The water abundance behind interstellar shocks: results from Herschel/PACS and Spitzer/IRS observations of H2O, CO, and H2
Abstract
We have investigated the water abundance in shock-heated molecular gas, making use of Herschel measurements of far-infrared CO and H2O line emissions in combination with Spitzer measurements of mid-IR H2 rotational emissions. We present far-infrared line spectra obtained with Herschel's PACS instrument in range spectroscopy mode towards two positions in the protostellar outflow NGC 2071 and one position each in the supernova remnants W28 and 3C391. These spectra provide unequivocal detections, at one or more positions, of 12 rotational lines of water, 14 rotational lines of CO, 8 rotational lines of OH (4 lambda doublets), and 7 fine-structure transitions of atoms or atomic ions. We first used a simultaneous fit to the CO line fluxes, along with H2 rotational line fluxes measured previously by Spitzer, to constrain the temperature and density distribution within the emitting gas; and we then investigated the water abundances implied by the observed H2O line fluxes. The water line fluxes are in acceptable agreement with standard theoretical models for nondissociative shocks that predict the complete vaporization of grain mantles in shocks of velocity v 25 km/s, behind which the characteristic gas temperature is 1300 K and the H2O/CO ratio is 1.2
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