Infrared CO Emission from Young Stars: Accretion Disks and Neutral Winds

Abstract

We have modeled the emission in the first-overtone rotation-vibration bands of CO from accretion disks and neutral winds. We compare our models with high-resolution spectra of five objects: DG Tau, SVS 13, WL 16, NGC 2024 IRS 2, and S106 IRS 4. The emission from accretion disks with accretion rates of ˜10^(-8) to 10^(-7) M_(sun) yr^(-1) successfully reproduce the fluxes, the profiles, and the optical depths of the observed spectra. We also find for several objects that the data are best reproduced by the disk model, with higher K-band extinctions to the central star than those measured by other methods. A simple wind model can also account for the flux and line profile in a number of cases, but fails to reproduce the high optical depth obtained by fitting low-resolution data. Furthermore, unreasonably high mass-loss rates are needed to reproduce the flux. However, if there are sources of heating in the wind preventing it from cooling adiabatically, the CO emission region is larger and lower mass-loss rates may be accommodated.