VLTI-GRAVITY measurements of cool evolved stars

II. Pulsation properties and mass-loss process of the Mira star R Car and the red supergiant VX Sgr

¹ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotláˇrská 2, 61137 Brno, Czech Republic
² European Southern Observatory (ESO), Karl-Schwarzschild Str. 2, 85748 Garching bei München, Germany
³ Université Côte d’Azur, Observatoire de la Côte d’Azur (OCA), CNRS, Lagrange, CS 34229, 06304, Nice Cedex 4, France
⁴ Max Planck Institute for Extraterrestrial Physics (MPE), Giessenbachstrasse 1, 85748 Garching bei München, Germany
⁵ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
⁶ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁷ Schmidt Sciences, New York, USA
⁸ National Science Foundation, 2415 Eisenhower Avenue, Alexandria, Virginia 22314, USA
⁹ Charles University, Faculty of Mathematics and Physics, Institute of Astronomy, V Holešovičkách 2, 18000 Prague, Czech Republic
¹⁰ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 January 2026
Accepted: 22 April 2026

Abstract

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 H₂O 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 (H₂O) 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 H₂O 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 H₂O 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.