Infrared spectra and visibilities as probes of the outer atmospheres of red supergiant stars

In the light of the recent results of the stellar interferometry, we examine the nature of the extra molecular layer outside the photosphere of red supergiant stars, so far studied mostly with the use of the infrared spectra. Although the visibility data are more direct probes of the spatial structure of the outer atmosphere, it is essential that they are analyzed in combination with the spectral data. In the case of the M2 supergiant mu Cephei, several sets of data, both spectra and visibilities, strongly suggested the presence of an extra molecular layer ( which we referred to as ''MOLsphere'' for simplicity), and the basic parameters of the MOLsphere are estimated to be excitation temperature T-ex approximate to 1600 K, column densities of CO and H2O molecules N-col approximate to 3.0 x 10(20) cm(-2), and located at about one stellar radius above the photosphere or R-in approximate to 2.0R(*). The result shows reasonable agreement with the one based on the infrared spectra alone, and the model inferred from the spectra is now fully supported with the recent visibility data. In the case of the M2 supergiant alpha Orionis, the infrared spectra and visibilities show a consistent picture in that its MOLsphere is closer to the photosphere (R-in approximate to 1.3R(*)) with higher gas temperature (T-ex approximate to 2250 K) and lower gas column density (N-col approximate to 10(20) cm(-2)), compared with that of mu Cep. Some controversy on the interpretation of the mid-infrared data of alpha Orionis can be reconciled. Given that the presence of the extra molecular layer is reasonably well established, the major unsolved problem is how to understand the origin of such a rather warm and dense layer in the outer atmosphere.