Abstract
We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal of contaminating features associated with common molecules ("weeds"), the HF spectrum shows a P-Cygni profile, with weak redshifted emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 x 10^13 cm^-2 for the HF column density responsible for the broad absorption component. Using our best estimate of the H2 column density within the low-velocity molecular outflow, we obtain a lower limit of ~1.6 x 10^-10 for the HF abundance relative to hydrogen nuclei, corresponding to 0.6% of the solar abundance of fluorine. This value is close to that inferred from previous ISO observations of HF J=2--1 absorption towards Sgr B2, but is in sharp contrast to the lower limit of 6 x 10^-9 derived by Neufeld et al. (2010) for cold, foreground clouds on the line of sight towards G10.6-0.4.
Highlights
Hydrogen fluoride (HF) is expected to be the main reservoir of fluorine in the interstellar medium because it is produced by the exothermic reaction of F with H2 (Neufeld et al 2005; Neufeld & Wolfire 2009) and its very strong chemical bond makes this molecule relatively insensitive to UV photodissociation
After the removal of features attributed to CH3OH and SO2, the HF J = 1−0 spectrum shows a P-Cygni profile with a broad, blueshifted absorption at LSR velocities between about −45 and 9 km s−1 and a redshifted emission component at LSR velocities in the range 12 to 50 km s−1
The HF spectrum is strikingly similar to that of another transition with an extremely high critical density: the CO fundamental vibrational band (Beuther et al 2010) which shows the same combination of absorption at LSR velocities between about −25 and 12 km s−1 and weak emission extending to VLSR ∼ 30 km s−1
Summary
Hydrogen fluoride (HF) is expected to be the main reservoir of fluorine in the interstellar medium because it is produced by the exothermic reaction of F with H2 (Neufeld et al 2005; Neufeld & Wolfire 2009) and its very strong chemical bond makes this molecule relatively insensitive to UV photodissociation. The J = 2−1 rotational transition was observed in absorption towards Sgr B2, at a low spectral resolution using the ISO long-wavelength spectrometer (LWS). The HIFI instrument (de Graauw et al 2010) aboard the Herschel Space Observatory (Pilbratt et al 2010) has allowed observations of the ground state rotational transition of HF at 1.232 THz to be performed for the first time, at high spectral resolution. This transition is expected to be generally observed in absorption because of the very large A coefficient (e.g. Neufeld et al 2010). Extremely dense regions could possibly generate enough collisional excitation to yield an HF feature with a positive frequency-integrated flux
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