Infrared CO Emission from Young Stars: High-Resolution Spectroscopy

We have detected the high-density, high-temperature, neutral material within 10-200 solar radii of five young stars through high-resolution spectroscopy of the first overtone bands of CO at 2.3 microns. We find a remarkable diversity in the band head profiles toward these sources, ranging from distinctly double-peaked (DG Tau), through broad bumps with emission extending up to 200 km/s from the stellar velocity (WL 16, NGC 2024 IRS 2, and S106 IRS 4), to narrow lines having a FWHM velocity of about 50 km/s (SVS 13). Both the profiles and the fluxes of WL 16, NGC 2024, and S106 can be fitted by emission from Keplerian accretion disks. DG Tau is well described by the superposition of emission from a Keplerian accretion disk and the diluted photospheric absorption features of an M0 dwarf. SVS 13 requires either a disk rotating at velocities slower than Keplerian, or a nearly face-on Keplerian disk plus an additional broad velocity component, possibly the acceleration region of a neutral disk wind.