Cloud-Cloud Collision Induced Star Formation in IRAS 18223-1243
  In the direction of l = 17.6 - 19 deg, the star-forming sites Sh 2-53 and
IRAS 18223-1243 are prominently observed, and seem to be physically detached
from each other. Sh 2-53 has been investigated at the junction of the molecular
filaments, while a larger-scale environment of IRAS 18223-1243 remains
unexplored. The goal of this paper is to investigate the star formation
processes in the IRAS site (area ~0.4 deg x 0.4 deg). Based on the GRS 13CO
line data, two molecular clouds, peaking at velocities of 45 and 51 km/s, are
found. In the position-velocity plots, a relatively weak 13CO emission is
detected at intermediate velocities (i.e. 47.5 - 49.5 km/s) between these two
clouds, illustrating a link between two parallel elongated velocity structures.
These clouds are physically connected in both space and velocity. The MAGPIS
data at 20 cm trace free-free continuum emission toward the IRAS 18223-1243
source. Using the Spitzer and UKIDSS photometric data, we have identified
infrared-excess young stellar objects (YSOs), and have observed their groups
toward the intersection zones of the clouds. IRAS 18223-1243 is also spatially
seen at an interface of the clouds. Considering these observational findings,
we propose the onset of the collision of two clouds in the IRAS site about 1
Myr ago, which triggered the birth of massive star(s) and the YSO groups. A
non-uniform distribution of the GPIPS H-band starlight mean polarization angles
is also observed toward the colliding interfaces, indicating the impact of the
collision on the magnetic field morphology.
