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Brown dwarfs and dark matters

Brown dwarfs and dark matters. L dwarfs, binaries and the mass function. Neill Reid, Univ. of Pennsylvania 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, Univ. of Pennsylvania 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 current and future constraints

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  4. 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

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

  6. Finding brown dwarfs(1) Initial discoveries - companions of known nearby stars: wide companion searches - van Biesbroeck VB 8, VB 10 (1943) coronagraphic searches - Gl 229B - serendipitous identifications in the field Kelu 1 Large scale catalogues - cool targets, T < 2000 K - require wide-field near-infrared surveys

  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 cf. Metal-poor subdwarfs

  14. 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)

  15. 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)

  16. The L/T transition (2) Early-type T dwarfs first identified from SDSS data - Leggett et al (2000) Unsaturated methane absorption

  17. 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

  18. 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

  19. 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

  20. 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)

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

  22. 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

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

  24. Searching for brown dwarf binaries The alternative model for browm dwarfs

  25. 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?

  26. 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

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

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

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

  30. 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

  31. 2M0850AB (1) 2M0850A has strong lithium absorption --> implies a mass below 0.06 M(sun) 2M0920A - no detectable lithium --> M > 0.06 M(sun)

  32. 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

  33. 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

  34. 2M0850A/B (4) Could 2M0850AB be an L/T binary? Probably not -- but cf. SDSS early T dwarfs

  35. What we’d really like... a brown dwarf eclipsing system

  36. 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

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

  38. L dwarf binary statistics (3) 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?

  39. 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

  40. 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

  41. 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

  42. 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

  43. 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%

  44. Substellar Mass function (6) Predictions vs. observations 10 Gyr-old disk constant star formation 0 < n < 2

  45. 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)

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

  47. 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

  48. 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

  49. 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

  50. 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.

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