Prof. Gregor Rauw
Space Technology and Astrophysics Research Institute, University of Liège, Belgium
The sleepy gamma Cas star PZ Gem, an ogre about to wake up?
Abstract:
Be stars are rapidly rotating massive stars surrounded by a circumstellar disc of material expelled by the star and showing up in the spectrum via prominent, often double-peaked, emission lines notably of the Balmer hydrogen lines. Currently, the properties of these stars are explained as the result of mass exchange in a binary system. In this picture, the Be star would be the mass gainer and the mass donor should be either a stripped He star, a white dwarf, a neutron star or a black hole. Gamma Cas stars are a subgroup of Be stars showing exceptionally bright and hard X-ray emission, although it is weaker and softer than the emission typically seen in so-called Be High Mass X-ray binaries which consist of a Be star and a neutron star accreting material from it. High resolution X-ray spectroscopy of gamma Cas showed that the X-ray emission arises most likely via accretion onto a magnetic white dwarf companion.
In this contribution, I will highlight the case of the gamma Cas star PZ Gem (= HD45314). Over the last thirty years, the optical spectrum of PZ Gem has changed quite dramatically, indicating a progressive disc dissipation. On top of this trend, there were episodes of semi-regular variations which are typically attributed to the gravitational action of a companion star on the Be disc. In parallel, the X-ray emission of PZ Gem switched from a bright and hard emission, with plasma temperatures of more than 100 MK typical of gamma Cas stars, to a weaker and softer state. In this talk, I will present the latest results of our observational campaign that offers a detailed view on the behaviour of this star. This campaign involved a wealth of data from amateur and professional observers.
Although binarity is suggested, both by the gamma Cas - like X-ray emission and the semi-regular variations of the emission line strengths and morphologies, attempts to establish an orbital solution have failed so far. The X-ray spectra during the low disc state indicates some residual hard emission, though of significantly lower temperature than during the gamma Cas episode. In the context of the accreting white dwarf paradigm for the gamma Cas phenomenon, this hints at some left over accretion but via a different mechanism than during the high state.
After a short episode of nearly pure absorption lines in 2023, the disc features in the spectrum of PZ Gem are growing again in strength, indicating that the disc is currently rebuilding. It will be highly valuable to follow the steps of this phase in the coming years, both in the optical (via spectroscopy, photometry and polarisation measurements) and in the X-ray domain. Such observations are not only important to better understand the cyclic growth and dissipation of Be discs, but will also help to establish a possible threshold in disc density and extend required to wake up the accretion mechanism onto the putative white dwarf companion.