Correlation of HI shells and CO clumps in the outer Milky Way
  HI shells, which may be formed by the activity of young and massive stars, or
connected to energy released by interactions of high-velocity clouds with the
galactic disk, may be partly responsible both for the destruction of CO clouds
and for the creation of others. It is not known which effect prevails. We study
the relation between HI shells and CO in the outer parts of the Milky Way,
using HI and CO surveys and a catalogue of previously identified HI shells. For
each individual location, the distance to the nearest HI shell is calculated
and it is specified whether it lies in the interior of an HI shell, in its
walls, or outside an HI shell. The method takes into account irregular shapes
of HI shells. We find a lack of CO clouds in the interiors of HI shells and
their increased occurrence in walls. Properties of clouds differ for different
environments: interiors of HI shells, their walls, and unperturbed medium. CO
clouds found in the interiors of HI shells are those that survived and were
robbed of their more diffuse gas. Walls of HI shells have a high molecular
content, indicative of an increased rate of CO formation. Comparing the CO
fractions within HI shells and outside in the unperturbed medium, we conclude
that HI shells are responsible for approx. 20 % increase in the total amount of
CO in the outer Milky Way.
