Optical interferometry and Gaia measurement uncertainties reveal the physics of asymptotic giant branch stars

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange, CS 34229, Nice, France
e-mail: andrea.chiavassa@oca.eu
² European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
³ The CHARA Array, Mount Wilson Observatory, Mount Wilson, CA 91023, USA
⁴ Department of Physics and Astronomy at Uppsala University, Regementsvägen 1, Box 516, 75120 Uppsala, Sweden
⁵ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁶ University of Exeter, School of Physics and Astronomy, Stocker Road, Exeter, EX4 4QL, UK
⁷ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
⁸ Steward Observatory, 933 N. Cherry Avenue, University of Arizona, Tucson, AZ 85721, USA
⁹ Institut de Planétologie et d’Astrophysique de Grenoble, CNRS, Univ. Grenoble Alpes, Grenoble, France
¹⁰ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 26 February 2020
Accepted: 3 June 2020

Abstract

Context. Asymptotic giant branch (AGB) stars are cool luminous evolved stars that are well observable across the Galaxy and populating Gaia data. They have complex stellar surface dynamics, which amplifies the uncertainties on stellar parameters and distances.

Aims. On the AGB star CL Lac, it has been shown that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error. We observed this star with the MIRC-X beam combiner installed at the CHARA interferometer to detect the presence of stellar surface inhomogeneities.

Methods. We performed the reconstruction of aperture synthesis images from the interferometric observations at different wavelengths. Then, we used 3D radiative hydrodynamics (RHD) simulations of stellar convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the spectral channels of MIRC-X observations. Then, we determined the stellar radius using the average 3D intensity profile and, finally, compared the 3D synthetic maps to the reconstructed ones focusing on matching the intensity contrast, the morphology of stellar surface structures, and the photocentre position at two different spectral channels, 1.52 and 1.70 μm, simultaneously.

Results. We measured the apparent diameter of CL Lac at two wavelengths (3.299 ± 0.005 mas and 3.053 ± 0.006 mas at 1.52 and 1.70 μm, respectively) and recovered the radius (R = 307 ± 41 and R = 284 ± 38 R_⊙) using a Gaia parallax. In addition to this, the reconstructed images are characterised by the presence of a brighter area that largely affects the position of the photocentre. The comparison with 3D simulation shows good agreement with the observations both in terms of contrast and surface structure morphology, meaning that our model is adequate for explaining the observed inhomogenities.

Conclusions. This work confirms the presence of convection-related surface structures on an AGB star of Gaia DR2. Our result will help us to take a step forward in exploiting Gaia measurement uncertainties to extract the fundamental properties of AGB stars using appropriate RHD simulations.