Interstellar H2O Masers from J Shocks
Abstract
We present a model in which the 22 GHz H2O masers observed in star-forming regions occur behind shocks propagating in dense regions (preshock density n0 106 - 108 cm-3). We focus on high-velocity (vs > 30 km s-1) dissociative J shocks in which the heat of H2 re-formation maintains a large column of 300-400 K gas; at these temperatures the chemistry drives a considerable fraction of the oxygen not in CO to form H2O. The H2O column densities, the hydrogen densities, and the warm temperatures produced by these shocks are sufficiently high to enable powerful maser action. The observed brightness temperatures (generally 1011 - 1014 K) are the result of coherent velocity regions that have dimensions in the shock plane that are 10 to 100 times the shock thickness of 1013 cm. The masers are therefore beamed towards the observer, who typically views the shock "edge-on", or perpendicular to the shock velocity; the brightest masers are then observed with the lowest line of sight velocities with respect to the ambient gas. We present numerical and analytic studies of the dependence of the maser inversion, the resultant brightness temperature, the maser spot size and shape, the isotropic luminosity, and the maser region magnetic field on the shock parameters and the coherence path length; the overall result is that in galactic H2O 22 GHz masers these observed parameters can be produced in J shocks with n0 106 - 108 cm-3 and vs 30 -200 km s-1. A number of key observables such as maser shape, brightness temperature, and global isotropic luminosity depend only on the particle flux into the shock, j=n0vs, rather than on n0 and vs separately.