Publication details
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
| Authors | |
|---|---|
| Year of publication | 2026 |
| Type | Peer-reviewed scientific article |
| Magazine / Source | ASTRONOMY & ASTROPHYSICS |
| MU Faculty or unit | |
| Citation | |
| web | https://www.aanda.org/articles/aa/full_html/2026/07/aa59237-26/aa59237-26.html |
| Doi | https://doi.org/10.1051/0004-6361/202659237 |
| Keywords | shock waves; techniques: interferometric; stars: AGB and post-AGB; stars: late-type; stars: mass-loss; stars: winds; stars: outflows |
| Attached files | |
| Description | 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 (?vis ~ 0.4–0.6). Inner atomic (Ti I, Sc I) and molecular (H2O) layers are further phase-shifted by ??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 (?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 ? 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 Re, 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. |
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