The Young Massive Stellar Objects of M17

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Abstract
We present a multiwavelength spectroscopic survey that reveals the ionizing cluster of M17 and uncovers a population of young stellar objects (YSOs) of high mass (M D 5È20 The masses of the M _). stars have been determined fairly accurately through optical or near-infrared spectral classiÐcation. We Ðnd strong circumstantial evidence for disks around the massive YSOs in the following forms : nearinfrared excess, optical veiling, CO band-head emission, and/or Pad emission. We Ðnd a direct correlation between those YSOs that show CO band-head emission at 2.3 km and those stars in our survey that show Pad emission ; in three of the four Pad emission stars this line is double peaked (suggestive of a bipolar wind or a rotating disk or envelope). Our data suggest that circumstellar material, possibly in the form of a disk, is prevalent among very young objects of fairly high mass. Based on considerations of disk lifetimes in other young clusters, the M17 cluster appears to be very young, perhaps less than 1 Myr. We have also identiÐed at least nine O stars and a couple of late-O/early-B stars, most behind more than 8 mag of visible extinction using either optical or near-infrared spectral types. Several stars have inferred masses in excess of 60 and they look to be very close to the predicted zero-age main M _ , sequence with an estimated age of about 1 Myr, consistent with the age of the massive YSOs in the cluster. We have used the O stars to determine the distance to M17, which assumes the stars to lie on the zero-age main sequence pc). While we attempt to determine an initial mass function for (1300 ~200 `400 the cluster, it is incomplete even at high masses because of regions of extremely high extinction (A V [ 20) in the cluster. We have also used the M17 O stars to study the dust properties in the local cloud and the behavior of the di †use interstellar bands (DIBs) along this sight line, over the extinction range of A V \ 3È10. The DIBs over this extinction range show little change in spectral shape nor a signiÐcant increase in strength. We suggest the features are already saturated at small or the material local to M17, A V , where the increased extinction is being traced, does not contain the carriers of the DIB feature.

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