Short-term variability and mass loss in Be stars

V. Space photometry and ground-based spectroscopy of γ Cas^⋆

¹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: cborre@phys.au.dk
² European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
e-mail: dbaade@eso.org
³ Instytut Astronomiczny, Uniwersytet Wrocławski, ul. Kopernika 11, 51-622 Wrocław, Poland
⁴ Observatório Nacional, Rua General Joé Cristino 77, São Cristóvão RJ 20921-400 Rio de Janeiro, Brazil
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁶ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Casilla 19001, Santiago 19, Chile
⁷ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
⁸ Observatoire Astronomique du Mont Mégantic, Departement de Physique, Université de Montréal, CP 6128, Succursale: Centre-Ville, Montréal, QC H3C 3J7, Canada
⁹ Silesian University of Technology, Institute of Automatic Control, Akademicka 16, Gliwice, Poland
¹⁰ Department of Physics & Space Science, Royal Military College of Canada, PO Box 17000 Station Forces, Kingston, ON K7K 0C6, Canada
¹¹ Institute for Astrophysics, University of Vienna, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
¹² Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstrasse 25, 6020 Innsbruck, Austria

Received: 5 November 2019
Accepted: 6 February 2020

Abstract

Context. Be stars are physically complex systems that continue to challenge theory to understand their rapid rotation, complex variability, and decretion disks. γ Cassiopeiae (γ Cas) is one such star but is even more curious because of its unexplained hard thermal X-ray emission.

Aims. We aim to examine the optical variability of γ Cas and thereby to shed more light on its puzzling behaviour.

Methods. We analysed 321 archival Hα spectra from 2006 to 2017 to search for frequencies corresponding to the 203.5 day orbit of the companion. Space photometry from the SMEI satellite from 2003 to 2011 and the BRITE-Constellation of nano-satellites from 2015 to 2019 were investigated in the period range from a couple of hours to a few days.

Results. The orbital period of the companion of 203.5 days is confirmed with independent measurements from the structure of the Hα line emission. A strong blue versus red asymmetry in the amplitude distribution across the Hα emission line could hint at a spiral structure in the decretion disk. With the space photometry, the known frequency of 0.82 d⁻¹ is confirmed in data from the early 2000s. A higher frequency of 2.48 d⁻¹ is present in the data from 2015 to 2019 and possibly in the early 2000s as well. A third frequency at 1.25 d⁻¹ is proposed to exist in both SMEI and BRITE data. Seemingly, only a non-radial pulsation interpretation can explain all three variations. The two higher frequencies are incompatible with rotation.