CO band emission from MWC 349 - I. First overtone bands from a disk or from a wind?

We have obtained spectra in the K band of the peculiar B[e]-star MWC349. A 1.85-2.50 μm spectrum, measured at medium resolution, contains besides the strong IR continuum the first overtone CO bands, the hydrogen recombination lines of the Pfund series, and a number of other neutral atomic lines and ionic lines of low ionization, all in emission. Portions of the CO band and superposed Pfund series lines were observed at resolutions of 10-15 km s^-1. The Pfund lines have gaussian profiles with FWHMs of ∼ 100km s^-1, are optically thin, and are emitted in LTE. The CO band heads are formed in LTE at temperatures of 3500 to 4000 K. The width of the 2 → 0 band head indicates kinematic broadening of 50 to 60 km s^-1. A number of CO emission geometries were investigated by spectral modeling. The emission may occur at the inner edge of the rotating circumstellar disk. In this case, the disk must have an outer bulge which partly blocks the radiation so that the observer sees only a sector on the far side where radial velocities are small. Alternatively, the CO emission originates in a wind and lines have gaussian profiles. Fits to both these geometries are of equally good quality. In a third alternative where the fit is less convincing, the CO emission is optically thin and comes from an extended Keplerian disk. In all successful fits the CO column density is ∼ 5 ·10²⁰ cm^-2.