Brown Dwarfs and Dark Matters

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


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

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


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

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 (3)

H-band Few identifiedatomic features

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

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


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 (1000K) --> dM(bol) ~ 1 mag similar diameters --> dT ~ 25% ---> T(L8) ~ 1250K

2M0850A/B

Could 2M0850ABbe 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 <a> than M dwarfs

How do these parameters mesh with overall binary statistics?


Brown dwarfsdon’t alwayshave brown dwarfcompanions


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


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


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

Extrapolate using n= 0, 1, 2 powerlaw

Miller-Scalo functions


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


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)


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?


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


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

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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

Brown dwarf atmospheres

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


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?

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

Brown Dwarfs and Dark Matters

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