Heating of dense interstellar clouds by magnetic ion slip - A constraint on cloud field strengths

Abstract

Observations suggest that fragmentation into stars occurs at densities comparable to those of massive interstellar clouds. However, fragmentation cannot occur if the magnetic field strength and density are related by B = B0n with y = 2/3, as for isotropic contraction with a frozen-in field. A semiempirical upper limit to the parameter y is derived by considering the thermal equilibrium of a cloud heated by magnetic ion slip (ambipolar diffusion) and requiring that the theoretical temperatures do not greatly exceed observed temperatures. Cooling by rotational transitions of CO, H2, and HD and by inelastic gas-grain collisions is included in the equilibrium calculations. It is found that a value of y as large as 2/3 would result in gas temperatures in excess of 100 K at densities greater than 10 and can therefore be excluded by the observations. A value of y < 0.55 is required if the scale of the magnetic field gradient is of the same order as the cloud radius. The calculated value of this upper limit is sensitive to the choice of B0. The value of y can be decreased by preferential flow of gas along field lines or by magnetic ion slip. The time scale for magnetic ion slip is reexamined. Subject headings: interstellar: matter - magnetic fields - nebulae: general - stars: formation