Hydrogen in diffuse molecular clouds in the Milky Way

Atomic column densities and molecular fraction along prominent lines of sight^⋆

¹ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: bwinkel@mpifr.de
² Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, 75014 Paris, France
⁴ Sorbonne Universités, UPMC Univ. Paris 06, UMR 8112, LERMA, 75005 Paris, France
⁵ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA

Received: 27 March 2016
Accepted: 30 December 2016

Abstract

Context. Recent submillimeter and far-infrared wavelength observations of absorption in the rotational ground-state lines of various simple molecules against distant Galactic continuum sources have opened the possibility of studying the chemistry of diffuse molecular clouds throughout the Milky Way. In order to calculate abundances, the column densities of molecular and atomic hydrogen, H i, must be known.

Aims. We aim at determining the atomic hydrogen column densities for diffuse clouds located on the sight lines toward a sample of prominent high-mass star-forming regions that were intensely studied with the HIFI instrument onboard Herschel.

Methods. Based on Jansky Very Large Array data, we employ the 21 cm H i absorption-line technique to construct profiles of the H i opacity versus radial velocity toward our target sources. These profiles are combined with lower resolution archival data of extended H i emission to calculate the H i column densities of the individual clouds along the sight lines. We employ Bayesian inference to estimate the uncertainties of the derived quantities.

Results. Our study delivers reliable estimates of the atomic hydrogen column density for a large number of diffuse molecular clouds at various Galactocentric distances. Together with column densities of molecular hydrogen derived from its surrogates observed with HIFI, the measurements can be used to characterize the clouds and investigate the dependence of their chemistry on the molecular fraction, for example.