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