A shock origin for interstellar H₂O masers

Abstract

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.