THE PHOTOSPHERES OF EMBEDDED YOUNG STELLAR OBJECTS

We present an extensive study of the infrared (Delta V = 2) CO absorpt ion bands in low-luminosity young stars at intermediate resolutions (R approximate to 1000), and at high resolution (R approximate to 17000) for a sub-sample. The bands are very common in YSOs and correlate wel l with SED class in the sense that Class II sources tend to show CO ab sorption while Class I sources do not. From the relative band strength s a lower limit on the CO excitation temperature of 2300 K is derived. The echelle spectra show that the majority of CO bandheads are broade ned by less than 17 km s(-1) and point to a stellar photospheric origi n rather than a disk; they also suggest that most YSOs are only slowly rotating. The band strengths are inversely correlated with the therma l infrared excess, which leads to a simple model in which the CO bands from the central PMS star are veiled by continuum emission from circu mstellar dust.