Dynamical Interactions and Brown Dwarfs

1. Michael F. Sterzik, ESO Richard H. Durisen, Indiana University Dynamical Interactions and Brown Dwarfs published 2003, Astron.&Astroph. 400, p.1031 • Hierarchical fragmentation and „two-step“ dynamical decay • Results and comparison w/ observations • Multiplicities and velocity dispersions • Companion fractions and separation distributions • Conclusions

2. Context: “Two-Step” Decay(Sterzik & Durisen, 2003) • Molecular clouds fragment into cores and clumps • Clump mass spectra (CMF) resemble stellar mass spectra • Clumps have flattish density profile (Bonnor-Ebert) • Turbulence(?) decays, produce N stars (SMF) • 1  N  “few”(10) non-hierarchical “mini-clusters” • N-body dynamical evolution (neglect: accretion, hydrodynamics) • End-state analysis: pairing statistics, kinematics • 1000’s of calculations yield a reliable benchmark for comparisons with observations and hydrodynamical simulations

3.   100-300AU Scenario  system scale  0.01 pc

4. Observed Multiplicities • Solar-type stars in the field: 57±10% (D&M 91) • M-type: 42±9% (F&M 92), 32±10% (Leinert et al 97) • late M-type: 31±5% (Marchal et al 03), 17±7% (Reid et al 97) • VLM: 20±11% (Reid et al 01), 15±7% (Close et al 03) • Observed Multiplicity Fractions  Evidence for a mass - multiplicity relation

5. Multiplicity Fractions(Sterzik & Durisen, 2003) • Increasing MF with increasing primary mass compatible with 2-step decay • VLM: 8 -18% • Solar type: 63% • 1-step models too “steep” • “Random” IMF sampling ruled out for M >0.5 Msol

6. Velocity Dispersions • Mass-velocity dependence • Single-Binary segregation • High velocity escape exist, but are not so frequent • Convolve w/ cloud motion! • Joergens (2001): ~2 km/sec • White (2003): ~1.9 km/sec ~2 km/sec (BD) ~1 km/sec (stars)

7. BD Companions … • … hardly found in direct imaging surveys… • Schroeder et al. (HST, 2000); Oppenheimer (2001): 1% • McCarthy (KECK, 2001); Lowrance (2001): 1 - few% • … and in radial velocity surveys (BD desert, Halbwachs 2000) • Rare when formed dynamically • Probably inconsistent with random pairing

8. Observed Separation Distributions • Reference distribution for solar-type stars in the field: Duquennoy & Mayor 91 • Lognormal, broad peak log P = 4.8 days (~ 30AU) • late M binaries: Fischer & Marcy 92; Marchal et al 03 (23 M2.5-M5.5) • VLM binaries: Bouy; Burgasser; Close 03 (34 later then M8) • Separations: 1 <  < 15AU, narrow peak ~ 3AU • Cumulative separation distributions  Mounting evidence for a mass-separation relation

9. Separation Distributions (Sterzik & Durisen, 2003) • IF the specific initial cluster energy E/M=const  Separations ~ System Mass • Dynamical decay model reproduces the mean of the observed separation distribution • Observed distributions are broader (initial conditions NOT constant, further evolution)

10. “Wide” BD Companions • … are “abundant” as CPM companions (Gizis et al. 2001) • GJ337, GJ570, GJ 584,… are multiple systems • Mass ratio vers. Separation Distribution  Do “wide” BD systems prefer a hierarchical configuration?

11. Mass ratios vers. Separations  Wide BD companions are outer member in hierarchical systems

12. Conclusions „Two-Step“ dynamical decay models predict: • High velocity escapers are rare, dispersion velocities ~ cloud motions • Increasing multiplicity fraction with increasing mass • VLM multiplicity fraction of 8-18% • Low BD secondary fractions, decreasing with increasing primary mass • Mean binary separations are correlated with their system mass, IF the progenitor systems have a constant specific energy (or a linear M ~ R), as e.g. in Bonnor-Ebert spheres  Dynamical decay models provide a valueable benchmark for the observed statistics