Identification of Ionizing Sources and Young Stellar Objects in M17

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Abstract
We have obtained high-quality 2 μm (K-band) spectra of 25 stars in the direction of the heavily extincted, Galactic star-forming region M17. Interloping cool field giants are a potential source of confusion, but K-band spectra combined with normal JHK colors can identify and distinguish these stars via their strong CO absorption features at λ ≥ 2.29 μm. Among the other sources, we have identified five stars as O type from their spectral lines of H I, He I, He II, and N III by using a K-band classification system that we have developed. These provide the number of Lyman-continuum photons required by radio continuum observations. The remaining stellar objects, all but one with strong near-infrared (NIR) excesses, show completely different spectral characteristics from known main-sequence stars. Three are completely featureless throughout the K band, four (possibly five) show CO bands in emission, and two (candidate FU Orionis-like objects) have CO bands in absorption combined with extreme NIR excesses. We suggest that these stellar objects are relatively massive young stellar objects, still shrouded by circumstellar material.

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