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

Ph. Stee (1) , O. Delaa (1) , J. D. Monnier (2) , A. Meilland (1) , K. Perraut (3) , D. Mourard (1) , X. Che (2) , G. H. Schaefer (4) , E. Pedretti (5) , M. A. Smith (6, 7) , R. Lopes de Oliveira (8) , C. Motch (9) , G. W. Henry (10) , N. D. Richardson (11) , K. S. Bjorkman (12) , R. Bücke (13) , E. Pollmann (14) , J. Zorec (15) , D. R. Gies (11) , T. ten Brummelaar (11) , H. A. Mcalister (11) , N. H. Turner (11) , J. Sturmann (11) , L. Sturmann (11) , S. T. Ridgway (16)

1 LAGRANGE - Joseph Louis LAGRANGE
2 Department of Astronomy [Ann Arbor]
3 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble
4 Institut für Theoretische Physik [Heidelberg]
5 SUPA School of Physics and Astronomy [University of St Andrews]
6 College of Natural Science and Mathematics [Houston]
7 Department of Chemistry and Biochemistry [Tucson]
8 Universidade Federal de Sergipe, Departamento de Física
9 ObAS - Observatoire astronomique de Strasbourg
10 Center of Excellence in Information Systems
11 CHARA - Center for High Angular Resolution Astronomy
12 Ritter Astrophysical Research Center
13 Anna-von-Gierke-Ring
14 Emil-Nolde-Str
15 IAP - Institut d'Astrophysique de Paris
16 NOAO - National Optical Astronomy Observatory

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.

Mots clés: techniques: interferometric; stars: emission-line; Be; stars: winds; outflows; circumstellar matter; binaries: close

Domaines: Astrophysique stellaire et solaire [astro-ph.SR]; Astrophysique stellaire et solaire [astro-ph.SR]

Ph. Stee, O. Delaa, J. D. Monnier, A. Meilland, K. Perraut, et al.. 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. Astronomy and Astrophysics - A&A, 2012, 545, pp.59. ⟨10.1051/0004-6361/201219234⟩. ⟨hal-00766118⟩