Identification of Ionizing Sources and Young Stellar Objects in M17

Abstract

We have obtained high-quality 2 mu m (K-band) spectra of 25 stars in thedirection of the heavily extincted, Galactic star-forming region M17.Interloping cool field giants are a potential source of confusion, butK-band spectra combined with normal JHK colors can identify anddistinguish these stars via their strong CO absorption features atlambda {\gt}= 2.29 mu m. Among the other sources, we have identified fivestars as O type from their spectral lines of H I, He I, He II, and N IIIby using a K-band classification system that we have developed. Theseprovide the number of Lyman-continuum photons required by radiocontinuum observations. The remaining stellar objects, all but one withstrong near-infrared (NIR) excesses, show completely different spectralcharacteristics from known main-sequence stars. Three are completelyfeatureless throughout the K band, four (possibly five) show CO bands inemission, and two (candidate FU Orionis--like objects) have CO bands inabsorption combined with extreme NIR excesses. We suggest that thesestellar objects are relatively massive young stellar objects, stillshrouded by circumstellar material.