A CO-multilayer outer atmosphere for eight evolved stars revealed with VLTI/AMBER

Abstract

We determine the physical parameters of the outer atmosphere of a sample of eight evolved stars, including the red supergiant α Scorpii, the red giant branch stars α Bootis and γ Crucis, the K giant λ Velorum, the normal M giants BK Virginis and SW Virginis, and the Mira star W Hydrae (in two different luminosity phases) by spatially resolving the stars in the individual carbon monoxide (CO) first overtone lines. We used the Astronomical Multi-BEam combineR (AMBER) instrument at the Very Large Telescope Interferometer (VLTI), in high-resolution mode ($\lambda /\Delta \lambda \approx 12000$) between 2.28 and 2.31 $\mu m$ in the K band.The maximal angular resolution is 10 mas, obtained using a triplet telescope configuration, with baselines from 7 to 48 m. By using a numerical model of a molecular atmosphere in a spherical shells (MOLsphere), called PAMPERO (an acronym for the "Physical Approach of Molecular Photospheric Ejection at high-angular-Resolution for evOlved stars"), we add multiple extended CO layers above the photospheric MARCS model at an adequate spatial resolution. We use the differential visibilities and the spectrum to estimate the size ($R$) of the CO MOLsphere, its column density ($N_{CO}$) and temperature ($T_{mol}$) distributions along the stellar radius. The combining of the ${\chi }^2$ minimization and a fine grid approach for uncertainty analysis leads to reasonable $N_{CO}$ and $T_{mol}$ distributions along the stellar radius of the MOLsphere.

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