The Origin of Brown Dwarfs

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. Where to measure the substellar IMF? star forming regions M L T

8. 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

9. 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

10. 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)

11. Brown Dwarf Binarity

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

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

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

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

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

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

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 model disk + photosphere BD photosphere

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

26. Brown dwarfs & stars have similar disk fractions disks no disks 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 Apai et al. 2005 Luhman et al. 2007

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