The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in the common envelope phase^⋆,⋆⋆

¹ Laboratoire Lagrange, UMR7293, Univ. Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
e-mail: Olivier.Chesneau@oca.eu
² Leiden Observatory, Leiden University Postbus 9513, 2300RA Leiden, The Netherlands
³ FNRS, Département AGO, Université de Liège, Allée du 6 Août 17, Bat. B5C, 4000 Liège, Belgium
⁴ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson AZ 85721, USA
⁵ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
⁶ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁷ Departamento de Física y Astronomá, Universidad de Valparaíso, Chile
⁸ South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
⁹ Geneva Observatory, Geneva University, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
¹⁰ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
¹¹ American Association of Variable Star Observers, 49 Bay State Road, Cambridge MA 02138, USA
¹² School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
¹³ Las Cumbres Observatory Global Telescope Network, Goleta CA 93117, USA
¹⁴ Department of Electrical Engineering and Center of Astro Engineering, Pontificia Universidad Catolica de Chile, Av. Vicuña Mackenna, 4860 Santiago, Chile
¹⁵ Astronomy, Cosmology and Gravitation Centre, Astronomy Department, University of Cape Town, 7700 Rondebosch, South Africa

Received: 1 August 2013
Accepted: 5 December 2013

Abstract

Context. Only a few stars are caught in the very brief and often crucial stages when they quickly traverse the Hertzsprung-Russell diagram, and none has yet been spatially resolved in the mass transfer phase.

Aims. We initiated long-term optical interferometry monitoring of the diameters of massive and unstable yellow hypergiants (YHG) with the goal of detecting both the long-term evolution of their radius and shorter term formation of a possible pseudo-photosphere related to proposed large mass-loss events.

Methods. We observed HR 5171 A with AMBER/VLTI. We also examined archival photometric data in the visual and near-IR spanning more than 60 years, as well as sparse spectroscopic data.

Results. HR 5171 A exhibits a complex appearance. Our AMBER data reveal a surprisingly large star for a YHG R_∗ = 1315 ± 260R_⊙  (or ~6.1 AU) at the distance of 3.6 ± 0.5 kpc. The source is surrounded by an extended nebulosity, and these data also show a large level of asymmetry in the brightness distribution of the system, which we attribute to a newly discovered companion star located in front of the primary star. The companion’s signature is also detected in the visual photometry, which indicates an orbital period of P_orb = 1304 ± 6 d. Modeling the light curve with the NIGHTFALL program provides clear evidence that the system is a contact or possibly over-contact eclipsing binary. A total current system mass of 39⁺⁴⁰_-22 M_⊙  and a high mass ratio q ≥ 10 is inferred for the system.

Conclusions. The low-mass companion of HR 5171 A is very close to the primary star that is embedded within its dense wind. Tight constraints on the inclination and vsini of the primary are lacking, which prevents us from determining its influence precisely on the mass-loss phenomenon, but the system is probably experiencing a wind Roche-Lobe overflow. Depending on the amount of angular momentum that can be transferred to the stellar envelope, HR 5171 A may become a fast-rotating B[e]/luminous blue variable/Wolf-Rayet star. In any case, HR 5171 A highlights the possible importance of binaries for interpreting the unstable YHGs and for massive star evolution in general.