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Paper: Star Formation in M17
Volume: 5, Handbook of Star Forming Regions:
Volume II, The Southern Sky
Page: 625
Authors: Chini, R.; Hoffmeister, V.
Abstract: M17 is a region of active high-mass star formation at a distance of 2.1 kpc. It consists of a luminous H II region, a huge adjacent molecular cloud and an embedded young cluster with an age of about 5 × 105 years. Spectroscopic investigations suggest that stars later than about B8 have not yet reached the main sequence. In its center two O4 and several later O-type stars seem to have triggered the formation of further young objects at the interface between the H II region and the surrounding molecular cloud. Altogether, there are several thousand infrared stars within a projected area of 3.6 × 3.7 pc. The local extinction law is characterized by a ratio of total-to-selective extinction of R = 3.9. About 74% of the investigated cluster members show infrared excess suggesting the presence of dense circumstellar material; the excess frequency is higher for fainter stars. CO band-head features indicate that accretion is still going on in a large number of objects. Nearly a thousand sources in the field are known to exhibit X-ray emission. The spatial distribution of infrared excess sources is suggestive of triggered star formation with the youngest objects populating the outer cluster regions. Spatially resolved images of circumstellar disks and shells have been obtained for a number of high-mass sources, suggesting the accretion scenario to work also in the high-mass regime.
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