Dark Matters Red dwarfs lowmass stars brown dwarfs
Dark Matters Red dwarfs, low-mass stars & brown dwarfs Neill Reid, Univ. of Pennsylvania in association with 2 MASS Core project: Davy Kirkpatrick, Jim Liebert, Conard Dahn, Dave Monet, Adam Burgasser
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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
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
Cool dwarf spectra (1) Early-type M dwarfs characterised by increasing Ti. O absorption Ca. OH present for sp > M 4
Cool dwarf spectra (2) Late M dwarfs: increasing Ti. O VO at sp > M 7 Fe. H at sp > M 8
Cool dwarf spectra (3) Spectral class L: decreasing Ti. O, VO - dust depletion increasing Fe. H, Cr. H, 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 L 3. 5/L 4 Latest dwarf 2 M 1507 -1627 L 5 Astrometry/photometry courtesy of USNO (Dahn et al)
The near-infrared HR diagram K I absorption leads to increasing (I-J) at sp > L 7
Cool dwarf spectra (5) : near-IR K, Fe, Na atomic lines water, CO molecular bands
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) [Burgasser - this meeting]
The near-IR HR diagram (2) Methane absorption eliminates JHK-only search for T dwarfs
Brown dwarf atmospheres Non-grey atmospheres - flux peaks at 1, 5 and 10 microns - bands and zones? - “weather”?
Brown dwarf “weather” Observations suggest that brown dwarfs have rapid rotation - v ~ 40 to 80 km/sec - P ~ 4 hrs to 90 mins If brown dwarfs had spots (giant storms? ), what would we see?
Clouds on an L 8? Gl 584 C - r ~ 17 pc - 2 G dwarf companions - a ~ 2000 AU - age ~ 100 Myrs - Mass ~ 0. 045 M(sun) - M(J) ~ 15. 0 Gl 229 B M(J) ~ 15. 4
Low mass binaries Why binaries? - dynamical mass estimates - coeval: calibration of relative properties Finding binary systems - direct imaging: wide systems ( > 5 AU) - HST + ground-based AO imaging - radial velocities: close systems - Keck spectroscopy, optical + IR Targets - low mass stars in open clusters - nearby low-luminosity dwarfs
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 (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
Binary surveys: the Hyades (3) Spectroscopic survey (Reid & Mahoney) 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 -> 1 st candidate brown dwarf
Binary surveys: the Hyades (4) Summary: 25% of low-mass Hyads have a stellar companion 1 candidate brown dwarf Another brown dwarf desert?
What about 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 584 C, G 196 -3 B, GJ 1001 B (& Gl 229 B 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 (>M 8) over cycles 8 & 9 (Reid + 2 MASS/SDSS consortium) Successful WFPC 2 observations of 20 targets to date --> only 4 binaries detected 2 M 0746 - L 0. 5 (brightest known L dwarf) 2 M 1146 - L 3 2 M 0920 - L 6. 5 2 M 0850 - L 6
Binary systems: L dwarfs (5) 2 M 0850: I-band V-band
Binary surveys: L dwarfs (6) Binary components lie close to L dwarf sequence: 2 M 0850 B M(I) ~0. 7 mag fainter than type L 8 M(J) ~0. 3 mag brighter than Gl 229 B
2 M 0850 AB (1) 2 M 0850 A has strong lithium absorption --> implies a mass below 0. 06 M(sun) 2 M 0920 A - no detectable lithium --> M > 0. 06 M(sun)
2 M 0850 AB(2) Mass limits: 2 M 0850 A: M < 0. 06 M(sun) q(B/A) ~ 0. 75 2 M 0920 A: M > 0. 06 M(sun) q(B/A) ~ 0. 95
2 M 0850 AB (3) Constraining brown dwarf models primaries have similar spectral type (Temp) -> similar masses ~0. 06 2 M 0850 B ~ 0. 045 M(sun) age ~ 1. 7 Gyrs
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
L dwarf binary statistics (2) 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?
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 M 7
Binary surveys: T dwarfs . . but 2 M 1237+68, a T dwarf, has strong H-alpha emission - no variation observed July, 1999 - February, 2000 Possible mechanisms: - Jovian aurorae? - flares? - binarity?
2 M 1237 : a vampire T dwarf Brown dwarfs are degenerate - increasing R, decreasing M - ensures continuous Roche lobe overflow
Summary 1. Photometric/spectroscopic characteristics of ultracool dwarfs are now well characterised 2. Gl 584 C provides the first detection of brown dwarf “weather” 3. Rhy 403 B is a candidate Hyades brown dwarf, but substellar-mass companions remain rare 4. First results from HST L dwarf survey - 4 of 20 are binary - 1 candidate L/T transition dwarf - L dwarf/L dwarf binaries rare disruption of primordial systems?
And our goal. . an eclipsing system
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