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Brown Dwarfs and Dark Matters

Brown Dwarfs and Dark Matters. L dwarfs, binaries and the mass function. Neill Reid, STScI in association with 2MASS Core project: Davy Kirkpatrick, Jim Liebert, Conard Dahn, Dave Monet, Adam Burgasser. Outline. Finding ultracool dwarfs

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Brown Dwarfs and Dark Matters

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  1. Brown Dwarfs and Dark Matters L dwarfs, binaries and the mass function Neill Reid, STScI in association with 2MASS Core project: Davy Kirkpatrick, Jim Liebert, Conard Dahn, Dave Monet, Adam Burgasser

  2. 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 brown dwarfs and dark matter Some results and a conundrum • Heavy halo white dwarfs?

  3. 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

  4. Lithium test Late-type dwarfs are fully convective  everything visits the core If core temperature > 2 x 10^6 K  lithium is destroyed If M < 0.06 M(sun), lithium survives

  5. Cool dwarf evolution (2) Rapid luminosity evolution for substellar-mass dwarfs

  6. 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

  7. Finding ultracool dwarfs Gl 406 = M6 dwarf (Wolf 359) Flux distribution peaks at ~ 1 micron ---> search at near-IR wavelengths

  8. 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)

  9. Finding ultracool dwarfs (3) Search for sources with red (J-K) and either red optical/IR colours or A-type colours

  10. Cool dwarf spectra (1) Early-type M dwarfs characterised by increasing TiO absorption CaOH present for sp > M4

  11. Cool dwarf spectra (2) Late M dwarfs: increasing TiO VO at sp > M7 FeH at sp > M8

  12. 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

  13. Cool dwarf spectra (4) Low opacity leads to high pressure broadening of Na D lines

  14. The L/T transition • Methane absorption • T ~ 1200/1300K • (Tsuji, 1964) • Blue JHK colours • Early-type T dwarfs • first identified from • SDSS data - • Leggett et al (2000) • Unsaturated methane • absorption

  15. 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

  16. 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)

  17. NIR Spectral Classification (3) H-band Few identified atomic features

  18. 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

  19. The 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)

  20. The near-infrared HR diagram Mid- and late-type L dwarfs can be selected using 2MASS JHK alone SDSS riz + 2MASS J permits identification of all dwarfs sp > M4 Note small offset L8  Gl 229B

  21. Searching for brown dwarf binaries The alternative model for brown dwarfs

  22. Binary surveys: L dwarfs (2) Why do we care about L dwarf binaries? 1. Measure dynamical masses  constrain models 2. Star formation and, perhaps, planet formation HST imaging survey of 160 ultracool dwarfs (>M8) over cycles 8 & 9 (Reid + 2MASS/SDSS consortium) Successful WFPC2 observations of 60 targets to date --> only 11 binaries detected

  23. Binary surveys: L dwarfs (3) 2M0746 (L0.5) 2M1146 (L3)

  24. Binary systems: L dwarfs (4) 2M0920 (L6.5): I-band V-band

  25. Binary systems: L dwarfs (5) 2M0850: I-band V-band

  26. 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

  27. 2M0850A/B Could 2M0850AB be an L/T binary? Probably not -- but cf. SDSS early T dwarfs

  28. L dwarf binary statistics (1) • Approximately 20% of L dwarfs are resolved • almost all are equal luminosity, therefore equal mass • 2M0850AB – mass ratio ~ 0.8 • none have separations > 10 AU • L dwarf/L dwarf binaries seem to be rarer, and/or have smaller <a> than M dwarfs How do these parameters mesh with overall binary statistics?

  29. L dwarf binary statistics (2) Brown dwarfs don’t always have brown dwarf companions

  30. L dwarf binary statistics (3) Known L dwarf binaries - high q, small <a> - low q, large <a> -> lower binding energy - preferential disruption? Wide binaries as minimal moving groups?

  31. The substellar mass function (1) Brown dwarfs evolve along nearly identical tracks in the HR diagram, at mass-dependent rates No single-valued M/L relation Model N(mag, sp. Type)  infer underlying Y(M) Require temperature scale bolometric corrections star formation history

  32. 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

  33. Bolometric corrections Given near-IR data --> infer M(bol) --> bol correction little variation in BC_J from M6 to T

  34. 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

  35. 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

  36. Substellar Mass function (6) Predictions vs. observations 10 Gyr-old disk constant star formation 0 < n < 2 All L: 14002100 K >L2 : 14001900K T : < 1300K

  37. 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)

  38. Substellar Mass Function (8) Miller-Scalo mass function --> log-normal Match observations for disk age 8 to 10 Gyrs

  39. 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 In general  observations appear consistent with n ~ 1  equal numbers of BDs (>0.01 M(sun)) and MS stars No significant contribution to dark matter……..but….

  40. A kinematic conundrum (1) Stellar kinematics are correlated with age  scattering through encounters with molecular clouds leads to 1. Higher velocity dispersions 2. Lower net rotational velocity, V e.g. Velocity distributions of dM (inactive, older) and dMe (active, younger)

  41. A kinematic conundrum (2) Stellar kinematics are usually modelled as Gaussian distributions  (s(U), s(V), s(W) ) But disk kinematics are more complex:  use probability plots Composite in V 2 Gaussian components in (U, W) local number ratio high:low ~ 1:10 thick disk and old disk?

  42. A kinematic conundrum (3) Kinematics of ultracool dwarfs (M7  L0) Hires data for 35 dwarfs ~50% trig/50% photo parallaxes Proper motions for all  (U, V, W) velocities We expect the sample to be dominated by long-lived low-mass stars – although there is at least one BD

  43. A kinematic conundrum (4) Ultracool M dwarfs have kinematic properties matching M0-M5 dMe dwarfs  t ~ 2-3 Gyrs Does this make sense? M7  L0 ~2600  2100K Where are the old V LM stars?

  44. A different kind of dark matter • Galaxy rotation curves at large radii are not Keplerian - heavy halos (Ostriker, Peebles & Yahil, 1974) - Milky Way M ~ 5 x 10^11 solar masses, R < 50 kpc visible material (disk + stellar halo) ~ 5 x 10^10 solar masses => 90% dark matter – particles? compact objects? • Microlensing surveys – MACHO, EROS, DUO,OGLE Given timescale, estimated velocity => mass MACHO: 13-17 events, t ~ 34-230 days, <V> ~ 200-300 km/s => can account for ~20% of the missing 90% <M> = 0.5+/- 0.3 solar masses Halo white dwarfs?

  45. Heavy halo white dwarfs? I • We are in the dark halo – local density ~ 10^-2 M_sun/pc^3 => search for local representatives in proper motion surveys • Oppenheimer et al. (Science Express, March 23) Photographic survey of ~12% of the sky near the SGP - 38 cool, high-velocity white dwarfs – 4 x 10^-4 stars/pc^3 - local mass density of ~3 x 10^-4 M_sun/pc^3 => could account for 3% of dark matter if they’re in the heavy halo But are they?

  46. Heavy halo white dwarfs? II • The Galactic disk has a complex kinematic structure - thin/old disk: 300 pc scaleheight, 90% of local stars - thick/extended disk: 700 pc scaleheight, 10% • Should we expect any high-velocity disk stars  consider a volume-complete sample of 514 M dwarfs (Reid, Hawley & Gizis, 1995)

  47. Heavy halo white dwarfs? III • Thick disk stars can have high velocities - Reid, Hawley & Gizis (1995): PMSU M dwarf survey 4% of the sample would be classed as dark halo by Oppenheimer et al => ~2 x 10^-4 white dwarfs / pc^3 • Most of the Oppenheimer et al. white dwarfs are remnants of the first stars which formed in the thick disk • White dwarfs from the stellar halo account for the rest • There is no requirement for a dark matter contribution

  48. What next? (1) • Better statistics for nearby stars  F(M), Y(M) • A 2MASS NStars survey • (with Kelle Cruz (Upenn), Jim Liebert (UA), John Gizis (Delaware) • Davy Kirkpatrick & Pat Lowrance (IPAC), Adam Burgasser (UCLA)) • Aim: find all dwarfs later than M4 within 20 parsecs • 2MASS/NLTT cross-referencing: (m(r) – K)  p • Deep van Biesbroeck survey for wide cpm companions • 2MASS-direct: (J-K)  p • 2MASS/POSS II: (I-J)  p

  49. What next? (2) • If n~1, •  equal numbers of stars • and brown dwarfs • Numerous cool (room temp.) BDs • brightest at 5 mm •  accessible to SIRTF • ~10 400K BDs /100 sq deg • F>10 mJy at 5 mm

  50. Summary • 1. Brown dwarfs are now almost commonplace • 2. Near-IR spectra show that the L dwarf sequence • L0…L8 is consistent with near-infrared variations •  probably well correlated with temperature • 3. First results from HST L dwarf binary survey • - L dwarf/L dwarf binaries relatively rare • - Maximum separation is correlated with total mass • nature or nurture? • Current detection rates are inconsistent with a steep IMF •  brown dwarfs are poor dark matter candidates • Neither are cool white dwarfs

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