  Dynamical few-body encounters in the dense cores of young massive star
clusters are responsible for the loss of a significant fraction of their
massive stellar content. Some of the escaping (runaway) stars move through the
ambient medium supersonically and can be revealed via detection of their bow
shocks (visible in the infrared, optical or radio). In this paper, which is the
second of a series of papers devoted to the search for OB stars running away
from young (several Myr) Galactic clusters and OB associations, we present the
results of the search for bow shocks around the star-forming region NGC 6357.
Using the archival data of the Midcourse Space Experiment (MSX) satellite and
the Spitzer Space Telescope, and the preliminary data release of the Wide-Field
Infrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometry
is consistent with the possibility that they are generated by stars expelled
from the young star clusters, Pismis 24 and AH03 J1725-34.4, associated with
NGC 6357. Two of the seven bow shocks are driven by the already known O stars.
Follow-up spectroscopy of three other bow shock-producing stars showed that
they are O-type stars as well, while the 2MASS photometry of the remaining two
stars suggests that they could be B0 V stars, provided that both are located at
the same distance as NGC 6357. Detection of numerous massive stars ejected from
the very young clusters is consistent with the theoretical expectation that
star clusters can effectively lose massive stars at the very beginning of their
dynamical evolution and lends strong support to the idea that probably all
field OB stars have been dynamically ejected from their birth clusters. A
by-product of our search for bow shocks around NGC 6357 is the detection of
three circular shells typical of luminous blue variable and late WN-type
Wolf-Rayet stars.
