The effects of ambipolar diffusion, dust, and ultraviolet radiation in the formation of cores and protostars in interstellar molecular clouds

Details The effects of ambipolar diffusion, dust, and ultraviolet radiation in the formation of cores and protostars in interstellar molecular clouds The effects of ambipolar diffusion, dust, and ultraviolet radiation in the formation of cores and protostars in interstellar molecular clouds

Jan 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt.........9c&link_type=abstract

Ph.D. Thesis Illinois Univ., Urbana-Champaign, IL.

Astronomy and Astrophysics

Astrophysics

4

Ambipolar Diffusion, Cosmic Dust, Magnetohydrodynamics, Molecular Clouds, Plasma Diffusion, Protostars, Star Formation, Stellar Cores, Ultraviolet Radiation, Gravitational Collapse, Interstellar Matter, Interstellar Radiation, Magnetic Fields, Space Plasmas, Stellar Envelopes

Scientific paper

Star formation, a central problem in astrophysics, takes place within magnetically supported interstellar molecular clouds. The observed inefficiency of star formation in molecular clouds, in which only a small fraction of a cloud's mass is converted into stars, is explained naturally by magnetic support of the envelope and slow, inward drift of neutral matter with respect to plasma and magnetic field (ambipolar diffusion), which allows a cloud to gradually increase its central mass-to-flux ratio until it reaches the critical value for gravitational collapse. Beyond this stage, the magnetically supercritical core contracts dynamically, while the envelope remains magnetically supported. Interstellar grains, a significant component of the interstellar medium, can become charged, couple to the magnetic field, and decrease the rate of ambipolar diffusion in a cloud. In this thesis we model the self-initiated formation and contraction of cloud cores due to ambipolar diffusion in axisymmetric, self-gravitating, isothermal, magnetic molecular clouds, accounting for a cosmic abundance of interstellar grains (both charged and neutral), and an external (stellar) ultraviolet radiation field. The basic microscopic and macroscopic effects of grains are discussed. The magnetohydrodynamic equations for a multicomponent plasma are derived, the dimensionless free parameters are obtained, and their meaning is explained physically. The ion and electron fluids combine to form a system of 'quasiparticles', each having an effective charge Zeff = e(ni - ne)/ni (where ni and ne are the number densities of ions and electrons, and e the electronic charge). Electrostatic attraction between quasiparticles, which are attached to the magnetic field, and negatively charged grains couple the grains to the field even at densities at which grain-neutral collisions would normally detect them from the field. Also, neutral grains can couple to the magnetic field through (inelastic) charge capture. Interstellar UV radiation increases the degree of ionization in a cloud's envelope, with a corresponding decrease in the rate of ambipolar diffusion in the envelope. The results and their application to the formation of cores and protostars in molecular clouds are discussed.

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