Spectral and spatial imaging of the Be+sdO binary ϕ Persei^⋆,⋆⋆

¹ Laboratoire Lagrange, UMR 7293 UNS-CNRS-OCA, Boulevard de l’Observatoire, BP 4229, 06304 Nice Cedex 4, France
e-mail: denis.mourard@oca.eu
² Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
³ CHARA, Georgia State University, PO Box 3969, Atlanta, GA 30302-3969, USA
⁴ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁵ CNRS, IPAG, 38000 Grenoble, France
⁶ Physics and Astronomy Department, Vanderbilt University, 6301 Stevenson Center, Nashville, TN 37235, USA
⁷ CHARA Array, Mount Wilson Observatory, Mount Wilson, CA 91023, USA
⁸ University of Exeter, School of Physics, Stocker Road, Exeter EX4 4QL, UK
⁹ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Heriot-Watt University, Edinburgh, Scotland EH4 4AS, UK
¹¹ Université de Lyon, 69003 Lyon, France
¹² Université Lyon 1, Observatoire de Lyon, 9 avenue Charles André, 69230 Saint Genis Laval Cedex, France
¹³ CNRS, UMR 5574, Centre de Recherche Astrophysique de Lyon, Université Lyon, 19 Avenue Charles André, 69561 Saint Genis Laval Cedex, France
¹⁴ École Normale Supérieure, 69007 Lyon, France
¹⁵ National Optical Astronomy Observatory, PO Box 26732, Tucson, AZ 85726, USA

Received: 10 October 2014
Accepted: 24 February 2015

Abstract

Aims. The rapidly rotating Be star ϕ Persei was spun up by mass and angular momentum transfer from a now stripped-down, hot subdwarf companion. Here we present the first high angular resolution images of ϕ Persei made possible by new capabilities in long-baseline interferometry at near-IR and visible wavelengths. We analyzed these images to search for the companion, to determine the binary orbit, stellar masses, and fluxes, and to examine the geometrical and kinematical properties of the outflowing disk surrounding the Be star.

Methods. We observed ϕ Persei with the MIRC and VEGA instruments of the CHARA Array. MIRC was operated in six-telescope mode, whereas VEGA was configured in four-telescope mode with a change of quadruplets of telescopes during two nights to improve the (u,v) plane coverage. Additional MIRC-only observations were performed to track the orbital motion of the companion, and these were fit together with new and existing radial velocity measurements of both stars to derive the complete orbital elements and distance. We also used the MIRC data to reconstruct an image of the Be disk in the near-IR H-band. VEGA visible broadband and spectro-interferometric Hα observations were fit with analytical models for the Be star and disk, and image reconstruction was performed on the spectrally resolved Hα emission line data.

Results. The hot subdwarf companion is clearly detected in the near-IR data at each epoch of observation with a flux contribution of 1.5% in the H band, and restricted fits indicate that its flux contribution rises to 3.3% in the visible. A new binary orbital solution is determined by combining the astrometric and radial velocity measurements. The derived stellar masses are 9.6 ± 0.3 M_⊙ and 1.2 ± 0.2 M_⊙ for the Be primary and subdwarf secondary, respectively. The inferred distance (186 ± 3 pc), kinematical properties, and evolutionary state are consistent with membership of ϕ Persei in the α Per cluster. From the cluster age we deduce significant constraints on the initial masses and evolutionary mass transfer processes that transformed the ϕ Persei binary system. The interferometric data place strong constraints on the Be disk elongation, orientation, and kinematics, and the disk angular momentum vector is coaligned with and has the same sense of rotation as the orbital angular momentum vector. The VEGA visible continuum data indicate an elongated shape for the Be star itself, due to the combined effects of rapid rotation, partial obscuration of the photosphere by the circumstellar disk, and flux from the bright inner disk.