Dark Matters Neill Reid, Univ. of Pennsylvania in association with 2MASS Core project: Davy Kirkpatrick, Jim Liebert, Conard Dahn, Dave Monet, Adam Burgasser Red dwarfs, low-mass stars & brown dwarfs Shameless plug. Now available from Amazon.com and in all the best bookstores 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 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 - 2M L5 Astrometry/photometry courtesy of USNO (Dahn et al) The near-infrared HR diagram K I absorption leads to increasing (I-J) at sp > L7 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 L8? Gl 584C - r ~ 17 pc - 2 G dwarf companions - a ~ 2000 AU - age ~ 100 Myrs - Mass ~ M(sun) - M(J) ~ 15.0 Gl 229B 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) Rhy 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 solar masses. 70% probability M < > 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? 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 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 2M L0.5 (brightest known L dwarf) 2M L3 2M L6.5 2M L6 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 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) ~ M0920A: 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 ~ M(sun) age ~ 1.7 Gyrs 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) 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? 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 2M , a T dwarf, has strong H-alpha emission - no variation observed July, 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 Summary 1. Photometric/spectroscopic characteristics of ultracool dwarfs are now well characterised 2. Gl584C provides the first detection of brown dwarf weather 3. Rhy 403B 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