Detection of hydrogen fluoride absorption in diffuse molecular clouds withHerschel/HIFI: an ubiquitous tracer of molecular gas

Abstract

We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along the sight line to the submillimeter continuum sources W49N and W51. We have used Herschel's HIFI instrument in dual beam switch mode to observe the 1232.4762 GHz J = 1−0 HF transition in the upper sideband of the band 5a receiver. We detected foreground absorption by HF toward both sources over a wide range of velocities. Optically thin absorption components were detected on both sight lines, allowing us to measure -as opposed to obtain a lower limit on -the column density of HF for the first time. As in previous observations of HF toward the source G10.6-0.4, the derived HF column density is typically comparable to that of water vapor, even though the elemental abundance of oxygen is greater than that of fluorine by four orders of magnitude. We used the rather uncertain N(CH) − N(H 2 ) relationship derived previously toward diffuse molecular clouds to infer the molecular hydrogen column density in the clouds exhibiting HF absorption. Within the uncertainties, we find that the abundance of HF with respect to H 2 is consistent with the theoretical prediction that HF is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen fluoride has the potential to become an excellent tracer of molecular hydrogen, and provides a sensitive probe of clouds of small H 2 column density. Indeed, the observations of hydrogen fluoride reported here reveal the presence of a low column density diffuse molecular cloud along the W51 sight line, at an LSR velocity of ∼24 km s −1 , that had not been identified in molecular absorption line studies prior to the launch of Herschel.

HERSCHEL /HIFI OBSERVATIONS OF HYDROGEN FLUORIDE TOWARD SAGITTARIUS B2(M)

“…The mean H 2 O/HF column density ratio in those intervals is 13.8 with a standard deviation of 0.58. The resulting H 2 O/HF ratio is thus significantly higher than that observed toward other submillimeter sources-such as W31C, W49N, and W51-where the H 2 O/HF ratio is close to unity and shows a remarkably small dispersion (Neufeld et al 2010;Sonnentrucker et al 2010). Our analysis indicates that the HF abundance along the sight line to Sgr B2(M) is similar to that inferred toward the other sources, and that the larger H 2 O/HF ratio in Sgr B2(M) reflects an enhanced H 2 O abundance along this sight line.…”

“…Based on the average gas-phase interstellar abundance in diffuse atomic gas clouds, N F /N H = 1.8 × 10 −8 (Snow et al 2007), our observations suggest a small F depletion factor of ∼3 along the line of sight to Sgr B2(M). Neufeld et al (2010) and Sonnentrucker et al (2010) estimated similar depletion factors toward G10.6-0.4 and W49N and W51, respectively. These results corroborate the theoretical prediction that HF is the main reservoir of gas-phase fluorine in a wide variety of interstellar conditions where the HF and H 2 column densities track each other closely, with the N(HF)/2N(H 2 ) ratio equal to the gas-phase elemental abundance of F relative to H. The HF abundance obtained here, which applies to relatively diffuse gas along the sight line to Sgr B2(M), is roughly two orders of magnitude larger than that inferred by Neufeld et al (1997) from observations of the HF J = 2-1 line.…”

“…In the upper panel of Figure 2 we present, for comparison, the spectrum of the 1 11 -0 00 1113.3430 GHz line of para-water, obtained using HIFI (Lis et al 2010). The para-water absorption spectrum shows a larger optical depth than that of HF at almost all LSR velocities, in contrast to what has been seen toward other Galactic sources, e.g., W51, W49N, and W31C, where the H 2 O/HF abundance ratio is remarkably constant (≈1) and the spectra have similar profiles (Sonnentrucker et al 2010;Neufeld et al 2010). In Figure 2 (lower panel) we compare the HF spectrum with 13 CO (1-0) and CS (2-1) absorption spectra toward Sgr B2(M) line of sight (observed with the IRAM 30 m telescope; Lis et al 2001).…”

“…The HIFI instrument, on board the Herschel Space Observatory, has enabled for the first time observations of the fundamental J = 1-0 rotational transition of HF at 1.232 THz to be performed at high spectral resolution (R > 10 6 ). The line has been detected in environments as diverse as Orion KL, OMC-1 (Phillips et al 2010), and in diffuse clouds on the line of sight toward W49N, W51 (Sonnentrucker et al 2010), and W31C (Neufeld et al 2010). This transition is generally observed in absorption, as expected, due to its very large A-coefficient, A 10 = 2.42 × 10 −2 s −1 .…”

Water Formation During the Epoch of First Metal Enrichment

“…This formation mechanism is generally not very efficient, and only a small fraction of the oxygen is converted into water; the rest remains in atomic form or freezes out as water ice (Hollenbach et al 2009). Sonnentrucker et al (2010) showed that the abundance of water vapor within diffuse clouds in the Galaxy is remarkably constant, with ∼ − is the H 2 O number density relative to the total hydrogen nuclei number density. Toward the Galactic center, this value can be enhanced by up to a factor of ∼3 (Monje et al 2011;Sonnentrucker et al 2013).…”