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

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

3 Scenario  system scale  0.01 pc   AU

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 M sol

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 … Primary Mass L (<0.08M s ) M (<0.47M s ) K/G (<1.2M s ) F+ (>1.2M s ) L (<0.08M s ) 2% (4%)3% (5%)2% (4%)1% (2%) T (<0.05M s ) 6% (17%)5% (10%)3% (6%)1% (5%) … 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  Wide BD companions are outer member in hierarchical systems Mass ratios vers. Separations

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