Brown dwarfs and dark matters

Brown dwarfs and dark matters

Neill Reid, Univ. of Pennsylvania

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 limitcurrent and future constraints

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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

Finding brown dwarfs(1)

Initial discoveries - companions of known nearby stars: wide companion searches - van Biesbroeck VB 8, VB 10 (1943) coronagraphic searches - Gl 229B - serendipitous identifications in the field Kelu 1Large scale catalogues - cool targets, T < 2000 K - require wide-field near-infrared surveys

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

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 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)

The L/T transition (2)

Early-type T dwarfs first identified from SDSS data - Leggett et al (2000)

Unsaturated methane absorption


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 (4)

Mid- and late-typeL dwarfs can be selectedusing 2MASS JHK alone

SDSS riz + 2MASS Jpermits identification ofall dwarfs sp > M4

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

Bolometric corrections

Given near-IR data --> infer M(bol) --> bol correction

little variation in BC_J from M6 to T

Searching for 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 - L3 2M0920 - L6.5 2M0850 - L6


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 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

2M0850A/B (4)

Could 2M0850ABbe an L/T binary?

Probably not -- but cf. SDSS early T dwarfs

What we’d really like...

a brown dwarf eclipsing system

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


Brown dwarfsdon’t alwayshave brown dwarfcompanions


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?

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 --> 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


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, 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%

Substellar Mass function (6)

Predictions vs. observations

10 Gyr-old disk constant star formation 0 < n < 2

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


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

Summary

1. Brown dwarfs are now almost commonplace2. Near-IR spectra show that the L dwarf sequence L0…L8, defined at far-red wavelengths, is consistent with near-infrared variations --> probably well correlated with temperature3. L dwarfs - 2000 > T > 1350 K T dwarfs - T < 1300K - brown dwarfs 4. First results from HST L dwarf binary survey - L dwarf/L dwarf binaries rare - Maximum separation correlated with total mass --> nature or nurture?5. Current detection rates are inconsistent with a steep IMF

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”?

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 Hyades cluster

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

Binary surveys: the Hyades (3)

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

Spectroscopic survey (Reid & Mahoney)


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

Another brown dwarf desert?


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

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)

Brown dwarfs and dark matters

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