Intriguing X-ray and optical variations of the γ Cassiopeiae analog HD 45314^★

¹ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liége, Allée du 6 Août, 19c, Bât B5c, 4000 Liège, Belgium
e-mail: rauw@astro.ulg.ac.be
² National Optical Astronomical Observatory, 950 N. Cherry Ave., Tucson, AZ, USA
³ Department of Physics and Astronomy, University of North Carolina at Greensboro, Greensboro, NC 27412, USA
⁴ Balmes, 2, 08784 Piera, Barcelona, Spain
⁵ Observatori Puig d’Agulles, Passatge Bosc 1, 08759 Vallirana, Barcelona, Spain
⁶ Kernersville Observatory, Kernersville, NC, USA
⁷ Hamburger Sternwarte, Universität Hamburg, Hamburg, Germany
⁸ Departamento de Astronomía, Universidad de Guanajuato, Guanajuato, Mexico
⁹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Ensenada 22800, Baja California, Mexico

Received: 16 August 2017
Accepted: 25 January 2018

Abstract

Context. A growing number of Be and Oe stars, named the γ Cas stars, are known for their unusually hard and intense X-ray emission. This emission could either trace accretion by a compact companion or magnetic interaction between the star and its decretion disk.

Aims. To test these scenarios, we carried out a detailed optical monitoring of HD 45314, the hottest member of the class of γ Cas stars, along with dedicated X-ray observations on specific dates.

Methods. High-resolution optical spectra were taken to monitor the emission lines formed in the disk, while X-ray spectroscopy was obtained at epochs when the optical spectrum of the Oe star was displaying peculiar properties.

Results. Over the last four years, HD 45314 has entered a phase of spectacular variations. The optical emission lines have undergone important morphology and intensity changes including transitions between single- and multiple-peaked emission lines as well as shell events, and phases of (partial) disk dissipation. Photometric variations are found to be anti-correlated with the equivalent width of the Hα emission. Whilst the star preserved its hard and bright X-ray emission during the shell phase, the X-ray spectrum during the phase of (partial) disk dissipation was significantly softer and weaker.

Conclusions. The observed behaviour of HD 45314 suggests a direct association between the level of X-ray emission and the amount of material simultaneously present in the Oe disk as expected in the magnetic star-disk interaction scenario.