Hydrogen Fluoride in High-Mass Star-forming Regions

TL;DR

This study uses Herschel/HIFI observations to analyze hydrogen fluoride in high-mass star-forming regions, revealing how HF abundance varies with environment and is affected by processes like freeze-out and sputtering.

Contribution

First direct measurements of HF abundance in dense high-mass star-forming regions, highlighting the impact of freeze-out and sputtering on interstellar fluorine chemistry.

Findings

HF abundance in diffuse clouds is ~1.5e-8

Lower limit of HF abundance in dense clouds is 5e-10

HF is abundant in outflows due to sputtering

Abstract

Hydrogen fluoride has been established to be an excellent tracer of molecular hydrogen in diffuse clouds. In denser environments, however, the HF abundance has been shown to be approximately two orders of magnitude lower. We present Herschel/HIFI observations of HF J=1-0 toward two high-mass star formation sites, NGC6334 I and AFGL 2591. In NGC6334 I the HF line is seen in absorption in foreground clouds and the source itself, while in AFGL 2591 HF is partially in emission. We find an HF abundance with respect to H2 of 1.5e-8 in the diffuse foreground clouds, whereas in the denser parts of NGC6334 I, we derive a lower limit on the HF abundance of 5e-10. Lower HF abundances in dense clouds are most likely caused by freeze out of HF molecules onto dust grains in high-density gas. In AFGL 2591, the view of the hot core is obstructed by absorption in the massive outflow, in which HF is also very abundant 3.6e-8) due to the desorption by sputtering. These observations provide further evidence that the chemistry of interstellar fluorine is controlled by freeze out onto gas grains.