Collapse and Fragmentation Models of Tidally Interacting Molecular Cloud Cores. IV. Initial Slow Rotation and Magnetic Field Support

Details Collapse and Fragmentation Models of Tidally Interacting Molecular Cloud Cores. IV. Initial Slow Rotation and Magnetic Field Support Collapse and Fragmentation Models of Tidally Interacting Molecular Cloud Cores. IV. Initial Slow Rotation and Magnetic Field Support

Mar 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000apj...531.1037s&link_type=abstract

The Astrophysical Journal, Volume 531, Issue 2, pp. 1037-1052.

Astronomy and Astrophysics

Astronomy

7

Stars: Binaries: General, Hydrodynamics, Ism: Clouds, Ism: Magnetic Fields, Magnetohydrodynamics: Mhd, Stars: Formation

Scientific paper

Fragmentation has long been advocated as the primary mechanism for explaining the observed binary frequency among pre-main-sequence stars and, more recently, for explaining the emerging evidence for binary and multiple protostellar systems. The role of magnetic fields and ambipolar diffusion is essential to understand how dense cloud cores begin dynamic collapse and eventually fragment into protostars. Here we consider new numerical models of the gravitational collapse and fragmentation of slowly rotating molecular cloud cores, including the effects of magnetic support and ambipolar diffusion. The starting point of the evolution is provided by a magnetically stable (subcritical) condensation that results from adding a magnetic field pressure, B2/8π [with the field strength given by the scaling relation B=B0(ρ/ρ0)1/2], to a reference state consisting of a thermally supercritical (α~0.36), slowly rotating (β~0.037), Gaussian cloud core of prolate shape and central density ρ0. The effects of ambipolar diffusion are approximated by allowing the reference field strength B0 to gradually decrease over a timescale of 10 free-fall times. The models also include the effects of tidal interaction due to a gravitational encounter with another protostar, and so they may apply to low-mass star formation within a cluster-forming environment. The results indicate that the magnetic forces delay the onset of dynamic collapse, and hence of fragmentation, by an amount of time that depends on the initial central mass-to-flux ratio. Compared with previous magnetic collapse calculations of rapidly rotating (β=0.12) clouds, lower initial rotation (β~0.037) is seen to result in much shorter delay periods, thus anticipating binary fragmentation. In general, the results show that the models are still susceptible to fragment into binary systems. Intermediate magnetic support (η~0.285) and low tidal forces (τ<~0.201) may lead to final triple or quadruple protostellar systems, while increasing the size of η and τ always results in final binary protostellar cores. The formed binary systems have separations of ~200-350 AU, suggesting that the recently observed peaks around ~90 AU and 215 AU for T Tauri stars may be explained by the collapse and fragmentation of initially slowly rotating magnetic cloud cores with β<~0.04.

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