The Origin of Brown Dwarfs

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The Origin of Brown Dwarfs
Kevin L. Luhman Penn State
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What makes it possible for brown dwarfs to form?
Lada et al. 2003
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Turbulent fragmentation - low-mass cores
e.g., Padoan Nordlund 2004
Lada et al. 2003
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Dynamical interactions - premature halting of
accretion
e.g., Reipurth Clark 2001, Bate et al. 2002
Lada et al. 2003
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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

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Dynamical interactions - premature halting of
accretion
e.g., Reipurth Clark 2001, Bate et al. 2002

• Initial Mass Function
• Binarity
• Spatial Distribution
• Circumstellar Disks

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Where to measure the substellar IMF?
star forming regions
M
T
L
8
N(stars)/N(brown dwarfs) 5-10
But this is sensitive to - errors in mass esti
mates
- real variations in peak of IMF
Chamaeleon
IC 348
brown dwarfs
stars
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IMFs of brown dwarfs field young clusters
- no large population of BDs ejected from young
clusters
Taurus
Chamaeleon
Luhman 2007
Luhman 2004
Allen et al. (2005)
IC348
Orion
Muench et al. 2002
Luhman et al. 2003
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Brown dwarfs found down to 10 MJup
No sign yet of the minimum mass of the IMF
Taurus
Chamaeleon
Luhman 2007
Luhman 2004
IC348
Orion
Muench et al. 2002
Luhman et al. 2003
See also ? BDs in Sigma Ori (Martin, Zapatero
Osorio, et al.)
? BDs in Orion (Lucas Roche)
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Brown Dwarf Binarity
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The brown dwarf desert few brown dwarfs among c
lose companions (stars
planets
BDs
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The brown dwarf desert at wide separations too?
stellar companions
brown dwarf companions
McCarthy Zuckerman 2004
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Binary brown dwarfs most have small separations
Young
Old
Burgasser et al. 2003
Kraus, White, Hillenbrand 2005
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Binary brown dwarfs most have small separations
Young
Old
Burgasser et al. 2003
Kraus, White, Hillenbrand 2005
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Binary brown dwarfs but a few are wide
Young
Old
Billeres et al. 2005
Luhman 2004
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Spatial Distribution of Brown Dwarfs
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Taurus
Luhman 2006
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Chamaeleon I
10 km/s for 1 Myr
Luhman 2007
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Circumstellar Disks around Brown Dwarfs
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H? profiles - accretion rates
Muzerolle et al. 2005 Mohanty et al. 2005
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Accretion rates continuous from stars to BDs
Muzerolle et al. 2005
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BD photosphere
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Brown dwarf disks hard to detect at model disk photosphere
BD photosphere
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SpitzerIRAC - best for finding brown dwarf disks
8 MJup
Luhman et al. 2006
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Brown dwarfs stars have similar disk fractions
disks
no disks
Luhman et al. 2006
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A young brown dwarf unusually faint for its
spectral type
Is it seen in scattered light (e.g., edge-on
disk)?
Luhman 2004
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Spitzer spectra - both silicate absorption
emission
Apai et al. 2005
Luhman et al. 2007
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Spitzer spectra - brown dwarf disk is nearly
edge-on
scattered light
disk
inner wall
mm Scholz et al. 2006
photosphere
Luhman et al. 2007
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Hubble images - confirm high inclination
Luhman et al. 2007
R 40 AU - larger than expected from ejection
models
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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 cant rule out ejection, but no
  evidence that it is necessary for the formation
  of brown dwarfs