2015ApJ...806...49S

We combine Herschel observations for a total of 12 sources to construct the most uniform survey of HF and H₂ O in our Galactic disk. Both molecules are detected in absorption along all sight lines. The high spectral resolution of the Heterodyne Instrument for the Far-infrared (HIFI) allows us to compare the HF and H₂O distributions in 47 diffuse cloud components sampling the disk. We find that the HF and H₂O velocity distributions follow each other almost perfectly and establish that HF and H₂O probe the same gas-phase volume. Our observations corroborate theoretical predictions that HF is a sensitive tracer of H₂in diffuse clouds, down to molecular fractions of only a few percent. Using HF to trace H₂in our sample, we find that the N(H₂O)-to-N(HF) ratio shows a narrow distribution with a median value of 1.51. Our results further suggest that H₂O might be used as a tracer of H₂–within a factor of 2.5–in the diffuse interstellar medium (ISM). We show that the measured factor of Å2.5 variation around the median is driven by true local variations in the H₂ O abundance relative to H₂throughout the disk. The latter variability allows us to test our theoretical understanding of the chemistry of oxygen-bearing molecules in the diffuse gas. We show that both gas-phase and grain-surface chemistry are required to reproduce our H₂O observations. This survey thus confirms that grain surface reactions can play a significant role in the chemistry occurring in the diffuse ISM ( 1000/cm3).