THE YOUNG MASSIVE STELLAR OBJECTS OF M17

We present a multiwavelength spectroscopic survey that reveals the ion izing cluster of M17 and uncovers a population of young stellar object s (YSOs) of high mass (M similar to 5-20 M.). The masses of the stars have been determined fairly accurately through optical or near-infrare d spectral classification. We find strong circumstantial evidence for disks around the massive YSOs in the following forms: near-infrared ex cess, optical veiling, CO band-head emission, and/or Pa delta emission . We find a direct correlation between those YSOs that show CO band-he ad emission at 2.3 mu m and these stars in our survey that show Pa del ta emission; in three of the four Pa delta emission stars this line is double peaked (suggestive of a bipolar wind or a rotating disk or env elope). Our data suggest that circumstellar material, possibly in the form of a disk, is prevalent among very young objects of fairly high m ass. Based on considerations of disk lifetimes in other young clusters , the M17 cluster appears to be very young, perhaps less than 1 Myr. W e have also identified at least nine O stars and a couple of late-O/ea rly-B stars, most behind more than 8 mag of visible extinction using e ither optical or near-infrared spectral types. Several stars have infe rred 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 u sed the O stars to determine the distance to M17, which assumes the st ars to lie on the zero-age main sequence (1300(-200)(+400) pc). While we attempt to determine an initial mass function for the cluster, it i s 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 sta rs 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 A(V) = 3-10. The DIBs over this extinction range show little change in spectral shape nor a significant increase in str ength. We suggest the features are already saturated at small A(V), or the material local to M17, where the increased extinction is being tr aced, does not contain the carriers of the DIB feature.