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

Abstract

We have modeled the emission in the first-overtone rotation-vibrationbands of CO from accretion disks and neutral winds. We compare ourmodels with high-resolution spectra of five objects: DG Tau, SVS 13, WL16, NGC 2024 IRS 2, and S106 IRS 4. The emission from accretion diskswith accretion rates of {\tilde}10^{-8} to 10^{-7}M_{sun} yr^{-1} successfully reproduce the fluxes, theprofiles, and the optical depths of the observed spectra. We also findfor several objects that the data are best reproduced by the disk model,with higher K-band extinctions to the central star than those measuredby other methods. A simple wind model can also account for the flux andline profile in a number of cases, but fails to reproduce the highoptical 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 fromcooling adiabatically, the CO emission region is larger and lowermass-loss rates may be accommodated.