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The Origin of Brown Dwarfs. Kevin L. Luhman Penn State. What makes it possible for brown dwarfs to form?. Lada et al. 2003. Turbulent fragmentation -> low-mass cores. e.g., Padoan & Nordlund 2004. Lada et al. 2003. Dynamical interactions -> premature halting of accretion.
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The Origin of Brown Dwarfs Kevin L. Luhman Penn State
What makes it possible for brown dwarfs to form? Lada et al. 2003
Turbulent fragmentation -> low-mass cores e.g., Padoan & Nordlund 2004 Lada et al. 2003
Dynamical interactions -> premature halting of accretion e.g., Reipurth & Clark 2001, Bate et al. 2002 Lada et al. 2003
Dynamical interactions -> premature halting of accretion e.g., Reipurth & Clark 2001, Bate et al. 2002 • no wide binaries • high velocities at birth • small circumstellar disks
Dynamical interactions -> premature halting of accretion e.g., Reipurth & Clark 2001, Bate et al. 2002 • Initial Mass Function • Binarity • Spatial Distribution • Circumstellar Disks
Where to measure the substellar IMF? star forming regions M L T
N(stars)/N(brown dwarfs) ~ 5-10 But this is sensitive to: - errors in mass estimates - real variations in peak of IMF Chamaeleon IC 348 brown dwarfs stars
IMFs of brown dwarfs: field ≈ young clusters -> no large population of BDs ejected from young clusters Taurus Chamaeleon Luhman 2004 Luhman 2007 Allen et al. (2005) IC348 Orion Luhman et al. 2003 Muench et al. 2002
Brown dwarfs found down to ~10 MJup No sign yet of the minimum mass of the IMF Taurus Chamaeleon Luhman 2004 Luhman 2007 IC348 Orion Luhman et al. 2003 Muench et al. 2002 See also BDs in Sigma Ori (Martin, Zapatero Osorio, et al.) BDs in Orion (Lucas & Roche)
The brown dwarf desert: few brown dwarfs among close companions (<5 AU) stars planets BDs
The brown dwarf desert: at wide separations too? stellar companions brown dwarf companions McCarthy & Zuckerman 2004
Binary brown dwarfs: most have small separations <20 AU Old Young Burgasser et al. 2003 Kraus, White, & Hillenbrand 2005
Binary brown dwarfs: most have small separations <200 AU Old Young Burgasser et al. 2003 Kraus, White, & Hillenbrand 2005
Binary brown dwarfs: but a few are wide <200 AU Old Young Billeres et al. 2005 Luhman 2004
Taurus Luhman 2006
Chamaeleon I 10 km/s for 1 Myr Luhman 2007
H profiles -> accretion rates Muzerolle et al. 2005 Mohanty et al. 2005
Accretion rates continuous from stars to BDs Muzerolle et al. 2005
Brown dwarf disks hard to detect at <4 m model disk + photosphere BD photosphere
Spitzer+IRAC -> best for finding brown dwarf disks ~ 8 MJup Luhman et al. 2006
Brown dwarfs & stars have similar disk fractions disks no disks Luhman et al. 2006
A young brown dwarf unusually faint for its spectral type Is it seen in scattered light (e.g., edge-on disk)? Luhman 2004
Spitzer spectra -> both silicate absorption & emission Apai et al. 2005 Luhman et al. 2007
Spitzer spectra -> brown dwarf disk is nearly edge-on scattered light disk inner wall mm = Scholz et al. 2006 photosphere Luhman et al. 2007
Hubble images -> confirm high inclination Luhman et al. 2007 R ~ 40 AU -> larger than expected from ejection models
Summary • N(stars)/N(BDs) ~ 5, but this is sensitive to: • Errors in mass estimates • Real variations in the peak mass of the IMF • IMF similar between young clusters & field • No sign of minimum mass of IMF down to 10 MJup • Most binary BDs are tight, but a few are wide • Young stars & BDs have similar spatial distributions • Accretion rates vary continuously from stars to BDs • Disks found around BDs down to ~8 MJup • Young stars & BDs have similar disk fractions • Edge-on disk around BD: disk radius ≥ 40 AU • Conclusion: can’t rule out ejection, but no evidence that it is necessary for the formation of brown dwarfs