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 Lada et al. 2003
Dynamical interactions -> premature halting of accretion e.g., Reipurth & Clark 2001, Bate et al no wide binaries high velocities at birth small circumstellar disks
Dynamical interactions -> premature halting of accretion e.g., Reipurth & Clark 2001, Bate et al Initial Mass Function Binarity Spatial Distribution Circumstellar Disks
star forming regions Where to measure the substellar IMF? MTL
IC 348 brown dwarfs stars Chamaeleon N(stars)/N(brown dwarfs) ~ 5-10 But this is sensitive to: - errors in mass estimates - real variations in peak of IMF
Orion Muench et al IMFs of brown dwarfs: field ≈ young clusters -> no large population of BDs ejected from young clusters Allen et al. (2005) TaurusChamaeleon Luhman 2007 Luhman et al Luhman 2004 IC348
Orion Muench et al Brown dwarfs found down to ~10 M Jup No sign yet of the minimum mass of the IMF See also BDs in Sigma Ori (Martin, Zapatero Osorio, et al.) BDs in Orion (Lucas & Roche) TaurusChamaeleon Luhman 2007 Luhman et al Luhman 2004 IC348
Brown Dwarf Binarity
The brown dwarf desert: few brown dwarfs among close companions (<5 AU) planets BDs stars
stellar companions brown dwarf companions The brown dwarf desert: at wide separations too? McCarthy & Zuckerman 2004
Binary brown dwarfs: most have small separations Burgasser et al. 2003Kraus, White, & Hillenbrand 2005 <20 AU YoungOld
Binary brown dwarfs: most have small separations Burgasser et al. 2003Kraus, White, & Hillenbrand 2005 <200 AU YoungOld
Binary brown dwarfs: but a few are wide Billeres et al. 2005Luhman 2004 <200 AU YoungOld
Spatial Distribution of Brown Dwarfs
Taurus Luhman 2006
Chamaeleon I 10 km/s for 1 Myr Luhman 2007
Circumstellar Disks around Brown Dwarfs
Muzerolle et al Mohanty et al H profiles -> accretion rates
Accretion rates continuous from stars to BDs Muzerolle et al. 2005
BD photosphere
Brown dwarf disks hard to detect at <4 m BD photosphere model disk + photosphere
Spitzer+IRAC -> best for finding brown dwarf disks ~ 8 M Jup Luhman et al. 2006
no disks disks Brown dwarfs & stars have similar disk fractions 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 Luhman et al Apai et al. 2005
Spitzer spectra -> brown dwarf disk is nearly edge-on photosphere scattered light disk inner wall mm = Scholz et al Luhman et al. 2007
Hubble images -> confirm high inclination Luhman et al 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 M Jup 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 M Jup 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