A Herschel/HIFI legacy survey of HF and H2 O in the galaxy: probing diffuse molecular cloud chemistry.
SONNENTRUCKER P., WOLFIRE M., NEUFELD D.A., FLAGEY N., GERIN M., GOLDSMITH P., LIS D. and MONJE R.
Abstract (from CDS):
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).