1. The Origin of Brown Dwarfs Kevin L. Luhman Penn State

2. What makes it possible for brown dwarfs to form? Lada et al. 2003

3. Turbulent fragmentation -> low-mass cores e.g., Padoan & Nordlund 2004 Lada et al. 2003

4. Dynamical interactions -> premature halting of accretion e.g., Reipurth & Clark 2001, Bate et al. 2002 Lada et al. 2003

5. 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

6. Dynamical interactions -> premature halting of accretion e.g., Reipurth & Clark 2001, Bate et al. 2002 Initial Mass Function Binarity Spatial Distribution Circumstellar Disks

7. star forming regions Where to measure the substellar IMF? MTL

8. 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

9. Orion Muench et al. 2002 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. 2003 Luhman 2004 IC348

10. Orion Muench et al. 2002 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. 2003 Luhman 2004 IC348

11. Brown Dwarf Binarity

12. The brown dwarf desert: few brown dwarfs among close companions (<5 AU) planets BDs stars

13. stellar companions brown dwarf companions The brown dwarf desert: at wide separations too? McCarthy & Zuckerman 2004

14. Binary brown dwarfs: most have small separations Burgasser et al. 2003Kraus, White, & Hillenbrand 2005 <20 AU YoungOld

15. Binary brown dwarfs: most have small separations Burgasser et al. 2003Kraus, White, & Hillenbrand 2005 <200 AU YoungOld

16. Binary brown dwarfs: but a few are wide Billeres et al. 2005Luhman 2004 <200 AU YoungOld

17. Spatial Distribution of Brown Dwarfs

18. Taurus Luhman 2006

19. Chamaeleon I 10 km/s for 1 Myr Luhman 2007

20. Circumstellar Disks around Brown Dwarfs

21. Muzerolle et al. 2005 Mohanty et al. 2005 H  profiles -> accretion rates

22. Accretion rates continuous from stars to BDs Muzerolle et al. 2005

23. BD photosphere

24. Brown dwarf disks hard to detect at <4  m BD photosphere model disk + photosphere

25. Spitzer+IRAC -> best for finding brown dwarf disks ~ 8 M Jup Luhman et al. 2006

26. no disks disks Brown dwarfs & stars have similar disk fractions Luhman et al. 2006

27. A young brown dwarf unusually faint for its spectral type Is it seen in scattered light (e.g., edge-on disk)? Luhman 2004

28. Spitzer spectra -> both silicate absorption & emission Luhman et al. 2007 Apai et al. 2005

29. Spitzer spectra -> brown dwarf disk is nearly edge-on photosphere scattered light disk inner wall mm = Scholz et al. 2006 Luhman et al. 2007

30. Hubble images -> confirm high inclination Luhman et al. 2007 R ~ 40 AU -> larger than expected from ejection models

31. 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