A new dimension in the variability of AGB stars: Convection patterns size changes with pulsation^⋆

¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, Ciudad de México 04510, Mexico
e-mail: jarosales@astro.unam.mx
² European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Santiago, Chile
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516 751 20 Uppsala, Sweden
⁵ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁶ Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, PO Box 5060 Atlanta, GA 30302-5060, USA
⁸ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, France
⁹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
¹⁰ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, Boulevard du Triomphe 1050, Brussels, Belgium
¹¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹² Dipartimento di Fisica e Astronomia Galileo Galilei, Universita‘ di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹³ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko shose, 1784 Sofia, Bulgaria

Received: 28 December 2023
Accepted: 15 May 2024

Abstract

Context. Stellar convection plays an important role in atmospheric dynamics, wind formation, and the mass-loss processes in asymptotic giant branch stars. However, a direct characterization of convective surface structures in terms of size, contrast, and lifespan is quite challenging, as spatially resolving these features requires the highest angular resolution.

Aims. We aim to characterize the size of convective structures on the surface of the O-rich AGB star R Car to test different theoretical predictions based on mixing-length theory from solar models.

Methods. We used infrared low-spectral resolution (R ∼ 35) interferometric data in the H-band (∼1.76 μm) obtained by the instrument PIONIER at the Very Large Telescope Interferometer (VLTI) to image the star’s surface at two epochs separated by approximately six years. Using a power spectrum analysis, we estimated the horizontal size of the structures on the surface of R Car. The sizes of the stellar disk at different phases of a pulsation cycle were obtained using parametric model fitting in the Fourier domain.

Results. Our analysis supports that the sizes of the structures in R Car are correlated with variations in the pressure scale height in the atmosphere of the target, as predicted by theoretical models based on solar convective processes. We observed that these structures grow in size when the star expands within a pulsation cycle. While the information is still scarce, this observational finding highlights the role of convection in the dynamics of those objects. New interferometric imaging campaigns with the renewed capabilities of the VLTI are envisioned to expand our analysis to a larger sample of objects.