Ambipolar Diffusion, Interstellar Dust, and the Formation of Cloud Cores and Protostars. IV. Effect of Ultraviolet Ionization and Magnetically Controlled Infall Rate

Details Ambipolar Diffusion, Interstellar Dust, and the Formation of Cloud Cores and Protostars. IV. Effect of Ultraviolet Ionization and Magnetically Controlled Infall Rate Ambipolar Diffusion, Interstellar Dust, and the Formation of Cloud Cores and Protostars. IV. Effect of Ultraviolet Ionization and Magnetically Controlled Infall Rate

Nov 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...454..194c&link_type=abstract

Astrophysical Journal v.454, p.194

Astronomy and Astrophysics

Astronomy

113

Diffusion, Ism: Dust, Extinction, Ism: Magnetic Fields, Magnetohydrodynamics: Mhd, Stars: Formation, Accretion, Accretion Disks

Scientific paper

We extend our previous studies of the self-initiated formation and contraction of protostellar cores (due to ambipolar diffusion) in axisymmetric, isothermal, self-gravitating, disklike, thermally supercritical but magnetically subcritical model molecular clouds, to include the effect of the external (interstellar) ultraviolet radiation field. UV ionization dominates cosmic-ray ionization up to optical depths of about 10 and increases the degree of ionization in the envelopes of model clouds by more than 2 orders of magnitude. It thereby decreases by a similar factor the rate at which ambipolar diffusion progresses in the envelopes. We
follow the evolution of four model clouds to a central density enhancement of 106 (e.g., from 2.6 × 103 to 2.6 × 109 cm-3). Magnetically supercritical cores form on the initial central flux-loss timescale, which exceeds the dynamical timescale (≌ free-fall time) by a factor 10-20. As in the case of no UV radiation, a typical magnetically supercritical core consists of a uniform-density central region and a "tail" of infalling matter with a power-law density profile nn ∝ rs, -1.5 ≳ s ≳ s ≳ -1.85. Models that include the macroscopic (collisional) effects of grains have the evolution of their cores retarded (typically by 50%) with respect to models that account only for neutral-ion drag, independently of the effects of UV radiation. Model clouds that account for the effect of UV ionization have envelopes that are even better supported by magnetic forces than envelopes of models ionized only by cosmic rays. The effect that a well-supported envelope has on an oblate cloud's central gravitational field is to increase the field strength, which speeds up the evolution of a core in a typical model cloud by 30%. In all cases, the mass infall (or accretion) rate in (or from) the magnetically supported envelope is controlled by slow ambipolar diffusion. Ambipolar diffusion is so ineffective in the envelopes of model clouds with UV ionization that mass infall decreases precipitously outside the supercritical protostellar cores.

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