An accreting dwarf star orbiting the S-type giant star π¹ Gru

¹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92195 Meudon CEDEX, France
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ University of Amsterdam, Anton Pannekoek Institute for Astronomy, 1090 GE Amsterdam, The Netherlands
⁴ Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 5060, Atlanta, GA 30302-5060, USA
⁵ School of Physics & Astronomy, Monash University, Wellington Road, Clayton 3800 Victoria, Australia
⁶ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Clayton 3800, Australia
⁷ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁸ California Institute of Technology, Jet Propulsion Laboratory, Pasadena, CA 91109, USA
⁹ Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹⁰ Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
¹¹ School of Mathematical and Physical Sciences, Macquarie University, Sydney, New South Wales, Australia
¹² Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
¹³ Departamento de Física, Universidad de Santiago de Chile, Av. Victor Jara 3659, Santiago, Chile
¹⁴ Center for Interdisciplinary Research in Astrophysics and Space Exploration (CIRAS), USACH, Chile
¹⁵ Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux, 33615 Pessac, France
¹⁶ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles (ULB), CP 226, 1060, Brussels, Belgium
¹⁷ Gothenburg University, Gothenburg, Sweden
¹⁸ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
¹⁹ University College London, Department of Physics and Astronomy, London WC1E 6BT, UK

^⋆ Corresponding author: miguel.montarges@observatoiredeparis.psl.eu

Received: 11 October 2024
Accepted: 23 April 2025

Abstract

Context. At the end of their lives, low- to intermediate-mass stars reach the asymptotic giant branch (AGB), during which their photospheres expand by up to several hundred times and strong stellar winds develop. These changes lead to various interactions with celestial bodies in their close circumstellar environments, including mass- and angular-momentum transfer.

Aims. We aim to characterize the properties of the inner companion of the S-type AGB star π¹ Gru and to identify plausible future evolutionary scenarios for this triple system.

Methods. We observed π¹ Gru with the Atacama Large Millimeter/sub-millimeter Array (ALMA) and the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument of the Very Large Telescope (VLT), collected archival photometric data, and used the HIPPARCOS-Gaia proper motion anomaly. We derived the best orbital parameters using Bayesian inference.

Results. In June-July 2019, the inner companion, π¹ Gru C, was located at 37.4±2.0 mas from the primary (a projected separation of 6.05±0.55 au at 161.7±11.7 pc). The best orbital solution yields a companion mass of 0.86^+0.22_−0.20 M_⊙ (using the derived mass of the primary) and a semi-major axis of 7.05_−0.57^+0.54 au, corresponding to an orbital period of 11.0_−1.5^+1.7 yr. The preferred solution is an elliptical orbit with eccentricity e = 0.35_−0.17^+0.18, although a circular orbit cannot be fully excluded. The close companion could be either a K1V_K7V^F9.5V star or a white dwarf (WD). Ultraviolet and millimeter continuum photometry are consistent with the presence of an accretion disk around the close companion. The ultraviolet emission may originate from hot spots in an overall cooler disk, or from a hot disk if the companion is a WD.

Conclusions. Although the close companion and the AGB star are interacting and an accretion disk is observed around the companion, the mass-accretion rate is too low to trigger a Type Ia supernova, but it could produce novæevery ≈900 yr. Short-wavelength, spatially resolved observations are required to further constrain the nature of the C companion. Searches for close-in companions similar to this system will improve our understanding of the physics of mass and angular momentum transfer, as well as orbital evolution during late evolutionary stages.