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 n 0 ~ 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 10-100 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 H2O 22 GHz masers, these observed parameters can be produced in J shocks with n 0 ~ 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.
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The research of D.J.H. and C.F.M. during the early portion of this work was supported in part by a NASA grant (RTOP 344-04-10-02) to the Center for Star Formation Studies, a consortium of theoretical researchers from NASA Ames, the University of California at Berkeley, and the University of California at Santa Cruz. C.F.M.'s research is also supported by NSF grants AST-0098365 and AST-1211729.
Hollenbach, David; Elitzur, Moshe; and McKee, Christopher F., "Interstellar H2O Masers from J Shocks" (2013). Physics and Astronomy Faculty Publications. 188.