2013ApJ...773...70H

We present a model in which the 22 GHz H₂ O masers observed in star-forming regions occur behind shocks propagating in dense regions (preshock density n₀∼ 10⁶-10⁸/cm3). We focus on high-velocity (v_s ≳ 30 km/s) dissociative J shocks in which the heat of H₂re-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 10-100 times the shock thickness of ∼10¹³ cm. The masers are therefore beamed toward 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 H₂ O 22 GHz masers, these observed parameters can be produced in J shocks with n₀∼ 10⁶-10⁸/cm3 and v_s∼ 30-200 km/s. 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 = n₀v_s, rather than on n₀ and v_s separately.