Hot Water In The ISM: Masing and Non-Masing Emission From Non-Dissociative Shocks

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Session 92 -- Dust and Gas in the ISM
Oral presentation, Friday, January 14, 10:15-11:45, Salon VI Room (Crystal Gateway)

[92.01] Hot Water In The ISM: Masing and Non-Masing Emission From Non-Dissociative Shocks

M.J. Kaufman, D.A. Neufeld (JHU)

We investigate the possibility that dense non-dissociative shocks may be a source of water maser emission in regions of active star formation. Recent observations of maser line ratios in several star forming regions (Melnick et al. 1993 ApJ 416, L37) indicate that water masers are excited in $\rm T>1000\,K$ gas, temperatures too high for molecular emission behind dissociative shocks. We solve for the structure of, and emission from, multi-fluid shocks in gas with $\rm n(H_2)>10^7\, cm^{-3}$ and $\rm V_{shock}< 50\, km\, s^{-1}$, using new treatments of molecular cooling and ion-neutral coupling in dense gas. Such high densities are required by maser collisional pumping schemes. In this gas, the fractional ionization is low and carried on grains; results are presented for a variety of assumed grain size distributions and as a function of shock velocity, magnetic field and preshock density. Suitable preshock conditions yield individual masing regions with sizes of $\rm \sim 10^{13} \,cm$, consistent with interferometric observations of 22 GHz maser spots, and peak masing gas temperatures of $\rm \sim few\times\, 10^{3} \, K$, consistent with the temperatures inferred from maser line ratios. Although these masers are an `exotic' manifestation of the passing shock waves, most of the shock energy emerges in non-masing rovibrational line emission from $\rm H_2O,\,\,OH,\,\,CO\,\, and \,\, H_2$, and we investigate this emission from shocks with densities as low as $\rm n(H_2)\sim 10^5\,cm^{-3}$. Our study of the expected $\rm H_2O$ far-IR line emissions is motivated, in particular, by the possibility of observing such emissions with the European Space Agency's Infrared Space Observatory.

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