Orbital period of the symbiotic system AX Persei determined by the method of radial velocities

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Variable stars are stellar objects that change their brightness. There are many reasons of these changes, and the classification of variable stars is based on these reasons. In general, variable stars are divided to geometrical and physical variables. The reason for brightness variations of geometric variable stars is a change in the geometric arrangement of the star or binary system (rotation, eclipses) with respect to the observer. In the case of physical variable stars, the reason for brightness variations is related to a change in the intrinsic properties of the star due to stellar pulsations or explosive instabilities. AX Persei belongs to symbiotic stars – a special group of eruptive variable stars. These systems consist of a late-type giant and a hotter companion object, which is usually a white dwarf – the final evolutionary state of stars like our Sun. Both components are surrounded by a complex nebula. Eruptive activity of AX Persei is characterized by an alternation of quiescence and active periods, the latter one consisting of a series of individual outbursts. The basic characteristic of binary system is the orbital period – the time during which the components orbit their centre of mass. There are many methods to determine the orbital period of binary star. In our previous research, we determined the orbital period of AX Persei to 680.4 ± 4.3 days based on the analysis of its light curve. In this article, we present our determination of the orbital period of the symbiotic system AX Persei based on the analysis of its radial velocity values. The motion of an object is manifested by a change in the wavelength of the spectral lines in its spectrum due to the Doppler effect, the magnitude of which depends on the radial velocity – a component of velocity in the direction of the line of sight. The radial velocity of a binary component varies regularly with the orbital period. To construct the radial velocity curve, we used 75 radial velocities, where 45 values were collected from literature and 30 ones were derived using the cross-correlation analysis of the spectra obtained by amateur astronomers. The result of the period analysis of the radial velocities of AX Persei is the orbital period 681.2 ± 4.2 days, which is in good agreement with the value determined by our previous analysis of photometric variability of the symbiotic system as well as with the values derived by other researchers.

Pavol Mártonfi is a student of Faculty of Science of Pavol Jozef Šafárik University in Košice. He studies the second year of the master’s degree of Astronomy and Astrophysics at the Department of Theoretical Physics and Astrophysics. He is interested in symbiotic binaries. In his bachelor thesis he aimed to investigation of the long-term photometric activity of the symbiotic binary AX Persei. The article concerning this research was published in the Open European Journal on Variable Stars. In his diploma thesis he would like to investigate the spectroscopic characteristics of this binary. The first part of this research aimed to analysis of the radial velocity curve of AX Persei is published in this article.