dark matters
DESCRIPTION
Dark Matters. Red dwarfs, low-mass stars & brown dwarfs. Neill Reid, Univ. of Pennsylvania in association with 2MASS Core project: Davy Kirkpatrick, Jim Liebert, Conard Dahn, Dave Monet, Adam Burgasser. Shameless plug…. Now available from Amazon.com and in all the best bookstores. - PowerPoint PPT PresentationTRANSCRIPT
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 fromAmazon.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 - 2M1507-1627 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 ~ 0.045 M(sun) - M(J) ~ 15.0 Gl 229B M(J) ~ 15.4
Low mass binaries
Why binaries? - dynamical mass estimates - coeval: calibration of relative propertiesFinding binary systems - direct imaging: wide systems ( > 5 AU) - HST + ground-based AO imaging - radial velocities: close systems - Keck spectroscopy, optical + IRTargets - low mass stars in open clusters - nearby low-luminosity dwarfs
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 (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 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)
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 massAre 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 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) ~ 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
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 tracesactivity in late-type dwarfs.
Binary surveys: T dwarfs
H-alpha activitydeclines 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
Summary
1. Photometric/spectroscopic characteristics of ultracool dwarfs are now well characterised2. Gl584C provides the first detection of brown dwarf “weather”3. Rhy 403B is a candidate Hyades brown dwarf, but substellar-mass companions remain rare4. 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