Preface

One of the most enigmatic groups of stars is the classical Be stars of the main sequence (MS), which include rapidly rotating stars of spectral class B with emission lines in the spectrum. Despite the more than 150-year history of studying these objects, their nature remains mysterious. The appearance of emission lines in the spectra of Be stars is associated with the formation of a circumstellar disk, the formation mechanisms of which are debated. Many authors consider the important role of the magnetic field in the formation of the disk, although up to date no magnetic field has been detected in any Be star. Among Be stars, a small (about 1%) group of gamma Cassiopeia type stars stands out. Specific features of these stars include high X-ray luminosity, intermediate between the luminosity of OB stars and that of X-ray binary stars with Be components, a hard spectrum with a flux ratio in the hard (2-10 keV) and soft (0.2-2 keV) energy bands greater than 1.6, and variability X-ray radiation, including on minute scales. Under the assumption that the X-ray spectrum of gamma Cas stars is completely thermal, the temperature of the plasma emitting in X-rays can reach 20-30 keV or more. There are several alternative models for the formation of the X-ray spectrum of gamma Cas stars, which indicates that the nature of the X-rays from this group of stars is hardly clear.

Now 25 gamma Cas stars and 2 candidates are known. Fifteen of the total 27 objects are available for observation in Russia. The present Project involves carrying out optical observations of gamma Cas type stars with high time resolution (less than a minute) to study the superfast variability of the line profiles in their visible spectra. It is planned to carry out such observations at the BTA, at the 2.5-m telescope of the KGO, and at the 1.25-m telescope of the Crimean station of the SAI. Studies of optical variability will help to understand how different and if the nature of the variability of gamma type Cas stars differs from the variability of other B and Be stars. To analyze the rapid photometric variability of gamma Cas stars, observational data from the TESS satellite will be used. Together with the analysis of optical variability, studies of the variability of X-ray light curves from the gamma Cas type stars will be carried out from an analysis of archival observations on the XMM-Newton and Chandra satellites. To study the relationship between X-ray and optical variability, we will also use the optical spectra of gamma Cas stars from the BESS database. Studying the correlation between X-ray and optical variability will allow us to investigate how differ the parameters of the regions where optical and X-ray radiation are formed. An important part of the proposed project will be the polarization observations of the bright gamma Cas type stars at the MSP spectrograph of the 6-m russian telescope BTA to determine the upper limit of the global magnetic field of these stars and looking for the local magnetic fields.

A new part of the Project will also be the expansion of the number of the known gamma Cas type stars. The archival observations on the XMM-Newton satellite with a low-resolution EPIC camera will be used for that.

The realization of the Project helps us to understand both the nature of mysterious gamma Cas type stars and the mechanisms of how their circumstellar disks are formed. This project will shed light on understanding similar mechanisms for other Be stars.

Results of work in the first year

Project's observations were made on January 12, 2023 under the on the 6-meter russian BTA telescope with the MSS spectrograph with a circular polarization analyzer. Spectra of 5 gamma type Cas stars were obtained: zet Tau, HD 45995, V558 Lyr, V2156 Cyg and V810 Cas (total of 112 spectra with exposures from 15 to 600 seconds.). Reducing of all obtained spectra was performed.
On nights of June 22 and 23, 2023, observations of the gamma Cas type star Cas V558 Lyr (HD183362) were made using the 1.25-m telescope of the Crimean station of the State Astronomical Institute. 650 low-resolution spectra of this star were obtained in the wavelength range 4500-7035 angstroms. Reducing of all obtained spectra was made.

The magnetic field of the gamma Cas star was measured using 144 pairs of spectra obtained on November 4, 2020 (73 spectra), January 2 and 3, 2021 (69 spectra), and May 30, 2021 (2 spectra). The rms magnetic fields obtained from all measurements made on these dates are 78+/-94 Gauss, 66+/-43 Gauss, 183+/-39 Gauss and 70+/-35 Gauss, respectively. The possible values of the local magnetic fields of gamma Cas are estimated. These values seems to be in the range from 50 G to 1 kG.

It has been established that at least five gamma Cas stars (HD 45314, HD 90563, HD 119682, NGC 6649-9, HD 110432) have states of low and high X-rays. In this case, the model characteristics of X-ray spectra (luminosity, hardness, temperature of the emitting plasma, contribution of the power-law component, spectral index, and others) change little. It is assumed that such changes in the X-ray spectra of gamma Cas type stars may be compelled with changes in the orientation of the decretion disks of stars or depend on the orbital phase of degenerate stellar satellites, if those are present.
A detailed analysis of the light curves of the bright gamma Cas star HD 110432 (BZ Cru), extracted from 6 archive X-ray observations made from 2002 to 2019, has been made. Three large X-ray quasi-flares (up to 20 kiloseconds) and several hundred short microflares were detected. The regular variations in the stellar X-rays with periods from 2 minutes to 6.5 hours and a fraction of periodic emission from 10 to 20% were discovered by the CLEAN Fourier analysis method. Some of these variations coincide with the periods of optical variations obtained by other authors. The X-ray spectra of the star were modeled during large quasi-flares and outside them. Significant changes in the model characteristics of these spectra were revealed. It is concluded that the most probable mechanism for the formation of X-ray emission of a gamma Cas star is the interaction of the magnetic field of the disk with the local magnetic fields of the star.

A catalog of all known gamma Cas stars and candidates to those is compiled. The catalog is available at http://www.astro.spbu.ru/LPV/?q=gcstars. The physical parameters of the stars listed in the catalog have been determined.

Results of the first year of the Project are presented on 5 oral and one invited reports at 5 conferences. 3 papers were published in peer-reviewed journals and an article in the proceedings of the All-Russian Conference “Solar and Solar-Terrestrial Physics-2023”.