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 ~ 5-20 M☉). The masses of the stars have been determined fairly accurately through optical or near-infrared spectral classification. We find strong circumstantial evidence for disks around the massive YSOs in the following forms: near-infrared excess, optical veiling, CO band-head emission, and/or Paδ emission. We find a direct correlation between those YSOs that show CO band-head emission at 2.3 μm and those stars in our survey that show Paδ emission; in three of the four Paδ 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 identified 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 M☉, and they look to be very close to the predicted zero-age main 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 (1300+ 400−200 pc). While we attempt to determine an initial mass function for the cluster, it is incomplete even at high masses because of regions of extremely high extinction (AV > 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 diffuse interstellar bands (DIBs) along this sight line, over the extinction range of AV = 3-10. The DIBs over this extinction range show little change in spectral shape nor a significant increase in strength. We suggest the features are already saturated at small AV, or the material local to M17, where the increased extinction is being traced, does not contain the carriers of the DIB feature.

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