1996A&A...306..427C

We present an extensive study of the infrared ({DELTA}V=2) CO absorption bands in low-luminosity young stars at intermediate resolutions (R≃1000), and at high resolution (R≃17000) for a sub-sample. The bands are very common in YSOs and correlate well with SED class in the sense that Class II sources tend to show CO absorption while Class I sources do not. From the relative band strengths a lower limit on the CO excitation temperature of 2300K is derived. The echelle spectra show that the majority of CO bandheads are broadened by less than 17 km/s and point to a stellar photospheric origin rather than a disk; they also suggest that most YSOs are only slowly rotating. The band strengths are inversely correlated with the thermal infrared excess, which leads to a simple model in which the CO bands from the central PMS star are veiled by continuum emission from circumstellar dust.