Triggered/sequential star formation? A multi-phase ISM study around the
  prominent IRDC G18.93-0.03
  G18.93-0.03 is a prominent dust complex within an 0.8deg long filament, with
the molecular clump G18.93/m being IR dark from near IR wavelength up to 160mu.
Spitzer composite images show an IR bubble spatially associated with G18.93. We
use GRS 13CO and IRAM 30m H13CO+ data to disentangle the spatial structure of
the region. From ATLASGAL submm data we calculate the gas mass, while we use
the H13CO+ line width to estimate its virial mass. Using HERSCHEL data we
produce temperature maps from fitting the SED. With the MAGPIS 20cm and
SuperCOSMOS Halpha data we trace the ionized gas, and the VGPS HI survey
provides information on the atomic hydrogen gas. We show that the bubble is
spatially associated with G18.93, located at a kinematic near distance of
3.6kpc. With 280Msun, the most massive clump within G18.93 is G18.93/m. The
virial analysis shows that it may be gravitationally bound and has neither
Spitzer young stellar objects nor mid-IR point sources within. Fitting the SED
reveals a temperature distribution that decreases towards its center, but
heating from the ionizing source puts it above the general ISM temperature. We
find that the bubble is filled by HII gas, ionized by an O8.5 star. Between the
ionizing source and the IR dark clump G18.93/m we find a layered structure,
from ionized to atomic to molecular hydrogen, revealing a PDR. Furthermore, we
identify an additional velocity component within the bubble's 8mu emission rim
at the edge of the infrared dark cloud and speculate that it might be shock
induced by the expanding HII region. While the elevated temperature allows for
the build-up of larger fragments, and the shock induced velocity component may
lead to additional turbulent support, we do not find conclusive evidence that
the massive clump G18.93/m is prone to collapse because of the expanding HII
region.
