dark matters neill reid, univ. of pennsylvania in association with 2mass core project: davy...

Dark Matters

Neill Reid, Univ. of Pennsylvania

in association with 2MASS Core project:

Davy Kirkpatrick, Jim Liebert, Conard Dahn, Dave Monet, Adam Burgasser

Red dwarfs, low-mass stars & brown dwarfs

Shameless plug….

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Who cares about brown dwarfs?

Cosmologistsbaryonic dark matter

Astrophysicistsstar formationGalactic dynamics

Planetary scientistsplanet formationstructure of planetary atmospheres

Cool dwarf evolution (1)

Low-mass stars: H fusion establishes equilibrium configuration

Brown dwarfs: no long-term energy supply T ~ 2 million K required for lithium fusion


Rapid luminosity evolution for substellar-mass dwarfs


Brown dwarfs evolve through spectral types M, L and T

L dwarfs encompass stars and brown dwarfs

Cooling rate decreases with increasing mass

Cool dwarf spectra (1)

Early-type M dwarfs characterised by increasing TiO absorption

CaOH present for sp > M4


Late M dwarfs: increasing TiO VO at sp > M7 FeH at sp > M8


Spectral class L: decreasing TiO, VO - dust depletion increasing FeH, CrH, water lower opacities - increasingly strong alkali absorption Na, K, Cs, Rb, Li


Low opacity leads to high pressure broadening of Na D lines

cf. Metal-poor subdwarfs

Optical HR diagram

Broad Na D lines lead to increasing (V-I) at spectral types later than L3.5/L4 Latest dwarf - 2M1507-1627 L5

Astrometry/photometry courtesy of USNO (Dahn et al)

The near-infrared HR diagram

K I absorption leads to increasing (I-J) at sp > L7

Cool dwarf spectra (5) : near-IR

K, Fe, Na atomic lines

water, CO molecular bands

The L/T transition

Onset of methane absorption at T~1200/1300 K leads to reduced flux at H, K

Radical change in colours (cf. Tsuji, 1964)

[Burgasser - this meeting]

The near-IR HR diagram (2)

Methane absorption eliminates JHK-only search for T dwarfs

Brown dwarf atmospheres

Non-grey atmospheres - flux peaks at 1, 5 and 10 microns - bands and zones? - “weather”?

Brown dwarf “weather”

Observations suggest that brown dwarfs have rapid rotation - v ~ 40 to 80 km/sec - P ~ 4 hrs to 90 mins

If brown dwarfs had spots (giant storms?), what would we see?

Clouds on an L8?

Gl 584C - r ~ 17 pc - 2 G dwarf companions - a ~ 2000 AU - age ~ 100 Myrs - Mass ~ 0.045 M(sun) - M(J) ~ 15.0 Gl 229B M(J) ~ 15.4

The substellar mass function (1)

Brown dwarfs cool/fade with time: essentially identical tracks in HR diagram, but mass-dependent rates --> the mass-luminosity relation is not single-valued

=> we can only model the observed N(mag, sp type) distribution and infer the underlying mass distribution

Require: 1. Temperature scale/sp type 2. Bolometric corrections 3. Star formation history


Major uncertainties:

1. Temperature scale - M/L transition --> 2200 to 2000 K L/T transition --> 1300 to 1200 K

2. Stellar birthrate --> assume constant on average

3. Bolometric corrections Scarce observations of cool dwarfs (>M6) beyond L band


Stellar mass function: dN/dM ~ M^-1(Salpeter n=2.35)

Extrapolate using n= 0, 1, 2


Observational constraints: from photometric field surveys for ultracool dwarfs - 2MASS, SDSS

L dwarfs: 17 L dwarfs L0 to L8 within 370 sq deg, J<16 (2MASS) --> 1900 all skyT dwarfs: 10 in 5000 sq deg, J < 16 (2MASS) 2 in 400 sq deg, z < 19 (SDSS) --> 80 to 200 all skyPredictions: assume L/T transition at 1250 K, M/L at 2000 K n=1 700 L dwarfs, 100 T dwarfs all sky to J=16 n=2 4600 L dwarfs, 800 T dwarfs all sky to J=16


Lithium in M dwarfs- identifies brown dwarfs with masses below 0.06 M(sun)

Two detections in 19 dwarfs M8 to M9.5

Predictions: n=1 16% n=2 33%


Caveats:

1. Completeness … 2MASS - early L dwarfs - T dwarfs (JHK) SDSS - T dwarfs (iz)2. Temperature limits … M/L transition3. Age distribution we only detect young brown dwarfs


Substellar mass function: n~1 --> equal numbers of stars and brown dwarfs--> 10% mass density--> no significant dark matter

1-4 400K BDs /100 sq deg F>10 microJanskys at 5 microns

Low mass binaries

Why binaries? - dynamical mass estimates - coeval: calibration of relative propertiesFinding binary systems - direct imaging: wide systems ( > 5 AU) - HST + ground-based AO imaging - radial velocities: close systems - Keck spectroscopy, optical + IRTargets - low mass stars in open clusters - nearby low-luminosity dwarfs

The Hyades cluster

Age ~ 625 MyrsDistance ~ 45.3 parsecsDiameter ~ 12 parsecs > 400 known membersUniform space motion V ~ 46.7 km/sec

Binary surveys: the Hyades (1)

Targets: 55 late-type M dwarfs Mv > 12, Mass < 0.3 M(sun)

HST imaging (with John Gizis, IPAC) - resolution 0.09 arcseconds, ~ 4 AU - capable of detecting 0.06 M(sun) brown dwarfs expect 2 to 3 detections - nine new stellar binaries detected - no brown dwarf companions


Radial velocity observations - 55 late-type M dwarfs - Keck optical spectroscopy (HIRES), 0.3 km/sec accuracy

Binary detection using - change in velocity between epochs - double-lined spectra - velocity offset w.r.t. Hyades space motion

(Reid & Mahoney, MNRAS, in press)


Rhy 403 - Period ~ 1.25 days - amplitude 40 km/secPrimary mass ~ 0.15 M(sun) single-lined system The secondary has a mass between 0.06 and 0.095 solar masses. 70% probability M < 0.075-> 1st candidate brown dwarf


Summary: 25% of low-mass Hyads have a stellar companion 1 candidate brown dwarf

Another brown dwarf desert?

What about brown dwarf binaries?

The alternative model for browm dwarfs

Binary surveys: L dwarfs (1)

Several L dwarfs are wide companions of MS stars: e.g. Gl 584C, G196-3B, GJ1001B (& Gl229B in the past).

What about L-dwarf/L-dwarf systems? - initial results suggest a higher frequency >30% for a > 3 AU (Koerner et al, 1999) - all known systems have equal luminosity --> implies equal massAre binary systems more common amongst L dwarfs? or are these initial results a selection effects?


HST imaging survey of 160 ultracool dwarfs (>M8) over cycles 8 & 9 (Reid + 2MASS/SDSS consortium)

Successful WFPC2 observations of 20 targets to date

--> only 4 binaries detected

2M0746 L0.5 (brightest known L dwarf)2M1146 L32M0850 L62M0920 L6.5


2M0746 (L0.5) 2M1146 (L3)

Binary systems: L dwarfs (4)

2M0920 (L6.5): I-band V-band

Binary systems: L dwarfs (5)

2M0850: I-band V-band


Binary components lie close to L dwarf sequence: 2M0850B M(I) ~0.7 mag fainter than type L8 M(J) ~0.3 mag brighter than Gl 229B

2M0850A has strong lithium absorption --> implies a mass below 0.06 M(sun)2M0920A - no detectable lithium --> M > 0.06 M(sun)

2M0850AB (1)

2M0850AB(2)

Mass limits:

2M0850A: M < 0.06 M(sun) q(B/A) ~ 0.75

2M0920A: M > 0.06 M(sun) q(B/A) ~ 0.95

2M0850AB (3)

Constraining brown dwarf models - primaries have similar spectral type (Temp) -> similar masses ~0.06

2M0850B ~ 0.045 M(sun) age ~ 1.7 Gyrs

L dwarf binary statistics (1)

Four detections from 20 targets --> comparable with detection rate in Hyades

but … <r> ~ 20 parsecs for L dwarfs ~ 46 parsecs for Hyades M dwarfs

Only 1 of the 4 L dwarf binaries would be resolved at the distance of the Hyades

=> L dwarf binaries rarer/smaller <a> than M dwarfs

L dwarf binary statistics (2)

Known L dwarf binaries - high q, small <a> a < 10 AU except Pl - low q, large <a>

-> lower binding energy - preferential disruption?

Wide binaries as minimal moving groups?

Binary surveys: T dwarfs

A digression:chromospheric activity is due to acoustic heating,powered by magnetic field. H-alpha emission tracesactivity in late-type dwarfs.


H-alpha activitydeclines sharply beyond spectral type M7


..but 2M1237+68, a T dwarf,has strong H-alpha emission - no variation observed July, 1999 - February, 2000

Possible mechanisms: - Jovian aurorae? - flares? - binarity?

2M1237 : a vampire T dwarf

Brown dwarfs are degenerate - increasing R, decreasing M - ensures continuous Roche lobe overflow

Summary

1. Photometric/spectroscopic characteristics of ultracool dwarfs are now well characterised2. Gl584C provides the first detection of brown dwarf “weather”3. Rhy 403B is a candidate Hyades brown dwarf, but substellar-mass companions remain rare4. First results from HST L dwarf survey - 4 of 20 are binary - 1 candidate L/T transition dwarf - L dwarf/L dwarf binaries rare disruption of primordial systems?

And our goal..

an eclipsing system

Finding brown dwarfs

Initial discoveries - companions of known nearby stars - serendipitous identifications in the fieldLarge scale catalogues - cool targets, T < 2000 K - require wide-field, deep, near-infrared surveys - DENIS (1996 - present) - 2MASS (1997 - present) - SDSS (1999/2000 - future)

dark matters neill reid, univ. of pennsylvania in association with 2mass core project: davy...

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