Hydrogen in diffuse molecular clouds in the Milky Way: Atomic column
  densities and molecular fraction along prominent lines of sight
  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,
HI, must be known. 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. Based on Jansky Very Large Array data, we employ
the 21 cm HI absorption-line technique to construct profiles of the HI opacity
versus radial velocity toward our target sources. These profiles are combined
with lower resolution archival data of extended HI emission to calculate the HI
column densities of the individual clouds along the sight lines. We employ
Bayesian inference to estimate the uncertainties of the derived quantities. 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.
