A shock origin for interstellar H2O masers
We present a comprehensive model for the powerful H2O masers observed in starforming regions. In this model the masers occur behind dissociative shocks propagating in dense regions (preshock density n o ≈ 106 – 108 cm−3). This paper focuses on high-velocity (ν s ≳ 30 km/s) dissociative shocks in which the heat of H2 reformation on dust grains maintains a large column of ≈ 300 – 400 K gas, where 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, where the maser is excited by thermal collisions with H atoms and H2 molecules. A critical ingredient in determining the shock structure is the magnetic pressure, and the fields required by our models are in agreement with recent observations. The observed brightness temperatures (generally ≈ 1011 – 1014 K) are the result of coherent velocity regions which have dimensions in the shock plane that are 5 to 50 times the postshock thickness.
KeywordsBrightness Temperature Shock Plane Shock Model Star Form Region Large Column
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