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 binariesSeparations and mass ratios
• The mass function below the hydrogen-burning limitbrown 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 coreIf core temperature > 2 x 10^6 K lithium is destroyedIf M < 0.06 M(sun), lithium survives
Lithium test
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
1969 - Neugebauer & Leyton - Mt. Wilson TMSS custom built 60-inch plastic mirror arc-minute resolution, K < 3rd magnitude
1996 - 2000 DENIS … southern sky ESO 1.3 metre, IJK to J~15, K~13.5
1997 - present 2MASS all-sky Mt. Hopkins/CTIO 1.5 metres, JHK J~16, K~14.5 (10-sigma)
Finding ultracool dwarfs (3)
Search for sourceswith red (J-K)and either redoptical/IR coloursor A-type colours
Cool dwarf spectra (1)
Early-type M dwarfs characterised by increasing TiO absorption
CaOH present for sp > M4
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
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: 1 - 1.35 microns Numerous atomic lines Na, K, Fe FeH bands
UKIRT CGS4 spectra: Leggett et al (2001) Reid et al (2001)
NIR Spectral Classification(4)
K-band Na I at 2.2 microns CO overtone bands molecular H_2(Tokunaga &Kobayashi)
--> H2O proves wellcorrelated with opticalspectral 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 - 2M1507-1627 L5
Astrometry/photometry courtesy of USNO (Dahn et al)
The near-infrared HR diagram
Mid- and late-typeL dwarfs can be selectedusing 2MASS JHK alone
SDSS riz + 2MASS Jpermits identification ofall dwarfs sp > M4
Note small offset L8 Gl 229B
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 (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
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 <a> than M dwarfs
How do these parameters mesh with overall binary statistics?
L dwarf binary statistics (3)
Known L dwarf binaries - high q, small <a> - low q, large <a>
-> 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, SDSSL 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
Substellar Mass function (6)
Predictions vs. observations
10 Gyr-old disk constant star formation 0 < n < 2
All L: 14002100 K>L2 : 14001900K 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 transition3. 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 GyrsDoes 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: 13-17 events, days, <V> ~ 200-300 km/s
=> can account for ~20% of the missing 90%
<M> = 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 parsecs1. 2MASS/NLTT cross-referencing: (m(r) – K) 2. Deep van Biesbroeck survey for wide cpm companions3. 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 commonplace2. Near-IR spectra show that the L dwarf sequence L0…L8 is consistent with near-infrared variations probably well correlated with temperature3. 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 candidates4. Neither are cool white dwarfs
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.
Binary surveys: T dwarfs
..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
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 massAre binary systems more common amongst L dwarfs? or are these initial results a selection effects?