Role of Dust in Protostar Formation

Abstract

We review the role of interstellar grains in the early, isothermal stages of self-initiated (due to ambipolar diffusion) star formation in massive, magnetically supported molecular clouds (up to a central density of about 1010 cm -3 ). Certain features of the formation and contraction of protostellar cores are virtually independent of the specific values of the (dimensionless) free parameters of the problem, provided that these values are kept within the observationally allowed range. (1) Core formation is a quasistatic process (i.e., one with negligible acceleration), occurring on the initial central flux-loss timescale. The neutrals contract through the essentially stationary magnetic field lines and the plasma (including grains). (2) Eventually a core becomes both thermally and magnetically supercritical and begins to contract dynamically. (3) Collapse, however, does not evolve into free fall at densities below 1010 cm -3. (4) The magnetically subcritical (but massive) cloud envelope evolves only on the much longer, local ambipolar-diffusion timescale. Beyond the well-known role of grains in the chemistry that determines the degree of ionization in static clouds, we quantify both their effect on ambipolar diffusion (hence, star formation) and the effect of the dynamics on grain abundance (hence, chemistry) in evolving dense cores. A number of commonly used assumptions are found to be incorrect.