Water Absorption From Line-of-Sight Clouds Toward W49A

We have observed 6 clouds along the line-of-sight toward W49A using the Submillimeter Wave Astronomy Satellite (SWAS) and several ground-based observatories. The ortho-H2O 1-0 and OH (1665 and 1667 MHz) transitions are observed in absorption, whereas the low-J CO, 13CO, and C18O lines, as well as the [CI] 1-0 transition, are seen in emission. By using both the o-H218O and o-H2O absorption lines, we are able to constrain the column-averaged o-H_2O abundances in each line-of-sight cloud to within about an order of magnitude. Assuming the standard N(H2)/N(CO) ratio of 10^4, we find N(o-H2O)/N(H2) = 8.1 x 10^-8 - 4 x 10^-7 for three clouds with optically thin water lines. In three additional clouds, the H$_2$O lines are saturated so we have used observations of the H218O ground-state transition to find upper limits to the water abundance of 8.2x 10^-8 - 1.5x10^-6. We measure the OH abundance from the average of the 1665 and 1667 MHz observations and find N(OH)/N(H2) = 2.3x10^-7 - 1.1x10^-6. The o-H2O and OH abundances are similar to those determined for line-of-sight water absorption features towards W51 and Sgr B2 but are higher than those seen from water emission lines in molecular clouds. However, the clouds towards W49 have lower ratios of OH relative to H2O column densities than are predicted by simple models which assume that dissociative recombination is the primary formation pathway for OH and H2O. Building on the work of Neufeld et al. (2002), we present photo-chemistry models including additional chemical effects, which can also explain the observed OH and H2O column densities as well as the observed H2O/CO abundance ratios.