Luminous blue variables: An imaging perspective on their binarity and near environment^⋆,⋆⋆

¹ European Organisation for Astronomical Research in the Southern Hemisphere, Alonso de Córdova 3107, Vitacura, 19001 Casilla, Santiago de Chile, Chile
e-mail: Christophe.Martayan@eso.org
² Royal Observatory of Belgium, 3 avenue Circulaire, 1180 Brussels, Belgium
³ European Organisation for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
⁴ Department of Astronomy, California Institute of Technology, 1200 E. California Blvd, MC 249-17, Pasadena, CA 91125, USA
⁵ Observatoire de Haute-Provence, CNRS/OAMP, 04870 Saint-Michel-l’ Observatoire, France
⁶ LESIA (UMR 8109), Observatoire de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁷ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁸ ESA/Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ ESO/ALMA − The Atacama Large Millimeter/Submillimeter Array, Alonso de Córdova 3107, Vitacura, Casilla 763 0355, Santiago, Chile
¹⁰ Sorbonne Universités, UPMC Université Paris 6 et CNRS, UMR 7095 Institut d’Astrophysique de Paris, 75014 Paris, France
¹¹ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, BP 53, 38041 Grenoble Cédex 9, France
¹² LUPM, Université de Montpellier, CNRS, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France

Received: 22 May 2015
Accepted: 1 December 2015

Abstract

Context. Luminous blue variables (LBVs) are rare massive stars with very high luminosity. They are characterized by strong photometric and spectroscopic variability related to transient eruptions. The mechanisms at the origin of these eruptions is not well known. In addition, their formation is still problematic and the presence of a companion could help to explain how they form.

Aims. This article presents a study of seven LBVs (about 20% of the known Galactic population), some Wolf-Rayet stars, and massive binaries. We probe the environments that surround these massive stars with near-, mid-, and far-infrared images, investigating potential nebula/shells and the companion stars.

Methods. To investigate large spatial scales, we used seeing-limited and near diffraction-limited adaptive optics images to obtain a differential diagnostic on the presence of circumstellar matter and to determine their extent. From those images, we also looked for the presence of binary companions on a wide orbit. Once a companion was detected, its gravitational binding to the central star was tested. Tests include the chance projection probability, the proper motion estimates with multi-epoch observations, flux ratio, and star separations.

Results. We find that two out of seven of LBVs may have a wide orbit companion. Most of the LBVs display a large circumstellar envelope or several shells. In particular, HD 168625, known for its rings, possesses several shells with possibly a large cold shell at the edge of which the rings are formed. For the first time, we have directly imaged the companion of LBV stars.