Hydromagnetic Shock Waves and Their Infrared Emission in H i Regions

Abstract

The steady-state equations for shocks with Mach numbers 5, 10, 15, and 20 (4-16 km/sec) are solved, including a magnetic field between 0 and 1O~ gauss parallel to the shock front. The structure of the shock is calculated, taking into account cooling mechanisms that are effective in H i regions, such as collisions of H atoms with 0 atoms and C+, Si+, and Fe+ ions. Electron collisions with ions are also included, as are collisions of H atoms with 112 molecules. Collisional excitation of fine-structure and rotational levels re- suits in an infrared emission spectrum of twelve lines, whose intensities are calculated for various 112/H ratios between 1O~ and 10-'. Cooling times for the gas to return to its preshock temperature (1000 K) are evaluated, and are in the range 5 to 15 X 10~ years for H2/H < 1O~. Even shorter times are found for 112/H > 1O~. The very short cooling times are largely due to collisions of H atoms with 0 atoms. They mean that shock waves generated by cloud collisions or by expanding H ii regions behave much like isothermal shocks. Two rotational lines of 112, 8 j.~ (J = 6 -*4) and 12 ~ (J = 4 -* 2), are emitted with considerable intensity. They may be observable by ground-based instruments if H2/H is large enough