Fundamental CO-bands from the disk of MWC 349

M. Kraus ¹, C. Thum ², & E. Krügel ¹

¹ Max-Planck-Instiut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

² IRAM, 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 Saint-Martin-d'Hères, France

The existence of a bulge if circumstellar dust around the peculiar B[e]-star MWC 349 has been known fro a long time. The proposition that MWC 349 also has a disk is suppoted by observations of double-peaked emission lines, and by infrared speckle interferometry which reveals a disk-like structure of the dust emission. The clearest indicator for the circumstellar disk, however, is the detection of strong hydrogen maser line emission, whihc is associated with a keplerian rotating disk. Moreover, the emission of the first-overtone CO-bandheads ( ) at wavelengths about 2.3  seems to come from the inner edge of the disk.

Now, with the ISO short wavelength spectrometer, we have detected the fundamental bands of CO ( , near ). The bands appear in meission with a low contrast, about 5%, above the continum. As far as we know, it is the first detection of these fundamental bands in emission from a young stellar object. Compared to the first-overtone spectrum we found that the fundamental bands arise in a completely different part of the circumstellar disk. The temperature of the CO is here about 1000 K and therefore 25000 K lower than in the first-overtone bands so that this cooler CO component must be located farther out in the disk.

In addition, we found that for the firts-overtone emission the CO is in LTE whereas the fundamental bands seem to deviate from LTE. Non-LTE effects such as excitation and de-excitation by strong IR radiation field might play a much more importnat role and have to be taken into account.

So we end up with a picture of the Keplerian disk around MWC 349 consisting of four parts: a) The ionized disk atmosphere with the hydrogen maser emission, b) the inner edge of the disk with a very hot ( ) CO component emitting the first overtone bands, c) the cooler CO fundamental bands combined with a hot dust region ( ) and d) a cool ( ) region where the CO rotation lines should come from. The latter, however, could not be detected because the dust might still be optically thick at 1 mm.