VLTI-GRAVITY measurements of cool evolved stars

Aims . The mass-loss process of red supergiant (RSG) and asymptotic giant branch (AGB) stars and its relation to variability are poorly constrained. We aim to study the photosphere and near-surface atmospheric structure, where the mass-loss is initiated. For this purpose, we studied two oxygen-rich evolved stars: the Mira-type AGB star R Car and the RSG VX Sgr. Methods . We used the VLTI-GRAVITY instrument operating in the near-infrared K -band. Our sample comprises 54 VLTI-GRAVITY snapshots (18 R Car, 36 VX Sgr) taken over about 7 years, making it the largest VLTI time series dataset to date. We determined the angular diameter as a function of time for the continuum (photosphere) and selected atomic and molecular bands, i.e., lines of Ti I and Sc I as well as bands of H 2 O and CO. Furthermore, we compared the variability and atmospheric structure to state-of-the-art radiative-hydrodynamics CO5BOLD 3D simulations. Results . The radii of photosphere ( R ⋆ ) and extended atmospheric layers are variable and relate to the light curve with phase shifts. The near-photospheric layers show a maximum radius near visual brightness minima ( φ vis ∼ 0.4–0.6). Inner atomic (Ti I , Sc I ) and molecular (H 2 O) layers are further phase-shifted by ∆φ vis ~ 0.05. The more extended CO layers show longer, irregular periods and maximum extensions of ∼1.3–1.7 R ⋆ for R Car and of ∼1.5–2.2 R ⋆ for VX Sgr. Comparison with synthetic interferometric data of an AGB model based on several pulsation cycles in CO5BOLD simulations revealed a similar behavior. The photosphere shows regular pulsations, but with maximum diameters preceding minimum brightness ( φ vis < 0.5). The H 2 O layer showed a much weaker extension compared to our observations, while CO showed a good agreement. Furthermore, during the 2020–2021 season, VX Sgr exhibited an extreme mass-loss event similar to that of Betelgeuse, preceded by two strong shocks and culminating with the extreme expansion of H 2 O and CO layers, both up to ∼2.2 R ⋆ . Unexpectedly, during this event, we also detected Brackett γ in interferometric data as well as strong Balmer emission in optical spectra, both of which are also signatures of a shock propagating through the atmosphere. Conclusions . The Mira R Car showed an estimated photospheric radius of R ⋆ = 280 ± 25 R ⊙ , with a regular fundamental mode (FM) pulsation amplitude of ∼13% of R ⋆ . During its active cycle, the extreme RSG VX Sgr showed R ⋆ = 1556 ± 110 R e , with an FM pulsation amplitude of ∼13% of R ⋆ , the same as R Car. During its quiescent cycle, it showed a smaller value, R ⋆ = 1456 ± 108 R ⊙ , and low-amplitude pulsations near the first overtone (O1), only ∼4% of R ⋆ . This supports a steady mass-loss process for Mira stars related to stable large-amplitude FM pulsation, whereas the mass-loss process for RSGs may be dominated by extreme events connected to changes in the pulsation mode from low-amplitude O1 to large-amplitude FM pulsations.