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Dynamical few-body encounters in the dense cores of young massive starclusters are responsible for the loss of a significant fraction of theirmassive stellar content. Some of the escaping (runaway) stars move through theambient medium supersonically and can be revealed via detection of their bowshocks (visible in the infrared, optical or radio). In this paper, which is thesecond of a series of papers devoted to the search for OB stars running awayfrom young (several Myr) Galactic clusters and OB associations, we present theresults of the search for bow shocks around the star-forming region NGC 6357.Using the archival data of the Midcourse Space Experiment (MSX) satellite andthe Spitzer Space Telescope, and the preliminary data release of the Wide-FieldInfrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometryis consistent with the possibility that they are generated by stars expelledfrom the young star clusters, Pismis 24 and AH03 J1725-34.4, associated withNGC 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 thatthey are O-type stars as well, while the 2MASS photometry of the remaining twostars suggests that they could be B0 V stars, provided that both are located atthe same distance as NGC 6357. Detection of numerous massive stars ejected fromthe very young clusters is consistent with the theoretical expectation thatstar clusters can effectively lose massive stars at the very beginning of theirdynamical evolution and lends strong support to the idea that probably allfield OB stars have been dynamically ejected from their birth clusters. Aby-product of our search for bow shocks around NGC 6357 is the detection ofthree circular shells typical of luminous blue variable and late WN-typeWolf-Rayet stars.

Search for OB stars running away from young star clusters. II. The NGC 6357 star-forming region