Dynamical Interactions and Brown Dwarfs

1
Dynamical Interactions and Brown Dwarfs
Michael F. Sterzik, ESO Richard H. Durisen,
Indiana University
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
• 1000s of calculations yield a reliable benchmark
  for comparisons with observations and
  hydrodynamical simulations

3
Scenario
?
system scale ? 0.01 pc
4
Observed Multiplicities

• Solar-type stars in the field 5710 (DM 91)
• M-type 429 (FM 92), 3210 (Leinert et al 97)
  
• late M-type 315 (Marchal et al 03), 177
  (Reid et al 97)
• VLM 2011 (Reid et al 01), 157 (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 gt0.5 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 lt ? lt 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/Mconst
• ? 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