THOR - The HI, OH, Recombination Line Survey of the Milky Way - The
  pilot study: HI observations of the giant molecular cloud W43
  To study the atomic, molecular and ionized emission of Giant Molecular Clouds
(GMCs), we have initiated a Large Program with the VLA: 'THOR - The HI, OH,
Recombination Line survey of the Milky Way'. We map the 21cm HI line, 4 OH
lines, 19 H_alpha recombination lines and the continuum from 1 to 2 GHz of a
significant fraction of the Milky Way (l=15-67deg, |b|<1deg) at ~20"
resolution. In this paper, we focus on the HI emission from the W43
star-formation complex. Classically, the HI 21cm line is treated as optically
thin with column densities calculated under this assumption. This might give
reasonable results for regions of low-mass star-formation, however, it is not
sufficient to describe GMCs. We analyzed strong continuum sources to measure
the optical depth, and thus correct the HI 21cm emission for optical depth
effects and weak diffuse continuum emission. Hence, we are able to measure the
HI mass of W43 more accurately and our analysis reveals a lower limit of
M=6.6x10^6 M_sun, which is a factor of 2.4 larger than the mass estimated with
the assumption of optically thin emission. The HI column densities are as high
as N(HI)~150 M_sun/pc^2 ~ 1.9x10^22 cm^-2, which is an order of magnitude
higher than for low mass star formation regions. This result challenges
theoretical models that predict a threshold for the HI column density of ~10
M_sun/pc^2, at which the formation of molecular hydrogen should set in. By
assuming an elliptical layered structure for W43, we estimate the particle
density profiles. While at the cloud edge atomic and molecular hydrogen are
well mixed, the center of the cloud is strongly dominated by molecular
hydrogen. We do not identify a sharp transition between hydrogen in atomic and
molecular form. Our results are an important characterization of the atomic to
molecular hydrogen transition in an extreme environment and challenges current
theoretical models.
