THE EXTENDED ENVIRONMENT OF M17: A STAR FORMATION HISTORY

Abstract

M17 is one of the youngest and most massive nearby star-formation regions in\nthe Galaxy. It features a bright H II region erupting as a blister from the\nside of a giant molecular cloud (GMC). Combining photometry from the Spitzer\nGLIMPSE survey with complementary infrared (IR) surveys, we identify candidate\nyoung stellar objects (YSOs) throughout a 1.5 deg x 1 deg field that includes\nthe M17 complex. The long sightline through the Galaxy behind M17 creates\nsignificant contamination in our YSO sample from unassociated sources with\nsimilar IR colors. Removing contaminants, we produce a highly-reliable catalog\nof 96 candidate YSOs with a high probability of association with the M17\ncomplex. We fit model spectral energy distributions to these sources and\nconstrain their physical properties. Extrapolating the mass function of 62\nintermediate-mass YSOs (M >3 Msun), we estimate that >1000 stars are in the\nprocess of forming in the extended outer regions of M17.\n From IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim\nof a large shell structure ~0.5 deg in diameter (~20 pc at 2.1 kpc). We present\nnew maps of CO and 13CO (J=2-1) emission, which show that the shell is a\ncoherent, kinematic structure associated with M17 at v = 19 km/s. The shell is\nan extended bubble outlining the photodissociation region of a faint, diffuse H\nII region several Myr old. We provide evidence that massive star formation has\nbeen triggered by the expansion of the bubble. The formation of the massive\ncluster ionizing the M17 H II region itself may have been similarly triggered.\nWe conclude that the star formation history in the extended environment of M17\nhas been punctuated by successive waves of massive star formation propagating\nthrough a GMC complex.\n