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Spectral lines which form in warm (several 1000 K) dense ($> 10^{10} \,{\rm cm}^{-3}$) gas such as the near IR vibration-rotation bands of CO are in principle powerful diagnostics of the inner regions of YSO disks and winds. Because of the similar densities and temperatures expected for inner disks and winds, high resolution line profiles are required to distinguish between a wind and disk origin for these spectral features.
In collaboration with J. Carr, F. Shu, and A. Tokunaga, we have obtained high resolution (R = 20,000) spectra of the CO vibrational overtone emission in a small sample of embedded YSOs and TTSs using the cryogenic echelle spectrograph at the IRTF (CSHELL). Thus far, several of the objects studied at high resolution show evidence for emission from rotating disks. The characteristic signature of bandhead emission from a rotating disk is clearly observed in the CO(v=2-0) bandhead emission from both the embedded YSO WL16 and the probable Herbig AeBe star 1548C27.
Recently, we have obtained high S/N spectra of both the CO(v=2-0) and CO(v=5-3) bandhead regions in 1548C27. The additional information provided by the CO(v=5-3) bandhead region is particularly valuable because lines from other CO overtone transitions also fall in this spectral region. These lines and the CO(v=5-3) bandhead together probe a much wider range of excitation temperatures than those contributing to the CO(v=2-0) bandhead. We demonstrate that by fitting both the CO(v=2-0) and CO(v=5-3) spectral regions we are able to place stringent constraints on the radial disk temperature distribution.
These observations constitute some of the most direct evidence to date for the existence of inner disks around YSOs and provide important constraints on the basic physical properties of YSOs and their surrounding disks.
