17 - Star-gas interactions

Summary

Gas dynamical processes

Many phenomena occur when stars plunge through clouds of gas. Among the most dramatic is the formation of shocks and ionized wakes, especially around stars which emit strongly in the ultraviolet. The ionizing radiation can be produced either by the star directly, or from the bow shock accompanying supersonic motion. Although we will not usually include gas dynamic and radiative processes, this one is an exception since it is important. So we give a brief general discussion of the phenomenon. Then we will describe collisionless accretion, the slowing down of stars by gas, and modifications of the Jeans and two-stream instabilities.

Suppose a star moves supersonically through a cloud of hydrogen. (The role of heavier atoms is mainly to increase and complicate the radiation processes.) In the direction of motion there is a bow shock which embraces the star more tightly at high Mach numbers (v_*/v_sound). To estimate the temperature immediately behind the shock front, equate the thermal energy 3kT/2 to the kinetic energy of an atom, giving, where v_⊥100 is the inflow velocity (in units of 100kms^-1) normal to the shock surface. As this shock-heated gas flows behind the star, it expands and cools. Part of the cooling will be from free–free radiation, and part from the expansion itself. From standard formulae for free-free emission by an ionized gas, one learns that the timescale for substantial radiation is τ_ff ≈ 10⁶v₁₀₀n^-1 yr, where n is the gas number density cm^-3.