A Herschel/HIFI Legacy Survey of HF and H_2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry

We combine Herschel observations for a total of 12 sources to construct the most uniform survey of HF and H_2O 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 H_2O 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 H_2O probe the same gas-phase volume. Our observations corroborate theoretical predictions that HF is a sensitive tracer of H_2 in diffuse clouds, down to molecular fractions of only a few percent. Using HF to trace H_2 in our sample, we find that the N(H_2O)-to-N(HF) ratio shows a narrow distribution with a median value of 1.51. Our results further suggest that H_2O might be used as a tracer of H_2—within a factor of 2.5—in the diffuse interstellar medium (ISM). We show that the measured factor of ~2.5 variation around the median is driven by true local variations in the H_2O abundance relative to H_2 throughout 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 H_2O observations. This survey thus confirms that grain surface reactions can play a significant role in the chemistry occurring in the diffuse ISM (n_H ≤ 1000 cm^(−3)).