Brown Dwarfs and Dark Matters Neill Reid, STScI in association with 2MASS Core project: Davy Kirkpatrick, Jim Liebert, Conard Dahn, Dave Monet, Adam Burgasser L dwarfs, binaries and the mass function

Outline Finding ultracool dwarfs The L dwarf sequence extending calibration to near-infrared wavelengths L-dwarf binaries Separations and mass ratios The mass function below the hydrogen-burning limit brown dwarfs and dark matter Some results and a conundrum Heavy halo white dwarfs?

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

Late-type dwarfs are fully convective  everything visits the core If core temperature > 2 x 10^6 K  lithium is destroyed If M < 0.06 M(sun), lithium survives Lithium test

Cool dwarf evolution (2) Rapid luminosity evolution for substellar-mass dwarfs

Cool dwarf evolution (3) Brown dwarfs evolve through spectral types M, L and T L dwarfs encompass stars and brown dwarfs Cooling rate decreases with increasing mass

Finding ultracool dwarfs Gl 406 = M6 dwarf (Wolf 359) Flux distribution peaks at ~ 1 micron ---> search at near-IR wavelengths

Finding ultracool dwarfs (2): Near-infrared sky surveys Neugebauer & Leyton - Mt. Wilson TMSS custom built 60-inch plastic mirror arc-minute resolution, K < 3rd magnitude DENIS … southern sky ESO 1.3 metre, IJK to J~15, K~ present 2MASS all-sky Mt. Hopkins/CTIO 1.5 metres, JHK J~16, K~14.5 (10-sigma)

Finding ultracool dwarfs (3) Search for sources with red (J-K) and either red optical/IR colours or A-type colours

Cool dwarf spectra (1) Early-type M dwarfs characterised by increasing TiO absorption CaOH present for sp > M4

Cool dwarf spectra (2) Late M dwarfs: increasing TiO VO at sp > M7 FeH at sp > M8

Cool dwarf spectra (3) Spectral class L: decreasing TiO, VO - dust depletion increasing FeH, CrH, water lower opacities - increasingly strong alkali absorption Na, K, Cs, Rb, Li

Cool dwarf spectra (4) Low opacity leads to high pressure broadening of Na D lines

The L/T transition Methane absorption T ~ 1200/1300K (Tsuji, 1964)  Blue JHK colours Early-type T dwarfs first identified from SDSS data - Leggett et al (2000) Unsaturated methane absorption

NIR Spectral Classification (1) Kirkpatrick scheme defined at far-red wavelengths Most of the flux is emitted at Near-IR wavelengths Is the NIR behaviour consistent? K, Fe, Na atomic lines water, CO, methane bands

NIR Spectral classification (2) J-band: microns Numerous atomic lines Na, K, Fe FeH bands UKIRT CGS4 spectra: Leggett et al (2001) Reid et al (2001)

NIR Spectral Classification (3) H-band Few identified atomic features

NIR Spectral Classification(4) K-band Na I at 2.2 microns CO overtone bands molecular H_2 (Tokunaga &Kobayashi) --> H2O proves well correlated with optical spectral type --> with temperature

The HR diagram Broad Na D lines lead to increasing (V-I) at spectral types later than L3.5/L4 Latest dwarf - 2M L5 Astrometry/photometry courtesy of USNO (Dahn et al)

The near-infrared HR diagram Mid- and late-type L dwarfs can be selected using 2MASS JHK alone SDSS riz + 2MASS J permits identification of all dwarfs sp > M4 Note small offset L8  Gl 229B

Searching for brown dwarf binaries The alternative model for brown dwarfs

Binary surveys: L dwarfs (2) Why do we care about L dwarf binaries? 1. Measure dynamical masses  constrain models 2. Star formation and, perhaps, planet formation HST imaging survey of 160 ultracool dwarfs (>M8) over cycles 8 & 9 (Reid + 2MASS/SDSS consortium) Successful WFPC2 observations of 60 targets to date --> only 11 binaries detected

Binary surveys: L dwarfs (3) 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 surveys: L dwarfs (6) 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 (1000K) --> dM(bol) ~ 1 mag similar diameters --> dT ~ 25% ---> T(L8) ~ 1250K

2M0850A/B Could 2M0850AB be an L/T binary? Probably not -- but cf. SDSS early T dwarfs

L dwarf binary statistics (1) Approximately 20% of L dwarfs are resolved almost all are equal luminosity, therefore equal mass 2M0850AB – mass ratio ~ 0.8 none have separations > 10 AU  L dwarf/L dwarf binaries seem to be rarer, and/or have smaller than M dwarfs How do these parameters mesh with overall binary statistics?

L dwarf binary statistics (2) Brown dwarfs don’t always have brown dwarf companions

L dwarf binary statistics (3) Known L dwarf binaries - high q, small - low q, large -> lower binding energy - preferential disruption? Wide binaries as minimal moving groups?

The substellar mass function (1) Brown dwarfs evolve along nearly identical tracks in the HR diagram, at mass-dependent rates No single-valued M/L relation Model N(mag, sp. Type)  infer underlying  (M) Require temperature scale bolometric corrections star formation history

The substellar mass function (2) Major uncertainties: 1. Temperature scale - M/L transition --> 2200 to 2000 K L/T transition --> 1350 to 1200 K 2. Stellar birthrate --> assume constant on average 3. Bolometric corrections: even with CGS4 data, few cool dwarfs have observations longward of 3 microns 4. Stellar/brown dwarf models

Bolometric corrections Given near-IR data --> infer M(bol) --> bol correction little variation in BC_J from M6 to T

The substellar mass function (3) Stellar mass function: dN/dM ~ M^-1 (Salpeter n=2.35) Extrapolate using n= 0, 1, 2 powerlaw Miller-Scalo functions

The substellar mass function (4) 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 sky T dwarfs: 10 in 5000 sq deg, J < 16 (2MASS) 2 in 400 sq deg, z < 19 (SDSS) --> 80 to 200 all sky Predictions: 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= L dwarfs, 800 T dwarfs all sky to J=16

Substellar Mass function (6) Predictions vs. observations 10 Gyr-old disk constant star formation 0 < n < 2 All L: 1400  2100 K >L2 : 1400  1900K T : < 1300K

Substellar mass function (7) Change the age of the Galactic disk Younger age ---> larger fraction formed in last 2 gyrs --> Flatter power-law (smaller n)

Substellar Mass Function (8) Miller-Scalo mass function --> log-normal Match observations for disk age 8 to 10 Gyrs

The substellar mass function (9) Caveats: 1. Completeness … 2MASS - early L dwarfs - T dwarfs (JHK) SDSS - T dwarfs (iz) 2. Temperature limits … M/L transition 3. Age distribution we only detect young brown dwarfs In general  observations appear consistent with n ~ 1  equal numbers of BDs (>0.01 M(sun)) and MS stars No significant contribution to dark matter……..but….

A kinematic conundrum (1) Stellar kinematics are correlated with age  scattering through encounters with molecular clouds leads to 1. Higher velocity dispersions 2. Lower net rotational velocity, V e.g. Velocity distributions of dM (inactive, older) and dMe (active, younger)

A kinematic conundrum (2) Stellar kinematics are usually modelled as Gaussian distributions  (  U),  (V),  (W) ) But disk kinematics are more complex:  use probability plots Composite in V 2 Gaussian components in (U, W) local number ratio high:low ~ 1:10 thick disk and old disk?

A kinematic conundrum (3) Kinematics of ultracool dwarfs (M7  L0) Hires data for 35 dwarfs ~50% trig/50% photo parallaxes Proper motions for all  (U, V, W) velocities We expect the sample to be dominated by long-lived low-mass stars – although there is at least one BD

A kinematic conundrum (4) Ultracool M dwarfs have kinematic properties matching M0-M5 dMe dwarfs   ~ 2-3 Gyrs Does this make sense? M7  L0 ~2600  2100K Where are the old V LM stars?

A different kind of dark matter Galaxy rotation curves at large radii are not Keplerian - heavy halos (Ostriker, Peebles & Yahil, 1974) - Milky Way M ~ 5 x 10^11 solar masses, R < 50 kpc visible material (disk + stellar halo) ~ 5 x 10^10 solar masses => 90% dark matter – particles? compact objects? Microlensing surveys – MACHO, EROS, DUO,OGLE Given timescale, estimated velocity => mass MACHO: events,  days, ~ km/s => can account for ~20% of the missing 90% = 0.5+/- 0.3 solar masses Halo white dwarfs?

Heavy halo white dwarfs? I We are in the dark halo – local density ~ 10^-2 M_sun/pc^3 => search for local representatives in proper motion surveys Oppenheimer et al. (Science Express, March 23) Photographic survey of ~12% of the sky near the SGP - 38 cool, high-velocity white dwarfs – 4 x 10^-4 stars/pc^3 - local mass density of ~3 x 10^-4 M_sun/pc^3 => could account for 3% of dark matter if they’re in the heavy halo But are they?

Heavy halo white dwarfs? II The Galactic disk has a complex kinematic structure - thin/old disk: 300 pc scaleheight, 90% of local stars - thick/extended disk: 700 pc scaleheight, 10% Should we expect any high-velocity disk stars  consider a volume-complete sample of 514 M dwarfs (Reid, Hawley & Gizis, 1995)

Heavy halo white dwarfs? III Thick disk stars can have high velocities - Reid, Hawley & Gizis (1995): PMSU M dwarf survey 4% of the sample would be classed as dark halo by Oppenheimer et al => ~2 x 10^-4 white dwarfs / pc^3 Most of the Oppenheimer et al. white dwarfs are remnants of the first stars which formed in the thick disk White dwarfs from the stellar halo account for the rest There is no requirement for a dark matter contribution

What next? (1) Better statistics for nearby stars   A 2MASS NStars survey (with Kelle Cruz (Upenn), Jim Liebert (UA), John Gizis (Delaware) Davy Kirkpatrick & Pat Lowrance (IPAC), Adam Burgasser (UCLA)) Aim: find all dwarfs later than M4 within 20 parsecs 1.2MASS/NLTT cross-referencing: (m(r) – K)   2.Deep van Biesbroeck survey for wide cpm companions 3.2MASS-direct: (J-K)   4.2MASS/POSS II: (I-J)  

What next? (2) If n~1,  equal numbers of stars and brown dwarfs  Numerous cool (room temp.) BDs brightest at 5  m  accessible to SIRTF ~10 400K BDs /100 sq deg F>10  Jy at 5  m

Summary 1. Brown dwarfs are now almost commonplace 2. Near-IR spectra show that the L dwarf sequence L0…L8 is consistent with near-infrared variations  probably well correlated with temperature 3. First results from HST L dwarf binary survey - L dwarf/L dwarf binaries relatively rare - Maximum separation is correlated with total mass nature or nurture? 4.Current detection rates are inconsistent with a steep IMF  brown dwarfs are poor dark matter candidates 4.Neither are cool white dwarfs

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

Binary surveys: T dwarfs H-alpha activity declines sharply beyond spectral type M7

Binary surveys: T dwarfs..but 2M , a T dwarf, has strong H-alpha emission - no variation observed July, 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

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

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 mass Are binary systems more common amongst L dwarfs? or are these initial results a selection effects?

Clouds on an L8? Gl 584C - r ~ 17 pc - 2 G dwarf companions - a ~ 2000 AU - age ~ 100 Myrs - Mass ~ M(sun) - M(J) ~ 15.0 Gl 229B M(J) ~ 15.4