Presentation on theme: "Brown dwarfs and dark matters Neill Reid, Univ. of Pennsylvania in association with 2MASS Core project: Davy Kirkpatrick, Jim Liebert, Conard Dahn, Dave."— Presentation transcript:
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 binaries Separations and mass ratios The mass function below the hydrogen-burning limit current 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
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 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
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 (4) Mid- and late-type L dwarfs can be selected using 2MASS JHK alone SDSS riz + 2MASS J permits identification of all 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 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
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 mass Are binary systems more common amongst L dwarfs? or are these initial results a selection effects?
Binary surveys: L dwarfs (2) 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
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 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 2M0850AB be 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 … ~ 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 than M dwarfs
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 a < 10 AU except Pl - low q, large -> 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
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
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=2 4600 L dwarfs, 800 T dwarfs all sky to J=16
The substellar mass function (5) 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
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
The substellar mass function (10) 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 commonplace 2. 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 temperature 3. 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 traces activity in late-type dwarfs.
Binary surveys: T dwarfs H-alpha activity declines sharply beyond spectral type M7
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”?
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 Myrs Distance ~ 45.3 parsecs Diameter ~ 12 parsecs > 400 known members Uniform space motion V ~ 46.7 km/sec
Binary surveys: the Hyades (3) Rhy 403 - Period ~ 1.25 days - amplitude 40 km/sec Primary 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)
Binary surveys: the Hyades (4) Summary: 25% of low-mass Hyads have a stellar companion 1 candidate brown dwarf Another brown dwarf desert?
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
Finding brown dwarfs Initial discoveries - companions of known nearby stars - serendipitous identifications in the field Large scale catalogues - cool targets, T < 2000 K - require wide-field, deep, near-infrared surveys - DENIS (1996 - present) - 2MASS (1997 - present) - SDSS (1999/2000 - future)
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