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 \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 } \approx 1600 K , column densities of CO and H _ { 2 } O molecules N _ { col } \approx 3.0 \times 10 ^ { 20 } cm ^ { -2 } , and located at about one stellar radii above the photosphere or R _ { in } \approx 2.0 R _ { * } .
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 \alpha Orionis , the infrared spectra and visibilities show a consistent picture in that its molsphere is closer to the photosphere ( R _ { in } \approx 1.3 R _ { * } ) with higher gas temperature ( T _ { ex } \approx 2250 K ) and lower gas column density ( N _ { col } \approx 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 , consistently with the spectral and visibility data , in at least two representative red supergiant stars \alpha Orionis and \mu Cephei , the major unsolved problem is how to understand the origin of such a rather warm and dense layer in the outer atmosphere .