2008ApJ...686..310H

The stellar content of M17 has been investigated by multicolor photometry and spectroscopy. Various independent estimates yield a distance of 2.1±0.2 kpc. The ratio of total-to-selective extinction is R=3.9. Within a projected area of 3.6x3.7 pc, there are several thousand stars. About 74% of them show infrared excess suggesting the presence of dense circumstellar material; the excess frequency is higher for fainter stars. The number of spectroscopically classified exciting stars could be enlarged from 13 to 46. The two central O4 stars are both spectroscopic binaries; multiplicity of other early O-type stars could also be established, increasing the number of high-mass stars even further. Our data suggest at least two episodes of star formation: There are about 500 ZAMS sources (2<A_V<7)–among them many spectroscopically classified OB stars and a significant fraction of lower mass sources with infrared excess (∼25%) and X-ray emission (∼6%). About 3350 heavily reddened sources with 10<A_V<40) are most likely deeply embedded pre-main-sequence objects with an age of less than 5x10⁵ yr. This group contains about 47% sources with infrared excess and 12% X-ray emitters. Cluster members later than about A0 have not yet reached the main sequence. In addition, a group of 647 protostellar candidates (1.5<K-L<6.9) has been detected in the cluster center as well as in the northern and southwestern bar. This population of accreting protostars argues in favor of ongoing star formation triggered by the central O stars in M17.