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Dynamical Interactions and Brown Dwarfs

Michael F. Sterzik, ESORichard H. Durisen, Indiana University

• Hierarchical fragmentation and „two-step“ dynamical decayHierarchical fragmentation and „two-step“ dynamical decay• Results and comparison w/ observationsResults and comparison w/ observations• Multiplicities and velocity dispersionsMultiplicities and velocity dispersions• Companion fractions and separation distributionsCompanion fractions and separation distributions• ConclusionsConclusions

published 2003, Astron.&Astroph. 400, p.1031

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Molecular clouds fragment into cores and clumpsClump mass spectra (CMF) resemble stellar mass spectraClumps 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, kinematics1000’s of calculations yield a reliable benchmark for comparisons with observations and hydrodynamical simulations

Context: “Two-Step” Decay(Sterzik & Durisen, 2003)

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Scenario

system scale 0.01 pc

100-300AU

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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

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Multiplicity Fractions(Sterzik & Durisen, 2003)

Increasing MF with increasing primary mass compatible with 2-step decayVLM: 8 -18%Solar type: 63%1-step models too “steep”“Random” IMF sampling ruled out for M >0.5 Msol

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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)

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BD Companions …

Primary Mass

L (<0.08Ms)

M(<0.47Ms)

K/G(<1.2Ms)

F+(>1.2Ms)

L (<0.08Ms) 2% (4%) 3% (5%) 2% (4%) 1% (2%)

T (<0.05Ms) 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

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Observed Separation Distributions

Reference distribution for solar-type stars in the field: Duquennoy & Mayor 91Lognormal, 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

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Separation Distributions (Sterzik & Durisen, 2003)

IF the specific initial cluster energy E/M=const

Separations ~ System MassDynamical decay model reproduces the mean of the observed separation distributionObserved distributions are broader (initial conditions NOT constant, further evolution)

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“Wide” BD Companions

… are “abundant” as CPM companions (Gizis et al. 2001)GJ337, GJ570, GJ 584,… are multiple systemsMass ratio vers. Separation Distribution

Do “wide” BD systems prefer a hierarchical configuration?

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Wide BD companions are outer member in hierarchical systems

Mass ratios vers. Separations

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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