of sky with 1-month to 1-yr coverage dependingon their ecliptic latitudes

TESS is poised to return improved statistics building uponthe original Kepler mission and its ongoing K2 reincarnationIn particular the large sky coverage of TESS will be excellent for thediscovery of rare phenomena of which Kepler has so far provided uswith few or even unique examples eg the first direct detection of

Ccopy 2017 The AuthorsPublished by Oxford University Press on behalf of the Royal Astronomical Society

Downloaded from httpsacademicoupcommnrasarticle-abstract472441734103558by University of Warwick useron 20 November 2017

4174 R Raddi et al

a disintegrating asteroid at a white dwarf (Vanderburg et al 2015)or the identification of stochastic outbursts in cool pulsating whitedwarfs (Bell et al 2015 2016 Hermes et al 2015) TESS is ex-pected to be launched in 2018 MarchndashJuly which is just beforeor at the same time of the second data release of the ESA Gaiamission (DR2 Gaia Collaboration 2016a Lindegren et al 2016)Therefore the need of an all-sky target list of bright white dwarfs isextremely urgent especially for the southern ecliptic hemispherewhich will not have the benefit of Gaia selection Bright whitedwarfs can also be monitored by upcoming photometric missionssuch as Evryscope (Law et al 2015) or the Next Generation TransitSurvey (NGTS West et al 2016)

Traditionally the identification of white dwarfs relies on theirblue colours and relatively large proper motions The availabilityof accurate digital photometry and astrometry has greatly improvedthe efficiency of white dwarf selection (eg Kawka amp Vennes 2006Kilic et al 2006 Limoges Lepine amp Bergeron 2013 GentileFusillo Gansicke amp Greiss 2015 Raddi et al 2016 Gentile Fusilloet al 2017) Numerous efforts have been undertaken to improvethe identification and characterization of bright and nearby whitedwarfs (Sion et al 2014 Holberg et al 2016) not only through spec-troscopy and photometry (Giammichele Bergeron amp Dufour 2012)but also including trigonometric parallaxes (eg DENSE project1

Subasavage et al 2009) So far the white dwarf census is stronglybiased towards the Northern hemisphere which Limoges Bergeronamp Lepine (2015) estimate to be 78 per cent complete at 40 pc

In the following sections we describe a collection of proper mo-tions and multiband all-sky photometry for potential TESS whitedwarf targets Out of 2990 known white dwarfs brighter than17 mag we re-identify 1864 stars ie 62 per cent of the totalWe discuss the properties of the sample and its completeness viacomparison with catalogues of spectroscopically confirmed whitedwarfs and the Lowell Observatory proper motion survey (GiclasBurnham amp Thomas 1980 GD sample) We present photometryand proper motions for 305 new high-probability white dwarf can-didates and 33 white dwarf candidates in the GD sample In the lastpart of the paper we discuss the properties of 82 white dwarfs and25 hot subdwarfs for which we acquired follow-up spectroscopy

2 TH E DATA

The photometric identification of white dwarfs hotter than 6000 Kis very effective with colours sampling the flux across the Balmerjump eg ugr photometry of the Sloan Digital Sky Survey (SDSSGirven et al 2011) Currently there is no single all-sky surveycovering this spectral region for magnitudes brighter than 17thmagnitude A useful alternative is offered by the combination offar- and near-ultraviolet photometry (FUV and NUV bands cen-tred at 1540 and 2270 Aring respectively) from the Galaxy EvolutionExplorer (GALEX Morrissey et al 2007) with optical photometryfrom the AAVSO Photometric All-Sky Survey (APASS Henden ampMunari 2014) in the Johnson BV and Sloan gprimerprimeiprime passbands

Bianchi et al (2011) discussed the properties of hybrid colourndashcolour diagrams of GALEX and SDSS photometry showing theirpotential at separating classes of Galactic and extragalactic sourcesLimoges et al (2013) have used similar colour combinations toselect white dwarfs candidates in the Northern hemisphere fromGALEX SDSS Hipparcos (van Leeuwen 2007) and Tycho-2 (Hoslashget al 2000) using proper motions from SUPERBLINK (Lepine

1 httpwwwDenseProjectcom

Table 1 Photometric bands used in this work Refer to the Spanish VirtualObservatory web pagesa for the definition of effective wavelength and widthλeff and Weff respectively

Survey Filters λeff (nm) Weff (nm)

GALEX FUV 154 25NUV 227 73

APASS B 430 84gprime 464 116V 539 87rprime 612 111iprime 744 104

2MASS J 1235 162H 1662 251Ks 2619 162

WISE W1 3353 663W2 4602 1042

ahttpsvo2cabinta-csicestheoryvosa4

amp Shara 2005) In the work described here we used the PPMXLcatalogue that includes positions and proper motions for 900 mil-lion stars derived from USNO-B 10 (Monet et al 2003) and theTwo-Micron All-Sky Survey (2MASS Skrutskie et al 2006) as-trometry in the International Coordinate Reference System In ad-dition to the ultraviolet optical and near-infrared photometry wealso added mid-infrared data from the Wide-field Infrared SurveyExplorer (WISE Wright et al 2010) In Table 1 we list the wave-length coverage of the used set of filters

21 The reference catalogue APASS

APASS data release 9 (DR9 Henden et al 2016) containsasymp61 million objects in the 10thndash17th magnitude range The sur-vey uses two pairs of 20-cm twin telescopes two in the north atDark Ridge Observatory in New Mexico and two in the south atthe Cerro Tololo Inter-American Observatory This set-up allowsautomated essentially simultaneous observations in the BV andgprimerprimeiprime bands The pixel scale at detectors is 257 arcsec and pho-tometry is extracted using a 15-arcsec aperture in the north and20 arcsec in the south due to differences in the focusing of tele-scopes APASS DR9 is suggested to be currently complete downto V = 16 and it is expected to be complete down to V = 17 atthe end of the survey The bright limit of APASS is of little con-cern here as there are only two known white dwarfs brighter than10 mag (ie Sirius B and 40 Eri B see next section) The astrometricaccuracy is 015 arcsec and the photometric accuracy is 002 mag at15 mag (Munari et al 2014) The release notes2 suggest a carefuluse of APASS photometry in crowded areas where the photometryextraction algorithm suffers from blending Furthermore the mostrecent measures of B gprime magnitudes in the north are known to carrylarge random errors due to a technical problem with the detectorsNevertheless APASS photometry was shown to be sufficiently ac-curate for tying other deeper surveys on to an absolute flux scale(eg Barentsen et al 2014 Drew et al 2014 Shanks et al 2015)

Duplicated entries are present in APASS DR9 due to mergingissues at the detector corners and for high proper motion objectsThus we generated a reduced data set of unique sources within aseparation of lt125 arcsec which corresponds to half a pixel ofAPASS detectors When more than one photometric value is iden-tified in this internal cross-match process we adopted the brightest

2 httpswwwaavsoorgapass

MNRAS 472 4173ndash4192 (2017)Downloaded from httpsacademicoupcommnrasarticle-abstract472441734103558by University of Warwick useron 20 November 2017

Bright white dwarfs for high-speed photometry 4175

Table 2 Results of the all-sky cross-match

Survey No of objects

APASS DR9 61 176 401APASS DR9 unique 58 756 366APASS + GALEX (FUV and NUV) 19 037 824APASS + GALEX NUV unique 15 558 208APASS + PPMXL 44 290 772APASS + 2MASS 42 901 844APASS + WISE 43 179 834

detection as suggested in the APASS DR9 release notes The so-defined unique list of APASS sources contained a little less than 59million objects

22 Matching with PPMXL GALEX and WISE

To cross-correlate APASS with GALEX and WISE detections weneeded to account for stellar proper motions Thus we first queriedthe PPMXL data base via Table Access Protocol (TAP) using theStarlink Tables Infrastructure Library Tool Set (STILTS Taylor 2006)The query was run by uploading the entire unique APASS data setto the online service3 split in to chunks of 100 000 rows each Foreach APASS source we identified all the PPMXL objects within45 arcsec Then assuming epoch 20130 for the APASS DR9 coor-dinates we forward-propagated the coordinates of PPMXL sourcesusing their proper motions Finally we identified good matches byselecting objects that fall within 2 arcsec from the APASS sourcesThe APASS-PPMXL cross-match produced asymp45 million objects

The next step was to cross-match the APASS-PPMXL sourceswith the GALEX DR6+DR7 data base4 accessing the CatalogArchive Server Jobs System (CasJobs) via Simple Object AccessProtocol In CasJobs we first queried the PhotoObjAll table tosearch for all the nearby sources within 45 arcsec from the positionof our APASS-PPMXL sources then we identified the correspond-ing observing dates in the photoExtract table to propagate APASScoordinates to the corresponding GALEX epochs again using thePPMXL proper motions The cross-match with GALEX reduced fur-ther the number of APASS-PPMXL sources with proper motionsand NUV photometry leaving asymp19 million detections (ie about 15million unique sources)

As last step we cross-matched the APASS-PPMXL source listwith WISE via TAP Again we shifted the APASS coordinatesbackwards in time by taking into account the epoch differencebetween APASS and WISE

In Table 2 we summarize the results of the cross-match proce-dure It is important to stress here that we used the APASS-PPMXL-GALEX (APG hereafter) cross-match to characterize and identifymost of the white dwarfs described in this paper We used WISEphotometry to probe for infrared excess eg the binary systemsdiscussed in Section 65

To visualize the cross-match procedure we display in Fig 1the identification of a newly confirmed white dwarf J0515+5021(GD 289) We note the starrsquos significant proper motion across dif-ferent epochs when the considered set of images was taken

3 PPMXL is hosted by the German Astronomical Virtual Observatory datacentre at httpdczahuni-heidelbergde4 GALEX DR6+DR7 is hosted by the Mikulski Archive for Space Tele-scopes (MAST) at httpgalexstscieduGR6

3 A SSESSING THE C RO SS-MATCH

To verify the completeness of the APG cross-match we comparedthe data to a set of catalogues from the literature Our first compari-son is done against the GD sample (Giclas et al 1980) that containsblue stellar objects with proper motions of 020 arcsec yrminus1 andincludes 311 previously known white dwarfs In this work we haveconfirmed 12 new GD stars as white dwarfs

During this operation we visually inspected finding charts toconfirm our re-identifications We download cut-out images fromthe NASA SkyView service5 ie blue and red frames of the STScIDigitized Sky Survey6 2MASS and GALEX and for known whitedwarfs we used finding charts available at the DENSE web pagesor those made available by Tom Marsh7

We make all the photometry and proper motions for the re-identified stars available on VizieR (hosted by the Strasbourg astro-nomical Data Center CDS Wenger et al 2000) Table A1 describesthe columns that are included in the online tables

31 The GD sample

The catalogue of blue stars from the Lowell Observatory propermotion survey contains 1710 unique objects For this test we usedthe list of coordinates and magnitudes that is available on VizieR(catalogue ID II299) these quantities are an updated collection ofdata from other surveys and not just from the original work Throughour cross-match described above we successfully retrieved APASSphotometry PPMXL proper motions and GALEX photometry for1479 1455 and 1175 stars respectively Of these stars we re-identify APASS photometry for 260 white dwarfs In Fig 2 thedistribution of these magnitudes is compared to those from the GDcatalogue as available on VizieR In Fig 2 we also display thedistribution of PPMXL proper motions (μ) and angular separa-tions between the APASS coordinates and those listed in VizieR Inthe bottom-left panel of Fig 2 we display the difference betweenthe APASS V magnitudes plotted against the angular separations(APASS ndash VizieR coordinates) We measure a scatter of plusmn015 magaround the zero Five white dwarfs are at more than 5σ from the zeroGD 123 which has an M-dwarf companion GD 167 for which theV-band photometry recorded in VizieR is asymp2 mag brighter thanthose measured in other bands available on SIMBAD (also hostedby CDS) GD 230 GD 272 and GD 546 for which we suspectthe APASS photometry to be blended with nearby objects In thebottom-right panel of Fig 2 we display the correlation betweenPPMXL proper motions and angular separations The majority ofGD stars follow a linear trend which is more evident for the knownwhite dwarfs in the sample After inspecting finding charts for thesestars we note that most of the scatter appears to be due to the limitedaccuracy of the reference positions that are listed in VizieR Themost discrepant white dwarfs GD 230 (with an angular separationof 9 arcsec off-scale in Fig 2) is likely blended with nearby stars

We inspected the literature on GD stars available through SIM-BAD and divided them into six groups white dwarfs white dwarfcandidates subdwarfs stars unknown objects and QSOs Morethan half of the GD stars have very little information available (ieno classification in the references) or no SIMBAD entry at all InFig 3 we display the colourndashcolour and reduced proper motion dia-grams of stars with APG data (the reduced proper motion is defined

5 httpskyviewgsfcnasagov6 httpsarchivestscieducgi-bindss_form7 httpdenebastrowarwickacukphsaapwdcharts


4176 R Raddi et al

Figure 1 In the first four panels we show the evolving position of the newly confirmed white dwarf J0515+5021 (GD 289) From left to right we displaythe cut-outs from the POSS-I and POSS-II blue plates and the 2MASS J and GALEX NUV images The rightmost panel displays the PPMXL proper motionvector (in arcsec yrminus1 multiplied by 100 to improve the visualization) In each panel we plot the large cone-search of 45 arcsec used to identify objects withlarge proper motions (in blue) and a 2-arcsec circle for the cross-match at the APASS position (in red)

Figure 2 Assessment of the APG cross-match for the GD sample Topsolid histograms represent the APASS V-band magnitudes proper motionsand angular separations between APASS coordinates and those availablein VizieR The step histograms represent the V-band magnitudes availableon VizieR Bottom on the left the differences between APASS magnitudesand those from the literature are plotted against the angular separation onthe right we show the correlation between PPMXL proper motions plottedagainst the angular separations We use dark points to represent known whitedwarfs and light coloured points for non-white dwarf sources

as Hg = gprime + 5 + 5 times log μ with the proper motion μ given inarcsec yrminus1) We note that confirmed white dwarfs and hot subd-warfs mostly separate from the main group of other stellar objectsBased on the colours reduced-proper motions and the informationavailable in SIMBAD we identify 46-candidate white dwarfs ofwhich we observed 13 spectroscopically In the GD sample thereare about 1100 objects with little or no information on SIMBADwhich have colours and proper motions that are compatible withthose of main-sequence stars

32 White dwarf samples

As seen from the previous test the re-identification of known objectsmay present some issues due to the poor accuracy of reference posi-tions This can be accentuated for bright white dwarfs because theircoordinates were traditionally recorded in equinox B19500 with anaccuracy of the order of 1 arcmin (eg McCook amp Sion 1999) andpossess on average large proper motions With the advance of tech-nology higher accuracy is required and there is an ongoing effortfor updating the astrometry and proper motions of known white

Figure 3 Colourndashcolour (top) and reduced proper motion (bottom) dia-grams of the GD sample The dark coloured contours in both panels rep-resent the colour distribution of the field population as inferred from ourall-sky cross-match The object classification is based on the informationavailable from SIMBAD (see text) Top white dwarf cooling sequences(log g = 70ndash95 in 05 dex steps) and the main sequence are plotted forreference both have been determined by convolving Koester (2010) syn-thetic spectra and Pickles (1998) spectra with the filter transmission profilesrespectively Bottom solid curves representing the expected reduced propermotions for log g = 8 disc white dwarfs are plotted at three distinct valuesof tangential velocities VT = 20 40 and 150 km sminus1 The curves follow therelation Hg = gprime + 5 times log VT minus 3379

dwarfs eg on SIMBAD and the Montreal White Dwarf Database(MWDD Dufour et al 2017)

To assess the completeness of the APG cross-match with re-spect to well-studied white dwarfs we took into consideration five



spectroscopic catalogues that include about 23 of the spectro-scopically confirmed white dwarfs within the APASS magnituderange These are as follows (i) the 25-pc sample (analysed bySion et al 2014) (ii) hydrogen (DA) and helium (DBDBA) whitedwarfs from the European Southern Observatory (ESO) supernovaeIa progenitor survey (SPY Voss et al 2007 Koester et al 2009 re-spectively) (iii) DA and DB white dwarfs analysed by the Montrealgroup (to be distinguished from the MWDD which contains a largefraction of the spectroscopically confirmed white dwarfs Gianni-nas et al 2010 Bergeron et al 2011) (iv) a bright selection (g le17) of white dwarf plus main-sequence binaries (WD+MS) fromSDSS (Rebassa-Mansergas et al 2012) Covering most of the skywith Galactic latitudes of |b| gt 30 deg these catalogues allow fora semi-independent assessment of the photometric proper motionand on-sky completeness of our APG data set We note that for eachcatalogue we used the coordinates available online through VizieRor SIMBAD Given that the accuracy of positions may be differentin each catalogue this can lead to possible mismatches andor tonon-identifications even for objects that are in more than one of theconsidered catalogues When assessing the global-completenesswe took into account the unique objects across the catalogues

321 The local 25-pc sample

For this comparison we use the list of Sion et al (2014) whichcontains 223 white dwarfs We identify APASS matches for 169stars The unmatched objects include Sirius B or Procyon B whichare outshined by their bright main-sequence companions In the topand bottom panels of Fig 4 we display the comparison betweenAPASS V photometry and data available on SIMBAD for the Sionet al (2014) white dwarfs in the same band For this sample wemeasure a mean difference of plusmn035 mag Most of the outliers aredue to contamination from nearby sources We note that our cross-match becomes increasingly incomplete below 165 mag and we donot retrieve any of the white dwarfs fainter than 17 mag (5 per centof the total) The brightest white dwarf we re-identify is 40 Eri B(APASS V = 93)

The completeness of the cross-match with the Sion et al (2014)sample is reduced to 116 white dwarfs when we consider PP-MXL proper motions The highest proper motion we retrieve is125 arcsec yrminus1 for WD 2248+293 From Fig 4 we note that we

Figure 4 Assessment of the APG cross-match for white dwarfs in the 25-pcsample Refer to Fig 2 for a description of each panel

are about 60 per cent complete for white dwarfs with proper motionsle1 arcsec yrminus1

In the top-right panel of Fig 4 we present the distribution of an-gular separations between APASS and SIMBAD coordinates for theSion et al (2014) white dwarfs 40 Eri B is the star with the largestangular separation (52 arcsec) We do not plot WD 0727+482A(with an angular separation of 90 arcsec off-scale in Fig 4) forwhich SIMBAD lists the coordinates of the companion

In the bottom-right panel of Fig 4 we display the correlationbetween angular separations (APASS ndash SIMBAD coordinates) andthe proper motions First we note that the correlation follows closelya linear trend Secondly we fail to retrieve proper motions for starswith separations that are larger than 20 arcsec Most of these objectsmove faster than 15 arcsec yrminus1 The lowest point not aligningwith the others is WD 0108+277 the astrometry of which may beaffected by the presence of a nearby background source (FarihiBecklin amp Zuckerman 2005 Holberg et al 2016)

322 White dwarfs from the SPY project

SPY was an ESO spectroscopic survey that targeted bright whitedwarfs (B le 165) south of δ = +25 deg (Napiwotzki et al 2003)We tested the completeness of APASS on the DA and DBDBAwhite dwarf catalogues published by Koester et al (2009) and Vosset al (2007) respectively We re-identified 575 of the 669 DA whitedwarfs and 62 of the 69 DBDBA white dwarfs from SPY Thecomparison between APASS magnitudes and those available in theliterature is shown in the left-hand panels of Fig 5

Figure 5 Validation of the APASS-PPMXL cross-match The colouredhistogram bars represent APASS magnitude distributions (V or gprime depend-ing on the subsample considered) for the five white dwarf catalogues weused as reference K+09 Koester et al (2009) V+07 Voss et al (2007)G+10 Gianninas et al (2010) B+11 Bergeron et al (2011) RM+12Rebassa-Mansergas et al (2012) The black outline histograms representthe magnitude distributions of the known white dwarfs as retrieved fromthe literature In the bottom-right panel we plot the PPMXL proper mo-tions against the angular separations between APASS coordinates and thepositions available on VizieRSIMBAD Symbols are colour-coded with thesame colours used of the histogram bars


4178 R Raddi et al

Table 3 Summary of the comparison between the APG cross-match with stars from the literature

Catalogue No of objects APASS PPMXL GALEX NUV Reference

GD sample (all objects) 1710 1479 1455 1175 Giclas et al (1980)GD sample (confirmed white dwarfs) 323 260 246 17425 pc 223 169 116 40 Sion et al (2014)SPY DA 669 575 547 394 Koester et al (2009)SPY DBDBA 69 62 59 37 Voss et al (2007)Montreal DA 1265 889 822 546 Gianninas et al (2010)Montreal DBDBA 108 72 70 48 Bergeron et al (2011)WD+MS 92 55 54 39 Rebassa-Mansergas et al (2012)

Total white dwarfs 1931 1388 1271 840

We retrieved PPMXL proper motions for 547 DA and 59DBDBA white dwarfs On average proper motions of these starsare smaller in comparison with those of white dwarfs in the localsample and therefore we have a higher success rate in re-identifyingthese stars through our selection method In the bottom-right cor-ner of Fig 5 we display the correlation between PPMXL propermotions and angular separations between APASS and Vizier co-ordinates for the SPY sample In the same panel we plot alsowhite dwarfs from the other samples discussed next Proper mo-tion and angular separations correlate well as found above for theSion et al (2014) sample Just a few objects (eg WD 1334minus678WD 1943+163 WD 2029+183 and WD2157+161) which areblended with nearby stars may have inaccurate positions andorproper motions

323 DA and DB white dwarfs analysed by the Montreal group

These two white dwarf samples overlap to some degree with SPYand consist of 1265 DA (Gianninas et al 2010) and 108 DBDBAwhite dwarfs (Bergeron et al 2011) The relevant differences withthe SPY sample are their spatial distribution that extends to theNorthern hemisphere and a fainter limiting magnitude (asymp175 mag)We identified APASS photometry for 889 DA and 72 DBDBAwhite dwarfs The magnitude distributions of both samples are dis-played in the top-right panels of Fig 5 We note that the com-pleteness starts decreasing for stars that are fainter than 165 magIn the bottom-right panel of Fig 5 we find a good correlationbetween proper motions and angular separations also for thesestars like before a few discrepant objects (eg WD 0927minus173 orWD 1545+244) are close to nearby sources that cause less accuratemeasures of proper motions or positions

324 White dwarf plus main-sequence star binaries

Rebassa-Mansergas et al (2012) have produced the so far largestcatalogue of WD+MS binaries8 from SDSS which has been re-cently updated to the data releases 9 and 12 (Rebassa-Mansergaset al 2016) We selected a sample of 92 binaries with gprime le 17re-identifying APASS and PPMXL data for 55 and 54 of themrespectively The magnitude distribution of the sample is displayedin the bottom-left corner of Fig 5 while in the bottom-right panelwe add the corresponding data points to the comparison betweenPPMXL proper motions and coordinate separations

8 httpwwwsdss-WDMSorg

Figure 6 Completeness of the data set expressed as fraction of the num-ber of stars retrieved with respect to the total number of objects in thecorresponding sample

33 Completeness

Table 3 summarizes the tests of our combined APG catalogues Thecompleteness of the cross-match depends on the intrinsic propertiesof the different test cases eg the magnitude limits and accuracy ofpositions In total we have retrieved APASS photometry for 1388white dwarfs out of a sample of 1931 stars (72 per cent) We notethat that the median magnitude of the white dwarfs for which wedo not retrieve APASS data is V asymp 165 below which we are about80 per cent complete The completeness with respect to PPMXLand GALEX photometry is 66 and 43 per cent respectively A moredetailed breakdown into survey passbands is displayed in Fig 6 forthe spectroscopic catalogues considered earlier

In Fig 7 we display the spatial distributions of the white dwarfsdiscussed in the previous sections We checked for spatial clusteringof the white dwarfs we could not re-identify Most of the uniden-tified white dwarfs that were not recovered in our merged APGcatalogue are in the Montreal sample and are located along a stripstretching from α = +0ndash5 h and δ = minus30 + 15 deg to α = 8ndash15 hand δ = +30 + 40 deg Some of them are from the Kiso survey(Limoges amp Bergeron 2010 and references therein) which over-laps with the fainter end of APASS We also note the traditionaldeficit of white dwarfs at low Galactic latitudes that is common toall the known samples of white dwarfs At low latitudes there isalso no GALEX photometry mainly due to the satellite avoiding



Figure 7 Spatial distribution in equatorial coordinates of the known white dwarfs from the literature discussed here Coloured symbols are used for the objectswe re-identify in our multiband photometric data set (squares DA white dwarfs circles DBDBA white dwarfs diamonds WD+MS stars 25-pc sample)White symbols correspond to white dwarfs with no APASS cross-match The light grey-shaded area delimits the sky coverage of GALEX Iso-Galactic latitudecurves are overplotted in blue

bright stars and crowded areas in the Galactic plane (note the whiteareas in Fig 7)

Finally we confirmed our results by checking all the remainingwhite dwarfs (where we define white dwarfs as objects with log g ge6) in the MWDD and SDSS (Kleinman et al 2013 Gentile Fusilloet al 2015 Kepler et al 2015 2016) that is asymp1000 objects brighterthan 17 mag The overall completeness decreases to 62 per cent(1864 out of 2990 white dwarfs) given that most of these additionalstars are in the range of g = 16ndash17

34 Candidate selection

Having characterized the photometry of known white dwarfs wecan use our merged APG catalogue to identify new white dwarfcandidates In Fig 8 we display three colour combinations anda reduced proper motion diagram for the known white dwarfs Wecorrected GALEX photometry for non-linearity effects using the re-sults of Camarota amp Holberg (2014) In the top-left panel of Fig 8DA white dwarfs are clearly distinct from main-sequence stars whenthey reach Teff lt 12 000 K (FUV minus NUV gt 13 NUV minus gprime gt 05)In the top-right panel of Fig 8 the (NUV minus gprime gprime minus rprime) combi-nation of colours is a proxy for the more commonly used SDSS(u minus g g minus r) diagram The GALEX NUV band samples a spectralrange that is further away from the Balmer jump resulting in a lessstretched distribution of colours We note a fairly good agreementbetween observed colours and the white dwarf cooling tracks withan amount of scatter that is mostly due to the larger uncertainties ofthe APASS rprime magnitudes It is worth noting the larger offset of someof the stars towards redder colours which is a clear signature of thepresence of late-type companions The colour excess due to late-type companions becomes markedly evident when using 2MASS

magnitudes ie in the (NUV minus gprime NUV minus H) panel of Fig 8 Fi-nally the reduced proper motion diagram in the bottom-left cornerof this figure confirms the bulk of the re-identified stars as nearbyfast moving objects

To select white dwarf candidates we applied an (NUV minus gprimegprime minus rprime) cut based on the colours of the re-identified stars (Fig 8)This colour cut reduces the contamination from main-sequence starsthat may share a similar colour space with white dwarfs of Teff 7000 K We then defined a selection criterion following the workof Gentile Fusillo et al (2015) on SDSS which establishes a prob-ability for an object to be a white dwarf This probability PWD isinferred from the colours and the reduced proper motion of knownwhite dwarfs in the (Hg gprime minus rprime) plane Given the larger uncertain-ties of PPMXL proper motions with respect to those from SDSSwe only define objects with δμμ le 03 and PWD gt 075 to begood white dwarf candidates Below this value we found that theAPASS-PPMXL probabilities disagree by more than plusmn 25 per centwith those measured by Gentile Fusillo et al (2015) for a sampleof asymp300 stars in common

We identified 8000 objects within our colour-cut of which 1200have PWD gt 075 Given the strict PWD requirement not all theknown white dwarf fall within this selection mostly because PP-MXL proper motion uncertainties weight down their probabilitiesIn this group of objects we identified 392 that had not yet beenspectroscopically confirmed as white dwarfs 10 of which are GDstars We observed 77 high priority objects confirming 71 whitedwarfs and 6 hot subdwarfs Due to scheduling constraints we alsoincluded in our target list objects that fall outside the colour-cut orwith PWD lt 075 achieving a lower rate of confirmed white dwarfs

After our spectroscopic follow-up we are left with 305 high-confidence white dwarf candidates with PWD gt 075 and 33 GD


4180 R Raddi et al

Figure 8 Colourndashcolour and reduced proper motion diagrams obtained by combining GALEX APASS and 2MASS photometry The number of objectsplotted in each panel depends on the availability of the data (refer to Table 3 and Fig 6) The dark coloured contours in both panels represent the colourdistribution of the field population as inferred from our all-sky cross-match Symbols used to distinguish distinct classes of white dwarfs are listed in thetop-left panel The dashed curves in the top-right panel delimit the colour space where most known white dwarfs are found White dwarf cooling sequences(log g = 75ndash95 in 05 dex steps) and main sequence are plotted for reference in the three colourndashcolour diagrams The expected proper motion curves forlog g = 8 Galactic disc white dwarfs are plotted in the bottom-left diagram Also refer to the caption of Fig 3 for further details

stars that do not have spectra or known spectral classification inthe literature In Fig 9 we display the (NUV minus gprime gprime minus rprime) colourndashcolour diagram of the candidate stars In the lower panel of Fig 9the histogram bars represent the PWD of all objects with gprime minus rprime le05 and NUV minus gprime le 4 ie about 15 million objects We overlaystep-histograms representing the PWD of the known white dwarfsdiscussed in the previous sections those for the 305 candidate whitedwarfs and those of white dwarfs and hot subdwarfs that we spec-troscopically followed up

The photometry and proper motions for the 305 white dwarfcandidates are made available through VizieR in the format as listedin Table A1 The photometry for the 135 stars that we have followedup will also be hosted on VizieR with the inclusion of an additionalcolumn lsquoshort namersquo which we use through the paper to refer tospecific stars

4 SPECTRO SCOPI C FOLLOW-UP

We obtained low-resolution spectra for 135 stars covering the wave-length range from the Balmer jump to slightly beyond the H α

(3700ndash7500 Aring) The observations were spread over 25 yr (2014JuneJuly 2015 MarchMay and 2016 NovemberDecember) andthe data were taken at three different telescopes the 36-m NewTechnology Telescope (NTT) at the ESO La Silla observatory the25-m Isaac Newton Telescope (INT) at the Roque de Los Mucha-chos Observatory and the 182-m Copernico Telescope at the EkarAsiago Observatory Our targets were drawn from the APG list ofcandidates the GD sample and some objects that have lower PWDWe note that some of the targets (8 white dwarfs and 10 subdwarfssee next sections) were previously known in the literature but withpoorly accurate coordinates or that other research groups obtainedclassification during our follow-up operations



Figure 9 Selection of white dwarf candidates Top panel colourndashcolourdiagram showing the colour-cut (dashed lines) applied to select white dwarfcandidates The side-bar codes the assigned probability of being white dwarfPWD based on the reduced proper motion information Bottom panel his-togram distribution on a logarithmic scale of 15 million objects with gprime minus rprimele 05 and NUV minus gprime le 4 which we bin in ranges of PWD We overlay thehistogram distribution of PWD for known white dwarfs candidate whitedwarfs and the white dwarfs and hot subdwarfs that we confirmed spectro-scopically

Table 4 Summary of instruments and number of spectra for the follow-updiscussed in this paper

Telescope Set-up λ Slit Date N(Aring) (arcsec)

NTT EFOSC2 11 95 070 20140613ndash15 4865ndash95 070 20140711ndash13 34

INT IDSR400V 35 1 20150303ndash06 4635 10 20150512ndash18 38

Copernico AFOSCVPH7 13 125 20161105 213 125 20161226ndash30 7

At the NTT we used the ESO Faint Object Spectrograph andCamera 2 (EFOSC2 Buzzoni et al 1984) with the 11 grism a slitof 07 arcsec and binning 2 times 1 and 2 times 2 obtaining a resolutionof 65ndash95 Aring The set-up adopted at the INT produced a 35 Aring res-olution mounting the Intermediate Dispersion Spectrograph (IDS)with the grating R400V a slit of 1 arcsec and the blue-sensitiveEEV10 CCD For the Copernico set-up we mounted the AsiagoFaint Object Spectrograph and Camera (AFOSC) with the volumephase holographic grating VPH7 (Zanutta et al 2014) and a slit of

Table 5 Summary of spectral classification

Type Number

White dwarfs (total) 82DA 60DA+MS 9DAH 1DAB 1DBDBA 4DC 7Hot subdwarfsBHB 25Other stars 28

125 arcsec which deliver a resolution of 13 Aring with a 2 times 2 pixelsbinning The log of the observations is summarized in Table 4

The observations were done under different sky illumination(greybright at the NTT darkbright at the INT darkgrey at theCopernico) with the seeing typically lt25 arcsec variable weatherconditions often with thin clouds and during non-photometricnights (especially during the second run at NTT the second runat INT when calima affecting the sky transparency occurred andthe November runs at Asiago that were affected by high humidity)We took two to four exposures per target with exposure times in therange of 300ndash1800 s On several occasions we obtained multiplespectra of the same object across different runs to improve the totalsignal-to-noise ratio

We took standard calibration frames (bias dome flats) duringeach run and arc lamps before and after the targets at the NTTand INT We obtained a few arc lamps at beginning of the Asiagorun (Ne and Hg+Cd separately later combined during the datareduction) which were sufficient for an approximate wavelengthcalibration of the low-resolution spectra We also observed two tothree spectrophotometric standards per night with a 5-arcsec slitand we placed the slit at the parallactic angle for all the science andstandard stars in order to minimize flux-loss and to obtain a relativeflux-calibration of the spectra shown to be in good agreement withAPASS photometry for most of the confirmed white dwarfs Afew exceptions are the visually resolved WD+MS pairs where werotated the slit to include both the white dwarf and the late-typecompanion

We used standard reduction routines to subtract the bias to applythe flat-field correction and wavelength calibration to optimallyextract the 1D spectrum and for the flux calibration Specifically weused the STARLINK suite of programs including PAMELA9 (Marsh 1989)and MOLLY10

41 Spectral classification

From a visual inspection of the multiple observations we selectedthe best spectra for each star that we used for a quantitative clas-sification These spectra have a typical signal-to-noise ratio largerthan 20 We confirmed 82 white dwarfs (8 of which were previouslyknown) and 25 hot subdwarfsblue horizontal branch (BHB) stars(10 of which were previously known) We classify the remaining28 objects as AFG dwarfs some of which have apparently largeproper motions and colours that overlap with those of cool whitedwarfs (see Fig 18)

The breakdown of the sample by subtype is summarized inTable 5 We identify 71 hydrogen atmosphere white dwarfs in

9 httpstarlink eaohawaiiedustarlink10 httpwwwwarwickacukgotrmarshsoftware


4182 R Raddi et al

Figure 10 Spectra of the single hydrogen-atmosphere (DA) white dwarfs sorted by temperature Wherever relevant subtypes are included between brackets

Fig 10 we show 60 single objects (DA) one magnetic white dwarfwith Zeeman splitting of the Balmer lines (DAH) and another thatpresents also helium lines (DAB) We classify nine white dwarfsas binaries with a late-type companion (DA+MS) either on thebasis of the near- and mid-infrared excess detected by 2MASS andWISE or from spectroscopic evidence (Fig 11) Three of these sys-tems were visually resolved at the NTT and thus we took spectraat non-parallactic angles to include both stars in the slit One starJ1022minus3226 displays also a weak H α emission line that could beproduced from a chromospherically active companion Finally wefound four DBA white dwarfs six objects that do not present any

clearly detectable spectral features (DC) at the quality of our data(with the exception of J1010minus2327 which has broad features inthe blue part of the spectrum that may be either due to the presenceof carbon in the atmosphere or to a strong magnetic field) theirspectra are shown in Fig 12

We will not discuss the spectral classification of the hot sub-dwarfs and BHB stars in detail here We roughly estimated theirTeff via a photometric fit to the NUVBVgprimerprime photometry usingDA models with log g = 6 We note that some of the starshave composite spectral energy distributions (SEDs) which in-dicate the presence of a late-type companion We display the



Figure 11 DA+MS binaries sorted by temperature

Figure 12 DBA and DC white dwarfs

spectra of these 25 stars in Fig 13 noting they span a rangeof temperatures between 11 000 and 24 000 K For the previ-ously known stars we advise the reader to refer to the existingliterature

42 Atmospheric parameters of the spectroscopicallyconfirmed white dwarfs

For the determination of temperature and surface gravity (Teff log g)of the white dwarfs we used the software FITSB2 which estimatesa best fit via χ2 minimization using a downhill simplex algorithm(Napiwotzki et al 2004) The Teff and log g uncertainties were esti-mated via a bootstrap method We used the Koester (2010) modelatmospheres which adopt a ML2α = 08 mixing-length prescrip-tion for convective atmospheres and the Stark broadening profilesof hydrogen lines computed by Tremblay amp Bergeron (2009)

For the 60 DA single white dwarfs we fitted six Balmer lines(H αndashH8) while we excluded the H α and H β from the fit for thenine DA+MS pairs when the spectral profiles were altered by thepresence of the companion star A few examples of the best-fittingresults are shown in Fig 14 illustrating the quality of both the dataand the fits We also display in Fig 15 an example of how thesignal-to-noise ratio in the proximity of Balmer lines and spectralresolution are the relevant factors in assessing the surface gravityof selected objects chosen at Teff asymp 10 000 K and Teff asymp 20 000 KTo estimate Teff of non-DA white dwarfs we used a photometrictechnique fitting the available ultravioletoptical data and keepinglog g = 8 fixed

From the CopernicoAFOSC observations we identifiedJ0130+5321 (GD 278) for which we had to arbitrarily fix log g = 6to obtain a satisfying fit of the spectrum and photometry Its spec-trum was acquired in good weather conditions and presents onlyhydrogen lines at the resolution of the AFOSC set-up

The atmospheric parameters of the 60 single DA whitedwarfs and nine DA+MS white dwarfs are listed in Table B1The eight stars that were previously known are J0347+4358J0502+5401 J0812+1737 J1455+5655 and J2233+2610 (Limo-ges et al 2013 2015) J0847minus7312 (Gianninas et al 2010)J1003minus0337 (GD 110 Kepler et al 1995) and J1128+5919 (Gir-ven et al 2011) Our results for Teff and log g agree within 2σndash3σ

with those reported in the literatureAmong the hydrogen-dominated atmospheres there are two pe-

culiar objects One is a magnetic white dwarf (DAH) J2109+6507for which we kept log g = 8 fixed and derived its Teff through a fitof the photometric data The second object is a DAB white dwarfJ2047minus1259 We obtained two spectra the first taken at the NTTon 2014 June and the second at the INT on 2015 May which do notshow sensible differences in the relative position and appearance ofspectral features We initially estimated a Teff = 17 000 plusmn 740 K andlog g = 90 plusmn 01 using only the Balmer lines We then attempted afit with a grid of DAB model spectra which sample different [HHe]compositions In Fig 16 we show the improved agreement with asynthetic spectrum of Teff = 18 630 plusmn 360 K log g = 837 plusmn 006and [HHe] = minus1 The quality of our radial velocity informationdoes not exclude the possibility that this system could be a closebinary pair

5 PH Y S I C A L PA R A M E T E R S

We estimated masses and ages for the DA white dwarfs the DABwhite dwarf and the DA+MS white dwarfs We interpolated thephysical parameters from the Montreal cooling tracks using thespectroscopic estimates of Teff and log g For Teff lt 30 000 K weused the carbonndashoxygen core cooling models with thick hydro-gen layers which are described by Fontaine et al (2001) For hotterstars we used the carbon-core cooling sequence of Wood (1995) Weapplied the Tremblay et al (2013) 3D corrections to the measured


4184 R Raddi et al

Figure 13 Hot subdwarfs and BHB stars sorted by temperature

Figure 14 Best fits of the Balmer lines for selected single DA white dwarfs (top row) and those in DA+MS pairs (bottom row) Synthetic spectra (black) areoverlaid on to observed spectra (grey)

atmospheric parameters of cool DA white dwarfs (Teff lt 15 000 K)which account for the inaccurate treatment of convection in 1Dmodel atmospheres The physical parameters and the 3D correc-tions are given in Table B1 The distribution of hydrogen-dominatedwhite dwarfs in the (Teff log g) plane is displayed in Fig 17 wheremost stars are found to have log g = 800 plusmn 025

Seven stars have atmospheric parameters that are compatiblewith the instability strip of DA white dwarfs (Gianninas et al 2015Tremblay et al 2015) One is the already known ZZ Ceti starJ0502+540 (Green et al 2015 Limoges et al 2015) for which wegive more details in Section 64 The other six stars are candidatepulsators (cZZ in Table B1)



Figure 15 Best fits of H β and H γ lines for selected objects with Teff asymp20 000 K (top row) and asymp10 000 K (bottom row) Objects are plotted in orderof decreasing log g Synthetic spectra (black) are overlaid on to observedspectral lines (grey)

Figure 16 Observed spectrum (grey) of the DAB white dwarfJ2047minus1259 and the best-fitting model spectrum (black) withTeff = 18 630 plusmn 360 log g = 837 plusmn 06 and [HHe] = minus1

Two low surface gravity stars J0130+5321 (GD 278) andJ1142minus1803 could potentially be extremely low mass whitedwarfs J0130+5321 would have a mass of asymp016 M (cf fig 3in Brown et al 2010) J1142minus1803 was previously classified asa subdwarf (sdB) by Kilkenny et al (1997) It has narrow spec-tral lines as expected for a cool white dwarf (Teff asymp 9000 K) Thisstar has an NUV minus gprime colour that locates it between the whitedwarf cooling tracks and the main sequence in the top-left panelof Fig 18 in agreement with the low surface gravity estimate(log g = 676 plusmn 028) corresponding to asymp021 M Given thatwe used low-resolution gratings we cannot rule out that at leastGD 278 is a metal-poor halo star (Brown Kilic amp Gianninas 2017)

Nevertheless we note that both stars have relatively large propermotions that are compatible with those of white dwarfs

6 D I SCUSSI ON

61 Interstellar reddening

The sample of white dwarfs discussed here is made up of rela-tively bright but intrinsically low-luminosity objects thus theseare mostly nearby and suffer very little reddening In Fig 18 wedisplay a selection of colourndashcolour and reduced proper motiondiagrams for the observed stars which offer a visual comparisonwith the same plots for the known white dwarfs from the literature(Fig 8) The confirmed white dwarfs and subdwarfs are all locatedwithin the dashed lines that we have chosen as colour cut for theidentification of white dwarf candidates in Section 34 The only ex-ception is the DC white dwarf J0201+1212 (GD 21) The contam-inants also shown in this figure are mostly outside the dashed linesand share the colour space of cool (Teff lt 7000 K) white dwarfsFinally we stress that the presence of cool companions to whitedwarfs and hot subdwarfs is clearly evident in rightmost panels ofFig 18 where these objects depart from their typical single-starcolours

To confirm stars we observed are affected only by a smallamount of interstellar reddening we estimated the colour excessin four different colours in Fig 19 The leftmost panel of this fig-ure displays the colour excess in the UV bands Although we at-tempted an empirical correction to reduce non-linearity effects ofGALEX photometry (Camarota amp Holberg 2014) a few hot stars(FUVminusNUV lt 02 mag) still have negative UV excess This couldbe due to a wrong fit of the Balmer lines leading us to overestimateTeff However in B minus V and gprime minus rprime most stars are consistent withE(B minus V) le 01 Finally the NUVminusH colour in Fig 19 clearly re-veals the colour excess of the DA+MS systems due to the strongH-band contribution of their companions We note that a few otherstars follow the same trend as the DA+MS stars although withsmaller colour excess However given that their 2MASS H and Ks

are only upper limits we exclude the presence of unseen compan-ions

62 Comparison with Gaia

Gaia data release 1 (DR1 Gaia Collaboration 2016b) photometrybecame public last year Due to known issues with the chromatic cor-rection in the astrometry of Gaia not all the identified sources havebeen released (1 per cent of the total number of detected sourcesvan Leeuwen et al 2017) This problem has affected especiallyblue objects such as white dwarfs including the TychoHipparcosobjects that could already have parallaxes (Tremblay et al 2017)

We searched the Gaia data base within 1 arcsec from the APASScoordinates of the 135 stars for which we have follow-up spec-troscopy We could find Gp photometry for only 61 of them includ-ing 42 white dwarfs or hot subdwarfs In Fig 20 we display thecomparison between Gaia and APASS magnitudes The underlyingdistribution represents the photometry for the GD sample whilewe overlay on top the stars we have followed up spectroscopicallyAbout 70 per cent of the GD sample was retrieved successfully

We find a that the good correlation in the range of gprime minus rprime colourscovered by our sample There are just a few outliers and the corre-lations appear to depart from linearity for gprime minus rprime lt minus03 howeverwe identified fewer objects in this magnitude range The most dis-crepant object is J1212minus3642 (not shown in Fig 20) whose APASS


4186 R Raddi et al

Figure 17 Atmospheric parameters of DA white dwarfs The Fontaine Brassard amp Bergeron (2001) cooling tracks are overplotted as dashndashdotted curves for02 03 04 05 06 08 and 10 M from top to bottom respectively The empirical red and blue Teff limits of the instability strip are shown (dashed linesGianninas et al 2015) We mark in black one confirmed ZZ Ceti star J0502+5401

Figure 18 Colourndashcolour and reduced proper motion diagrams of the 135 stars we have followed up Symbols are clarified in the legend For a description ofsynthetic tracks refer to Fig 8



Figure 19 Assessment of the colour excess for the white dwarf sample Each panel represents a set of colours which combine from left to right GALEXAPASS and GALEX-2MASS bands On the y-axis is the colour excess ie the observed minus the intrinsic colours The dashed line marks the E(B minus V) = 01colour excess in each panel DA white dwarfs are plotted as white squares red symbols are used for the DA+MS pairs which appear with strong NUV minus Hexcesses Generally this spectroscopic follow-up sample has minimal interstellar reddening

Figure 20 Comparison between Gaia Gp APASS gprime and rprime magnitudesplotted against gprime minus rprime colours

gprime and rprime photometry is asymp1 mag fainter with respect to Gaia Gp GaiaDR1 contains only one source at 06 arcsec from the coordinates ofJ1212minus3642 It does not present detectable infrared excess or spec-tral peculiarities With this information in hand we suggest thatAPASS photometry could be systematically off Other two spec-troscopic objects that differ by 02ndash03 mag from the one-to-onecorrelation J1003minus3145 and J1438minus3127 are DA+MS binariesfor which the presence of a companion can be the source of photo-metric variability

63 White dwarf distances

We estimated spectroscopic distances by considering no extinctionbut we assumed an average scatter of E(B minus V) = 003 that cor-responds to 01 mag of extinction in the gprime band We estimated theintrinsic gprime-band magnitudes by convolving the filter profile to ap-propriately scaled (Koester 2010) synthetic spectra We propagatedthe Teff and log g uncertainties in order to assess the scatter in syn-thetic magnitudes We found that about 12 DA white dwarfs havespectroscopic parallaxes placing them within 40 pc from the SunOnly J0812+1737 was previously known to be at less than 40 pc

Figure 21 Confirmation of pulsations for J0502+540 The 45-h lightcurve taken at the Warwick 1-m Telescope is shown in the top panel thediscrete Fourier transform is displayed in the bottom panel The two mainpeaks are labelled and the 3σ threshold is displayed as a red line In theright corner is the window function that illustrates the frequency resolutionof the discrete Fourier transform

from the Sun (Limoges et al 2013) Of the other 11 stars 6 arein the Northern hemisphere and 5 in the south The nearest whitedwarf we identify is J1407minus0626 for which we estimate a distanceof 25 pc This is a cool white dwarf (Teff = 8400 K) which haslikely escaped previous searches due to its modest proper motion(asymp01 arcsec yrminus1) Next year the ESA Gaia mission will deliveraccurate parallaxes for this object confirming or refuting its mem-bership to the local sample within 25 pc from the Sun

The remaining DA white dwarfs are all within 100 pc from theSun The most distant white dwarf in the spectroscopic follow-upsample is in the hot DA+MS system J1022minus3226 at 288 pc

64 ZZ Ceti stars

Fig 17 identifies seven white dwarfs that fall within the instabilitystrip of hydrogen atmospheres One of the stars J0502+540 waspreviously classified as ZZ Ceti by Limoges et al (2015) and itspulsations confirmed by Green et al (2015) We re-observed thisstar on 2016 January 6 at the Warwick 1-m Telescope in La Palmafor asymp45 h with a 20-s cadence In Fig 21 we display the Fourier


4188 R Raddi et al

Table 6 Physical parameters of the low-mass companions

Name Teff (K) Age (Gyr) Mass (M)

J0219minus6306 Ba 2900 plusmn 170 205 009J0220minus0736 B 2300 plusmn 620 1170 008J0510+2315 B 2000 plusmn 380 040 005J1003minus3145 B 3000 plusmn 200 1155 008J1022minus3226 B 2800 plusmn 1050 124 008J1420minus2457 Ba 3000 plusmn 360 337 010J1438minus3127 Bb 3300 plusmn 140 gt230J2221minus0408 Ba 3200 plusmn 870 183 015J2308+0658 B 4000 plusmn 480 180 07

Notes aNot observed at parallactic angle Resolved binarybSuggested post-common envelope binary

transform of the light curve where we identify two main periods inagreement with Green et al (2015)

Among our sample J2233+2610 was also observed by Greenet al (2015) and found not to be variable to above 007 per centThis star is about 1000 K hotter than the empirical instability stripthus it is not expected to vary The remaining candidates will beexcellent targets for TESS which will acquire detailed light curvesenabling an unprecedented level of characterization for these andother ZZ Ceti stars Furthermore we stress that the brightest stars inour sample (g lt 16) are ideal targets for ground-based observationseg with Evryscope (Law et al 2015) or NGTS (West et al 2016)which currently provide detailed time-domain photometry with alarge coverage of the Southern hemisphere

65 Late-type companions

We identified nine white dwarfs in our sample that display reddercolours than isolated objects (Fig 19) which suggest the presenceof late-type companion stars We confirmed six of them based onspectroscopic evidence (eg composite spectra or the weak H α

emission seen in the spectrum of J1022minus3226 Fig 11) For threeof these stars labelled in Table 6 we could visually resolve the com-panions when we acquired their spectra Three further systems areproposed to have companions that are too cool with respect to thewhite dwarf to reveal their presence in optical spectra (J0520+2315J0220minus0736 and J2308+0658) but displaying infrared excess al-ready in the 2MASS J band

We derived the Teff of companion stars via fits to the compositeSEDs including GALEX APASS 2MASS and WISE W1W2 pho-tometry We kept the atmospheric parameters of the white dwarfsfixed to those determined from the spectroscopic analysis (Table B1)and we built a grid of spectra for low-mass stars and substellar ob-jects taken from the latest version of the BT-Settl models (Allardet al 2012) We fixed the modelsrsquo metallicity to solar and log g = 5We derived the intrinsic magnitudes of white dwarfs and main-sequence stars by convolving the filter profiles with the appropriatemodels from the Koester (2010) library of synthetic spectra andthe BT-Settl grid respectively Finally we estimated the best fitsthrough a χ2 minimization of the available photometry with threefree parameters ie an absolute scaling factor the relative flux ofwhite dwarf and companion and the atmospheric temperature ofthe secondary The results of our fits are displayed in Fig 22

We also estimated the companion masses under the assump-tion they are dwarf stars Thus we first considered a semi-empiricalinitial-to-final mass relation (Catalan et al 2008) from which we in-ferred the white dwarf progenitor mass We then estimated the totalage of the system by approximating the stellar lifetime with a simple

Figure 22 Photometric fits of the DA+MS binaries sorted by descendingwhite dwarf temperature (from top to bottom) We display the white dwarfspectra (in blue) the low-mass companions spectra (in red) and the com-bined spectra (in cyan) We display all the photometric bands available asblack dots In the top panel we also plot a cut-out of the H α region forJ 1022minus3226 for which we detect a weak emission

analytical expression (eg Age = 10 times Mminus25prog Gyr Wood 1992) Fi-

nally we determined the companion mass via interpolation of Teff

and age on the Baraffe et al (1998 2003) tracks for low-mass andsubstellar mass objects In Table 6 we list the relevant physicalproperties of the companions (Teff age and mass)

We found that most of the proposed companions have masses inthe range of 01 M ie corresponding to late-M spectral typesSpectroscopic follow-up aiming at a better characterization of these



companions is necessary especially for the least massive of themJ0510+2315 B that we only identify through its infrared excess Forthis object we infer a mass of 005 M compatible with a browndwarf Although the first brown dwarf candidate to be discoveredGD 165 B is a companion to a known white dwarf (Becklin ampZuckerman 1988) substellar white dwarf companions are rare withan occurrence rate of at most a few per cent estimated from highcontrast imaging techniques (Farihi et al 2005 Hogan Burleigh ampClarke 2009) transit surveys (Faedi et al 2011) and searches forinfrared excess (Girven et al 2011 Steele et al 2011)

Another system worth mentioning is J1438minus3127 for which weestimated a white dwarf mass of 036 plusmn 004 M A white dwarfof such a low mass could not have been formed in isolation withina Hubble time implying it could be the product of binary interac-tion Rebassa-Mansergas et al (2011) showed that low-mass whitedwarfs are more frequent in short-period (post-common envelope)binaries than in the field or in wide binary systems bringing ob-servational evidence in support to binary interactions during thecommon envelope phase Such low-mass white dwarfs are sug-gested to have gone through the common envelope phase while onthe first giant branch implying that their progenitors had an initialmass of 18 M (Zorotovic amp Schreiber 2013) and consequentlya lifetime of 23 Gyr Given that the evolution of such systemsis complicated by the common envelope phase we do not derive acompanion mass as done for the other binaries

7 C O N C L U S I O N

We have produced a catalogue of bright white dwarf targets forthe upcoming NASA TESS mission as well as long-baseline pho-tometric surveys such as Evryscope and NGTS The target list wascompiled by cross-matching APASS PPMXL and GALEX The cat-alogue includes proper motions and multiband photometry for 1864known white dwarfs brighter than 17 mag Having validated ourcross-match against the Lowell Observatory proper motion survey(Giclas et al 1980) and catalogues of spectroscopically confirmedwhite dwarfs we identified 428 high confidence white dwarf candi-dates (including 46 GD stars) We presented follow-up spectroscopyfor 82 white dwarfs and 25 hot subdwarfsBHB stars (8 white dwarfsand 10 subdwarfs in the sample were previously known) Amongthe confirmed white dwarfs we have identified a few interestingobjects such as a DAB white dwarf six bright ZZ Ceti star candi-dates nine WD+MS binaries and one white dwarf at 25 pc fromthe Sun This leaves 305 high-confidence white dwarf candidatesand 33 GD stars requiring follow-up spectroscopy to ascertain theirnature

The uninterrupted light curves that TESS will obtain for the pro-posed stars will provide unprecedented insight on their variability(ZZ Ceti stars) binary eclipses as well as important statistical infor-mation on transits from exoplanets or debris around white dwarfsThe 30-min full-frame exposures of the TESS mission will increasethe chances of finding serendipitous transits by also monitoringstars that are not included in the short-cadence list of targets

We publish the collection of photometric data for all the previ-ously know white dwarfs that we re-identified in our analysis whichwill become particularly useful with Gaia DR2 in spring 2018 Sta-tistical analysis of spectroscopy photometry and parallaxes caneventually be used to obtain a very accurate estimate of coolingages (eg OrsquoMalley von Hippel amp van Dyk 2013 Si et al 2017)for this sample of known white dwarfs

AC K N OW L E D G E M E N T S

This research was made possible through the use of the AAVSOPhotometric All-Sky Survey (APASS) funded by the Robert MartinAyers Sciences Fund

Some of the data presented in this paper were obtained from theMikulski Archive for Space Telescopes (MAST) STScI is operatedby the Association of Universities for Research in Astronomy Incunder NASA contract NAS5-26555 Support for MAST for non-HST data is provided by the NASA Office of Space Science viagrant NNX09AF08G and by other grants and contracts

This publication uses VOSA developed under the Spanish VirtualObservatory project supported from the Spanish MICINN throughgrant AyA2011-2

The Digitized Sky Surveys were produced at the Space TelescopeScience Institute under US Government grant NAG W-2166 Theimages of these surveys are based on photographic data obtainedusing the Oschin Schmidt Telescope on Palomar Mountain and theUK Schmidt Telescope The plates were processed into the presentcompressed digital form with the permission of these institutions

This research has used the SIMBAD data base and the VizieRcatalogue access tool operated at CDS Strasbourg France

Based on observations made with ESO Telescopes at the La SillaObservatory under programme ID 093D-0431 We thank JavierAlarcon for operating the NTT The INT is operated on the islandof La Palma by the Isaac Newton Group in the Spanish Observato-rio del Roque de los Muchachos of the Instituto de Astrofısica deCanarias The IDS spectroscopy was obtained as part of I15AN007and I2015A04 Based on observations collected at Copernico tele-scope (Asiago Italy) of the INAF ndash Osservatorio Astronomico diPadova

The research leading to these results has received funding fromthe European Research Council under the European Unionrsquos Sev-enth Framework Programme (FP2007-2013)ERC Grant Agree-ment no 320964 (WDTracer) Support for this work was providedby NASA through Hubble Fellowship grant HST-HF2-51357001-A awarded by the Space Telescope Science Institute which is oper-ated by the Association of Universities for Research in AstronomyIncorporated under NASA contract NAS5-26555

R E F E R E N C E S

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H Oslashstensen R Castanheira B G Winget D E 2015 ApJ 809 14Bell K J et al 2016 ApJ 829 82Bergeron P et al 2011 ApJ 737 28Bianchi L Efremova B Herald J Girardi L Zabot A Marigo P Martin

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Farihi J Becklin E E Zuckerman B 2005 ApJS 161 394Fontaine G Brassard P 2008 PASP 120 1043Fontaine G Brassard P Bergeron P 2001 PASP 113 409Gaia Collaboration 2016a AampA 595 A1Gaia Collaboration 2016b AampA 595 A2Gentile Fusillo N P Gansicke B T Greiss S 2015 MNRAS 448 2260Gentile Fusillo N P et al 2017 MNRAS 469 621Giammichele N Bergeron P Dufour P 2012 ApJS 199 29Gianninas A Bergeron P Dupuis J Ruiz M T 2010 ApJ 720 581Gianninas A Kilic M Brown W R Canton P Kenyon S J 2015 ApJ

812 167Giclas H L Burnham Jr R Thomas N G 1980 Lowell Obs Bull 8 157Girven J Gansicke B T Steeghs D Koester D 2011 MNRAS 417

1210Green E M et al 2015 in Dufour P Bergeron P Fontaine G eds ASP

Conf Ser Vol 493 High-speed Photometric Observations of ZZ CetiWhite Dwarf Candidates Astron Soc Pac San Francisco p 237

Greiss S et al 2016 MNRAS 457 2855Henden A Munari U 2014 Contrib Astron Obs Skalnate Pleso 43 518Henden A A Templeton M Terrell D Smith T C Levine S Welch D

2016 VizieR Online Data Catalog 2336Hermes J J et al 2011 ApJ 741 L16Hermes J J et al 2015 ApJ 810 L5Hoslashg E et al 2000 AampA 357 367Hogan E Burleigh M R Clarke F J 2009 MNRAS 396 2074Holberg J B Oswalt T D Sion E M McCook G P 2016 MNRAS

462 2295Howell S B et al 2014 PASP 126 398Kawka A Vennes S 2006 ApJ 643 402Kepler S O Giovannini O Kanaan A Wood M A Claver C F 1995

Balt Astron 4 157Kepler S O et al 2015 MNRAS 446 4078Kepler S O et al 2016 MNRAS 455 3413Kilic M et al 2006 AJ 131 582Kilkenny D OrsquoDonoghue D Koen C Stobie R S Chen A 1997

MNRAS 287 867Kleinman S J et al 2013 ApJS 204 5Koester D 2010 Mem Soc Astron Ital 81 921Koester D Voss B Napiwotzki R Christlieb N Homeier D Lisker T

Reimers D Heber U 2009 AampA 505 441Law N M et al 2015 PASP 127 234Lepine S Shara M M 2005 AJ 129 1483Limoges M-M Bergeron P 2010 ApJ 714 1037Limoges M-M Lepine S Bergeron P 2013 AJ 145 136Limoges M-M Bergeron P Lepine S 2015 ApJS 219 19Lindegren L et al 2016 AampA 595 A4McCook G P Sion E M 1999 ApJS 121 1Marsh T R 1989 PASP 101 1032Monet D G et al 2003 AJ 125 984Morrissey P et al 2007 ApJS 173 682Munari U et al 2014 AJ 148 81Napiwotzki R et al 2003 The Messenger 112 25Napiwotzki R et al 2004 in Hilditch R W Hensberge H Pavlovski K

eds ASP Conf Ser Vol 318 Spectroscopically and Spatially Resolv-ing the Components of the Close Binary Stars Astron Soc Pac SanFrancisco p 402

OrsquoMalley E M von Hippel T van Dyk D A 2013 ApJ 775 1Oslashstensen R H et al 2011 MNRAS 414 2860Oswalt T D Smith J A Wood M A Hintzen P 1996 Nature 382 692

Pauli E-M Napiwotzki R Altmann M Heber U Odenkirchen M KerberF 2003 AampA 400 877

Pickles A J 1998 PASP 110 863Raddi R et al 2016 MNRAS 457 1988Rebassa-Mansergas A Nebot Gomez-Moran A Schreiber M R Girven

J Gansicke B T 2011 MNRAS 413 1121Rebassa-Mansergas A Nebot Gomez-Moran A Schreiber M R Gansicke

B T Schwope A Gallardo J Koester D 2012 MNRAS 419 806Rebassa-Mansergas A Ren J J Parsons S G Gansicke B T Schreiber

M R Garcıa-Berro E Liu X-W Koester D 2016 MNRAS 4583808

Reid I N Sahu K C Hawley S L 2001 ApJ 559 942Ricker G R et al 2015 J Astron Telesc Instrum Syst 1 014003Shanks T et al 2015 MNRAS 451 4238Si S van Dyk D A von Hippel T Robinson E Webster A Stenning D

2017 MNRAS 468 4374Sion E M Holberg J B Oswalt T D McCook G P Wasatonic R

Myszka J 2014 AJ 147 129Skrutskie M F et al 2006 AJ 131 1163Steele P R Burleigh M R Dobbie P D Jameson R F Barstow M A

Satterthwaite R P 2011 MNRAS 416 2768Subasavage J P Jao W-C Henry T J Bergeron P Dufour P Ianna P A

Costa E Mendez R A 2009 AJ 137 4547Taylor M B 2006 in Gabriel C Arviset C Ponz D Enrique S eds ASP

Conf Ser Vol 351 Astronomical Data Analysis Software and SystemsXV Astron Soc Pac San Francisco p 666

Tremblay P-E Bergeron P 2009 ApJ 696 1755Tremblay P-E Ludwig H-G Steffen M Freytag B 2013 AampA 559

A104Tremblay P-E Kalirai J S Soderblom D R Cignoni M Cummings J

2014 ApJ 791 92Tremblay P-E Gianninas A Kilic M Ludwig H-G Steffen M Freytag

B Hermes J J 2015 ApJ 809 148Tremblay P-E et al 2017 MNRAS 465 2849van Leeuwen F 2007 AampA 474 653van Leeuwen F et al 2017 AampA 599 A32Vanderburg A et al 2015 Nature 526 546Voss B Koester D Napiwotzki R Christlieb N Reimers D 2007 AampA

470 1079Wenger M et al 2000 AampAS 143 9West R G et al 2016 The Messenger 165 10Winget D E Kepler S O 2008 ARAampA 46 157Winget D E Hansen C J Liebert J van Horn H M Fontaine G Nather

R E Kepler S O Lamb D Q 1987 ApJ 315 L77Wood M A 1992 ApJ 386 539Wood M A 1995 in Koester D Werner K eds Lecture Notes in Physics

Vol 443 White Dwarfs Springer-Verlag Berlin p 41Wright E L et al 2010 AJ 140 1868Zanutta A Bianco A Landoni M Tomasella L Benetti S Giro E 2014

in Ramsay S K McLean I S Takami H eds Proc SPIE Conf SerVol 9147 Ground-based and Airborne Instrumentation for AstronomyV SPIE Bellingham p 91474E

Zorotovic M Schreiber M R 2013 AampA 549 A95

S U P P O RT I N G IN F O R M AT I O N

Supplementary data are available at CDS via anonymous ftp tocdsarcu-strasbgfr (130791285) or via httpcdsarcu-strasbgfrviz-binqcatJMNRAS

Table A1 Header of the online tables containing photometry andproper motions of white dwarfs we have presented in this work

Please note Oxford University Press is not responsible for thecontent or functionality of any supporting materials supplied bythe authors Any queries (other than missing material) should bedirected to the corresponding author for the article



APPENDIX A O NLINE TABLES

Table A1 Header of the online tables containing photometry and propermotions of white dwarfs we have presented in this work

APASS Name based on APASS positionNames Simbad namesra_apass deg APASS Right Ascensiondec_apass deg APASS DeclinationB mag APASS B magnitudeerr_B mag APASS B magnitude errorV mag APASS V magnitudeerr_V mag APASS V magnitude errorg mag APASS gprime magnitudeerr_g mag APASS gprime magnitude errorr mag APASS rprime magnitudeerr_r mag APASS rprime magnitude errori mag APASS primei magnitudeerr_i mag APASS iprime magnitude errorfuv mag GALEX FUV magnitudeerr_fuv mag GALEX FUV magnitude errornuv mag GALEX NUV magnitudeerr_nuv mag GALEX NUV magnitude errorpmra mas yrminus1 μαcos δ

err_pmra mas yrminus1 μαcos δ errorpmdec mas yrminus1 μδ

err_pmdec mas yrminus1 μδ errorJ mag 2MASS J magnitudeerr_J mag 2MASS J magnitude errorH mag 2MASS H magnitudeerr_H mag 2MASS H magnitude errorK mag 2MASS Ks magnitudeerr_K mag 2MASS Ks magnitude errorW1 mag WISE W1 magnitudeerr_W1 mag WISE W1 magnitude errorW2 mag WISE W2 magnitudeerr_W2 mag WISE W2 magnitude error

A P P E N D I X B PH Y S I C A L PA R A M E T E R S O F SP E C T RO S C O P I C A L LY C O N F I R M E D W H I T E DWA R F S

Table B1 Properties of the follow-up white dwarf sample Columns are as follows short name of the object based on APASS coordinates SIMBAD nameif known telescope and observing date of spectrum used to assess the atmospheric parameters effective temperature surface gravity and the corresponding3D corrections interpolated from the tables in Tremblay et al (2013) masses and cooling ages interpolated from Fontaine et al (2001) cooling sequencesspectroscopic distances

Short Name SIMBAD Telescope Date SpT Teff T3D log g log g3D Mass τ cool d(K) (K) (cgs) (cgs) (M) (Gyr) (pc)

J0008+3645 GD 4 Copernico 20161227 cZZ 12520 plusmn 620 minus160 804 plusmn 014 minus003 062 plusmn 005 037 plusmn 007 75 plusmn 3J0018minus0956 PHL 778 NTT 20140615 DBA 16320 plusmn 100 800J0033+1021 2MASS J00335106+1021404 NTT 20140712 DC 11640 plusmn 300 800J0106+3930 GD 10 Copernico 20161107 DA 9830 plusmn 100 minus130 839 plusmn 005 minus027 068 plusmn 006 082 plusmn 013 38 plusmn 1J0126minus6556 NTT 20140614 DA 13400 plusmn 340 50 818 plusmn 006 001 074 plusmn 006 038 plusmn 006 56 plusmn 2J0128minus6915 NTT 20140614 DA 8720 plusmn 90 minus30 818 plusmn 010 minus025 056 plusmn 006 082 plusmn 012 49 plusmn 2J0130+2638 NTT 20140712 DA 13470 plusmn 400 50 825 plusmn 007 001 077 plusmn 006 041 plusmn 007 52 plusmn 2J0130+5321 GD 278 Copernico 20161230 DA 8840 plusmn 30 600 plusmn 000 016 plusmn 000 301 plusmn 009 138 plusmn 5J0201+1212 GD 21 Copernico 20161228 DC 7800 plusmn 200 800J0204+0726 GD 23 Copernico 20161229 DA 18630 plusmn 1000 836 plusmn 013 084 plusmn 006 019 plusmn 005 98 plusmn 4J0219minus6306 NTT 20140615 DAMS 9610 plusmn 600 minus150 799 plusmn 032 minus026 047 plusmn 005 053 plusmn 010 70 plusmn 4J0220minus0736 2MASS J02202493minus0736271 NTT 20140615 DAMS 12680 plusmn 260 minus60 786 plusmn 006 minus001 054 plusmn 005 027 plusmn 004 83 plusmn 3J0242minus8206 NTT 20140615 DA 9200 plusmn 240 minus60 867 plusmn 019 minus028 085 plusmn 006 161 plusmn 029 47 plusmn 2J0347+4358 Wolf 226 INT 20150304 DA 13230 plusmn 190 90 794 plusmn 004 058 plusmn 005 026 plusmn 004 33 plusmn 1J0421+4607 INT 20150304 DA 7650 plusmn 50 800 plusmn 010 minus014 053 plusmn 005 106 plusmn 013 27 plusmn 1J0502+5401 LP 119minus10 INT 20150305 ZZ 11720 plusmn 220 minus270 833 plusmn 004 minus011 074 plusmn 006 061 plusmn 010 41 plusmn 2J0510+2315 INT 20150303 DAMS 22710 plusmn 820 832 plusmn 011 083 plusmn 006 009 plusmn 002 65 plusmn 3J0515+5021 GD 289 INT 20150305 DA 9520 plusmn 90 minus120 826 plusmn 008 minus026 060 plusmn 006 074 plusmn 012 37 plusmn 2


4192 R Raddi et al

Table B1 ndash continued


J0529minus0941 INT 20150305 DA 14550 plusmn 370 826 plusmn 007 077 plusmn 006 033 plusmn 006 71 plusmn 3J0603+4518 INT 20150304 DA 16710 plusmn 290 785 plusmn 005 054 plusmn 005 011 plusmn 002 69 plusmn 3J0634+3848 GD 260 INT 20150304 cZZ 12610 plusmn 240 minus300 850 plusmn 005 minus005 089 plusmn 006 071 plusmn 012 49 plusmn 2J0753+6122 GD 453 INT 20150304 DC 9330 plusmn 230 800J0812+1737 GALEX J0812378+173701 INT 20150306 DA 16730 plusmn 140 795 plusmn 003 060 plusmn 005 013 plusmn 002 30 plusmn 1J0823+7158 GD 458 INT 20150304 DA 15300 plusmn 420 789 plusmn 007 056 plusmn 005 016 plusmn 003 88 plusmn 3J0838minus2146 INT 20150305 DA 21990 plusmn 440 795 plusmn 007 061 plusmn 005 004 plusmn 001 85 plusmn 3J0847minus7312 WD 0848minus730 NTT 20140613 DA 17620 plusmn 610 798 plusmn 010 061 plusmn 005 011 plusmn 002 73 plusmn 3J0857minus2245 NTT 20140613 DA 10730 plusmn 290 minus240 795 plusmn 010 minus018 049 plusmn 005 042 plusmn 005 50 plusmn 2J0859minus3124 NTT 20140615 DA 31020 plusmn 650 834 plusmn 015 085 plusmn 006 003 plusmn 001 133 plusmn 6J0900minus0909 INT 20150305 DA 21110 plusmn 320 780 plusmn 005 053 plusmn 004 004 plusmn 001 73 plusmn 3J0905minus2342 NTT 20140615 DA 18270 plusmn 250 775 plusmn 005 051 plusmn 005 007 plusmn 001 98 plusmn 3J0919+7723 INT 20150306 DA 9160 plusmn 70 minus100 791 plusmn 008 minus027 043 plusmn 005 054 plusmn 006 41 plusmn 1J1003minus3145 NTT 20140614 DAMS 13560 plusmn 750 130 795 plusmn 008 002 060 plusmn 005 026 plusmn 006 116 plusmn 5J1003minus0337 GD 110 NTT 20140614 DA 8420 plusmn 50 minus20 806 plusmn 007 minus023 051 plusmn 005 079 plusmn 008 37 plusmn 1J1010minus2427 INT 20150306 DC 20370 plusmn 300 800J1021minus3448 NTT 20140713 DA 13240 plusmn 720 80 804 plusmn 012 064 plusmn 006 031 plusmn 007 81 plusmn 3J1022minus3226 NTT 20140713 DAMS 60180 plusmn 5270 777 plusmn 034 063 plusmn 004 000 plusmn 000 288 plusmn 14J1040minus3123 NTT 20140615 DA 8270 plusmn 100 minus10 815 plusmn 012 minus02 057 plusmn 005 095 plusmn 013 48 plusmn 2J1052minus1137 NTT 20140615 DA 18520 plusmn 350 783 plusmn 006 055 plusmn 005 008 plusmn 001 145 plusmn 5J1058+5132 INT 20150303 cZZ 12320 plusmn 640 minus230 807 plusmn 012 minus004 063 plusmn 005 040 plusmn 008 88 plusmn 4J1128+5919 SDSS J11280530+5919579 INT 20150303 DA 14890 plusmn 670 758 plusmn 015 041 plusmn 004 012 plusmn 002 140 plusmn 5J1131minus1225 NTT 20140613 cZZ 12160 plusmn 420 minus280 820 plusmn 009 minus006 069 plusmn 006 048 plusmn 009 51 plusmn 2J1142minus1803 EC 11396-1746 INT 20150513 DA 8860 plusmn 140 676 plusmn 028 021 plusmn 002 116 plusmn 044 115 plusmn 6J1212minus3642 NTT 20140712 DA 20150 plusmn 360 775 plusmn 005 051 plusmn 004 005 plusmn 001 90 plusmn 3J1223minus1852 EC 12206-1835 INT 20150305 DC 10660 plusmn 300 800J1229minus0432 LP 675-43 NTT 20140712 DC 9950 plusmn 400 800J1254minus3856 NTT 20140615 DA 13030 plusmn 330 40 801 plusmn 008 minus001 061 plusmn 005 030 plusmn 005 87 plusmn 3J1331+6809 GD 487 Copernico 20161229 DA 10750 plusmn 250 minus130 900 plusmn 014 minus02 108 plusmn 005 171 plusmn 021 36 plusmn 2J1342minus1413 NTT 20140615 DA 13870 plusmn 410 minus70 864 plusmn 006 001 101 plusmn 006 075 plusmn 015 51 plusmn 2J1401minus0553 NTT 20140712 DA 15930 plusmn 860 792 plusmn 011 058 plusmn 005 015 plusmn 004 107 plusmn 4J1402minus0736 SDSS J14023397minus0736506 NTT 20140615 DA 8590 plusmn 100 minus10 851 plusmn 010 minus023 078 plusmn 006 156 plusmn 029 51 plusmn 2J1407minus0626 INT 20150303 DA 8390 plusmn 70 minus10 824 plusmn 010 minus022 062 plusmn 006 104 plusmn 017 25 plusmn 1J1418+0929 2MASS J14183529+0929195 INT 20150513 DA 17150 plusmn 290 784 plusmn 006 055 plusmn 005 010 plusmn 002 104 plusmn 4J1420minus2457 2MASS J14200652minus2457026 NTT 20140614 DAMS 13530 plusmn 1020 90 817 plusmn 014 001 072 plusmn 006 036 plusmn 009 104 plusmn 4J1438minus3127 NTT 20140613 DAMS 14520 plusmn 510 744 plusmn 009 036 plusmn 003 010 plusmn 002 75 plusmn 3J1455+5655 SBSS 1453+571 INT 20150304 DA 15230 plusmn 520 797 plusmn 006 060 plusmn 005 019 plusmn 003 53 plusmn 2J1517+1030 SDSS J15175462+1030443 NTT 20140613 DA 19920 plusmn 600 786 plusmn 010 056 plusmn 005 006 plusmn 001 122 plusmn 5J1528minus2515 NTT 20140613 DA 14960 plusmn 370 840 plusmn 008 086 plusmn 006 038 plusmn 007 49 plusmn 2J1549minus3954 NTT 20140712 DB 12760 plusmn 200 800J1614+1046 NTT 20140613 DA 16800 plusmn 970 803 plusmn 012 064 plusmn 006 015 plusmn 004 95 plusmn 4J1620minus1901 NTT 20140615 DA 13920 plusmn 160 10 835 plusmn 004 001 084 plusmn 006 044 plusmn 007 47 plusmn 2J1638minus2035 INT 20150513 DA 8480 plusmn 110 minus10 828 plusmn 014 minus023 063 plusmn 006 105 plusmn 019 40 plusmn 1J1700minus8723 NTT 20140614 cZZ 10900 plusmn 350 minus230 820 plusmn 008 minus018 062 plusmn 005 054 plusmn 009 33 plusmn 1J1729+0208 INT 20150513 DA 15460 plusmn 760 785 plusmn 009 055 plusmn 005 015 plusmn 003 113 plusmn 4J1730+1346 GD 208 INT 20150513 DA 10500 plusmn 180 minus200 827 plusmn 005 minus025 062 plusmn 006 059 plusmn 009 39 plusmn 1J1944minus3603 NTT 20140615 DA 12970 plusmn 150 10 803 plusmn 003 minus001 063 plusmn 005 032 plusmn 004 30 plusmn 1J1956+6413 INT 20150513 DA 17780 plusmn 480 781 plusmn 009 053 plusmn 005 009 plusmn 002 81 plusmn 4J2017minus1712 INT 20150514 DA 42800 plusmn 840 797 plusmn 009 067 plusmn 005 000 plusmn 000 103 plusmn 4J2023minus1115 INT 20150515 DA 16620 plusmn 310 804 plusmn 004 064 plusmn 006 015 plusmn 003 51 plusmn 2J2025+7900 INT 20150516 DA 24030 plusmn 460 806 plusmn 006 067 plusmn 006 003 plusmn 001 113 plusmn 4J2047minus1259 EC 20444minus1310 INT 20150515 DAB 18630 plusmn 360 837 plusmn 006 085 plusmn 006 020 plusmn 004 80 plusmn 3J2101minus0527 SDSS J21011016minus0527512 INT 20150516 DBA 50530 plusmn 800 800J2109+6507 INT 20150517 DAH 18220 plusmn 500 800J2112minus2922 EC 21096minus2934 NTT 20140614 DC 10550 plusmn 110 800J2151minus0502 HE 2149minus0516 NTT 20140614 DBA 12240 plusmn 30 800J2202+3848 GD 399 Copernico 20161230 DA 9270 plusmn 60 minus100 804 plusmn 009 minus027 049 plusmn 005 060 plusmn 007 57 plusmn 2J2221minus0408 LP 700minus34 NTT 20140614 DAMS 9940 plusmn 830 minus100 874 plusmn 036 minus029 089 plusmn 006 146 plusmn 045 44 plusmn 3J2228minus3105 HE 2149minus0516 NTT 20140615 cZZ 12200 plusmn 220 minus260 802 plusmn 006 minus004 060 plusmn 005 038 plusmn 005 60 plusmn 2J2233+2610 PM J22331+2610 INT 20150518 DA 13430 plusmn 760 140 802 plusmn 009 002 064 plusmn 006 030 plusmn 007 67 plusmn 3J2256minus1319 PB 7361 NTT 20140614 DA 20080 plusmn 400 797 plusmn 005 062 plusmn 005 007 plusmn 001 98 plusmn 4J2308+0658 SDSS J23081305+0658368 NTT 20140712 DAMS 34640 plusmn 250 780 plusmn 005 058 plusmn 004 001 plusmn 000 147 plusmn 6J2331minus6656 2MASS J23314533minus6656078 NTT 20140614 DA 7430 plusmn 60 10 786 plusmn 010 minus013 046 plusmn 005 098 plusmn 010 43 plusmn 1J2341+2924 NTT 20140712 DA 13120 plusmn 230 10 810 plusmn 005 minus001 067 plusmn 006 034 plusmn 005 64 plusmn 3

This paper has been typeset from a TEXLATEX file prepared by the author


MNRAS 472 4173ndash4192 (2017) doi101093mnrasstx2243Advance Access publication 2017 September 4

Multiband photometry and spectroscopy of an all-skysample of bright white dwarfs

R Raddi1lsaquo N P Gentile Fusillo1 A F Pala1 J J Hermes2dagger B T Gansicke1

P Chote1 M A Hollands1 A Henden3 S Catalan4 S Geier5 D Koester6

U Munari7 R Napiwotzki8 and P-E Tremblay1

1Department of Physics University of Warwick Gibbet Hill Road Coventry CV4 7AL UK2Department of Physics and Astronomy University of North Carolina Chapel Hill NC 27599 USA3AAVSO 49 Bay State Rd Cambridge MA 02138 USA4Centre for Astronomy National University of Ireland Galway Galway Ireland5Institute for Astronomy and Astrophysics Eberhard Karls University Sand 1 D-72076 Tubingen Germany6Institut fur Theoretische Physik und Astrophysik Universitat Kiel D-24098 Kiel Germany7INAFndashAstronomical Observatory of Padova I-36012 Asiago (VI) Italy8Centre for Astrophysics Research STRI University of Hertfordshire College Lane Hatfield AL10 9AB UK

Accepted 2017 August 28 Received 2017 August 28 in original form 2017 February 27

ABSTRACTThe upcoming NASA Transiting Exoplanet Survey Satellite (TESS) will obtain space-baseduninterrupted light curves for a large sample of bright white dwarfs distributed across theentire sky providing a very rich resource for asteroseismological studies and the search fortransits from planetary debris We have compiled an all-sky catalogue of ultraviolet opticaland infrared photometry as well as proper motions which we propose as an essential toolfor the preliminary identification and characterization of potential targets We present datafor 1864 known white dwarfs and 305 high-probability white dwarf candidates brighter than17 mag We describe the spectroscopic follow-up of 135 stars of which 82 are white dwarfs and25 are hot subdwarfs The new confirmed stars include six pulsating white dwarf candidates(ZZ Cetis) and nine white dwarf binaries with a cool main-sequence companion We identifyone star with a spectroscopic distance of only 25 pc from the Sun Around the time TESS islaunched we foresee that all white dwarfs in this sample will have trigonometric parallaxesmeasured by the ESA Gaia mission next year

Key words catalogues ndash binaries spectroscopic ndash binaries visual ndash subdwarfs ndash whitedwarfs

1 IN T RO D U C T I O N

White dwarfs are the final evolutionary stage for the vast majorityof single stars Characterized by long steady cooling rates whitedwarfs can be used to infer properties of the Milky Way like the starformation history (Tremblay et al 2014) the age of the disc (Wingetet al 1987 Oswalt et al 1996) or their contribution to baryonic darkmatter (Reid Sahu amp Hawley 2001 Pauli et al 2003)

Asteroseismology is a powerful tool for probing the internal struc-ture of white dwarfs through the study of multiperiodic variabil-ity that occurs during short-lived instability phases on the whitedwarf cooling sequence (Fontaine amp Brassard 2008 Winget ampKepler 2008 Althaus et al 2010) Over the last few years the

E-mail rraddiwarwickacukdaggerHubble Fellow

NASA Kepler mission (Borucki et al 2010) and its re-purposedtwo-wheeled K2 mission (Howell et al 2014) have enabled high-precision asteroseismology of white dwarfs (Hermes et al 2011Oslashstensen et al 2011 Greiss et al 2016) Its successor the NASATransiting Exoplanet Survey Satellite (TESS Ricker et al 2015)will acquire all-sky times-series photometry for at least 2 yr search-ing for exoplanet transits and enabling asteroseismology for manyclasses of stars (Campante et al 2016) Roughly 200 000 selectedtargets including bright white dwarfs may be observed with 2-mincadence Full-frame images will be taken every 30 min The missionwill survey areas of sky with 1-month to 1-yr coverage dependingon their ecliptic latitudes

TESS is poised to return improved statistics building uponthe original Kepler mission and its ongoing K2 reincarnationIn particular the large sky coverage of TESS will be excellent for thediscovery of rare phenomena of which Kepler has so far provided uswith few or even unique examples eg the first direct detection of

Ccopy 2017 The AuthorsPublished by Oxford University Press on behalf of the Royal Astronomical Society

Downloaded from httpsacademicoupcommnrasarticle-abstract472441734103558by University of Warwick useron 20 November 2017

4174 R Raddi et al





2 TH E DATA








2MASS J 1235 162H 1662 251Ks 2619 162

WISE W1 3353 663W2 4602 1042
























31 The GD sample





4176 R Raddi et al























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33 Completeness















4180 R Raddi et al

















Type Number











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6 D I SCUSSI ON










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64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al






4174 R Raddi et al





2 TH E DATA








2MASS J 1235 162H 1662 251Ks 2619 162

WISE W1 3353 663W2 4602 1042
























31 The GD sample





4176 R Raddi et al























4178 R Raddi et al












33 Completeness















4180 R Raddi et al

















Type Number











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4184 R Raddi et al











6 D I SCUSSI ON










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64 ZZ Ceti stars



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R E F E R E N C E S









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4192 R Raddi et al






















31 The GD sample





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33 Completeness















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Type Number











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6 D I SCUSSI ON










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64 ZZ Ceti stars



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R E F E R E N C E S









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64 ZZ Ceti stars



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R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al



















4178 R Raddi et al












33 Completeness















4180 R Raddi et al

















Type Number











4182 R Raddi et al




















4184 R Raddi et al











6 D I SCUSSI ON










4186 R Raddi et al













64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al

















4180 R Raddi et al

















Type Number











4182 R Raddi et al




















4184 R Raddi et al











6 D I SCUSSI ON










4186 R Raddi et al













64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al















Type Number











4182 R Raddi et al




















4184 R Raddi et al











6 D I SCUSSI ON










4186 R Raddi et al













64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al






4182 R Raddi et al




















4184 R Raddi et al











6 D I SCUSSI ON










4186 R Raddi et al













64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al




















4184 R Raddi et al











6 D I SCUSSI ON










4186 R Raddi et al













64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al











6 D I SCUSSI ON










4186 R Raddi et al













64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al






4186 R Raddi et al













64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al















64 ZZ Ceti stars



4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al






4188 R Raddi et al

































R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al





















R E F E R E N C E S









4190 R Raddi et al




















































4192 R Raddi et al






4190 R Raddi et al




















































4192 R Raddi et al

















4192 R Raddi et al






multiband photometry and spectroscopy of an all-sky sample

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