A Funnel Flow Origin for CO Bandhead Emission in Young Stellar Objects

Abstract

Many young stellar objects show near-infrared CO overtone emission. The observed bandhead profiles are consistent with gas motion in the gravitational field of a central mass and have been interpreted as arising from the innermost regions of a circumstellar disk. However, there is evidence that many of these pre-main-sequence stars possess strong magnetic fields that disrupt the inner disk and channel the gas onto the stellar surface, thereby forming an accretion funnel. It is demonstrated that this funnel can naturally give rise to the observed CO emission. It is shown that gas free-falling along the field lines yields bandhead profiles in agreement with those observed, with the shape of the profile determined mainly by the viewer's inclination. At large inclinations (with respect to the stellar rotation axis) the bandhead has a blue shoulder and a redshifted peak relative to the rest wavelength of the bandhead, whereas at small inclinations the bandhead is narrow. Using stellar parameters appropriate to a low-mass young stellar object and an accretion rate typical of a classical T Tauri star (~10-7 M☉ yr-1), the computed luminosity of the v = 2-0 bandhead is near the low end of the observed range. Since the predicted bandhead luminosity is proportional to the accretion rate, it is concluded that only young stellar objects with large accretion rates should have detectable CO emission, which is consistent with the observations. The lower overtone transitions of CO (e.g., v = 2-0) are not strongly affected by the ambient radiation field since the densities in the funnel flow (nH ≈ 1012 cm-3) are large enough to ensure LTE. However, higher transitions (v ≥ 4) are more susceptible to IR and UV pumping. The calculations imply that radiative effects leading to deviations from LTE will be most noticeable for sources with large bolometric luminosities.