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 of a wide spectral coverage. In the case of the M2 supergiant μ 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 Tex ≈ 1600K, column densities of CO and H2O molecules Ncol ≈ 3.0×10 cm, and located at about one stellar radii above the photosphere or Rin ≈ 2.0R∗. The result shows reasonable agreement with the one based on the infrared spectra alone, and this may be because the infrared spectra already include some information on the spatial structure of the outer atmosphere. It is important, however, that the model inferred from the spectra is now fully supported with the recent visibility data. In the case of the M2 supergiant α Orionis, the infrared spectra and visibilities show a consistent picture in that its molsphere is closer to the photosphere (Rin ≈ 1.3R∗) with higher gas temperature (Tex ≈ 2250K) and lower gas column density (Ncol ≈ 10 cm), compared with that of μ Cep. Some controversy on the interpretation of the mid infrared data of α Orionis can be reconciled. Given that the presence of the extra molecular layer is reasonably well established, consistently with the spectral and visibility data, in at least two representative red supergiant stars α Orionis and μ Cephei, the major unsolved problem is how to understand the origin of such a rather warm and dense layer in the outer atmosphere. Subject headings: molecular processes – stars: individual (α Ori, μ Cep) – stars: supergiants – stars: late-type