The relationship between γ Cassiopeiae’s X-ray emission and its circumstellar environment

II. Geometry and kinematics of the disk from MIRC and VEGA instruments on the CHARA Array

¹ Laboratoire Lagrange, UMR 7293 UNS-CNRS-OCA, Boulevard de l’Observatoire, BP 4229, 06304 Nice Cedex 4, France
e-mail: Philippe.Stee@oca.eu
² UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
³ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
⁴ Catholic University of America, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁵ Universidade Federal de Sergipe, Departamento de Física, Av. Marechal Rondon s/n, 49100-000 São Cristóvão, SE, Brazil
⁶ Université de Strasbourg, CNRS, Observatoire Astronomique, 11 rue de l’Université, 67000 Strasbourg, France
⁷ Center of Excellence in Information Systems, Tennessee State University, 3500 John A. Merritt Blvd., Box 9501, Nashville, TN 37209, USA
⁸ Center for High Angular Resolution Astronomy, Department of Physics & Astronomy, Georgia State University, PO Box 4106, Atlanta, GA 30202-4106, USA
⁹ Ritter Astrophysical Research Center, Department of Physics & Astronomy, University of Toledo, 2801 W. Bancroft, Toledo, OH 43606, USA
¹⁰ Anna-von-Gierke-Ring 147, 21035 Hamburg, Germany
¹¹ Emil-Nolde-Str. 12, 51375 Leverkusen, Germany
¹² National Optical Astronomical Observatory, 950 North Cherry Ave., Tucson, AZ 85719, USA
¹³ UPMC Univ. Paris 06, UMR 7095, Institut d’Astrophysique de Paris, 98bis Bd. Arago, 75014 Paris, France
¹⁴ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK

Received: 16 March 2012
Accepted: 1 August 2012

Abstract

Context.γ Cas is thought to be the prototype of classical Be stars and is the most studied object among this group. However, as for all Be stars, the origin and the physics of its circumstellar disk responsible for the observed near IR-excess, emission lines, and peculiar X-ray emission is still being debated.

Aims. We constrain the geometry and kinematics of its circumstellar disk from the highest spatial resolution ever achieved on this star. This investigation is a part of a large multi-technique observing campaign to obtain the most complete picture of γ Cas which emphasizes the relation of the circumstellar environment to the star’s X-ray flux.

Methods. We present new observations in the near infrared (MIRC) and in the visible (VEGA) obtained with the CHARA interferometer. The VEGA instrument allows us to not only obtain a global disk geometry but also spectrally dispersed visibility modulus and phases within the Hα emission line, which enables us to study the kinematics within γ Cas’s disk.

Results. We obtain a disk extension in the nearby Hα continuum of 1.72 stellar diameter and 1.86 stellar diameter in the H band at 1.65 μm assuming a Gaussian disk model but also compatible with an elliptical ring model with a minor internal diameter of 1.38 stellar diameter in H. For the first time we demonstrate that the rotation mapped by the emission in the Hα line within the disk of γ Cas and up to 10 R_⋆ is Keplerian.

Conclusions. These observations have pushed the size of the disk to greater proportions. γ Cas was also confirmed to be a nearly critical rotator. The disk imaging gives neither indication of a 1-arm spiral feature nor evidence of a secondary star reinforcing the interpretation that the secondary is certainly a low-mass and low-luminosity star or a degenerate companion.