Statistics – Computation
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...425..142c&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 425, no. 1, p. 142-160
Statistics
Computation
145
Computational Astrophysics, Diffusion, Interstellar Extinction, Magnetic Fields, Magnetohydrodynamics, Plasmas (Physics), Protostars, Star Formation, Mass Ratios, Numerical Analysis, Stellar Envelopes, Stellar Models
Scientific paper
In a previous paper we formulated the problem of the formation of protostellar cores by ambipolar diffusion in axisymmetric, isothermal, self-gravitating, thermally supercritical but magnetically subcritical model molecular clouds, accounting for a cosmic abundance of interstellar grains (both charged and neutral). Using an implicit code with an adaptive mesh, we follow the evolution to a central density enhancement of 106 (e.g., from 2.6 x 103/cu cm to 2.6 x 109/cu cm). First, ambipolar diffusion slowly increases the mass-to-flux ratio of a cloud's central flux tubes, leading to the formation and contraction of thermally supercritical but magnetically subcritical cores. The timescale for this process is essentially the initial central flux-loss timescale, which exceeds the dynamical timescale (approximately equals free-fall timescale) typically by a 10-20. Eventually, the mass-to-flux ratio exceeds the critical value for collapse. The subsequent contraction of the thermally and magnetically supercritical cores becomes progressively more dynamic, while the envelopes remain relatively well supported by magnetic forces, in agreement with early theoretical predictions by Mouschovias. A typical supercritical core consists of a uniform-density central region and a 'tail' of infalling matter with a power-law density profile nn proportional to r5, -1.5 is approximately greater than s is approximately greater than -1.85. The mass infall (or accretion) rate from the subcritical envelopes is controlled by ambipolar diffusion, and differs both qualitatively and quantitatively from estimates based on nonmagnetic models and their extrapolations to magnetic clouds. Model clouds that include the macroscopic (collisional) effects of grains have their evolution retarded (typically by 50%) with respect to models accounting only for neutral-ion drag. Neutral-grain drag typically dominates the neutral-ion drag at core densities nn, c is approximately greater than 108/cu cm. Electrostatic attraction by electron-shielded ions ('quasiparticles') keeps charged grains partially attached to the magnetic field for densities nn, c is approximately greater than 3 x 105/cu cm, at which detachment would otherwise occur because of collisions with neutrals. Neutral grains also couple to the magnetic field by inelastic charge-capture processes. The grains lengthen the timescale for the formation of a core, accentuate the core-envelope separation, and, by any given central density enhancement, increase a core's size, mass, and magnetic flux.
Ciolek Glenn E.
Mouschovias Telemachos Ch.
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