STEREO observations of hybrid stars V775 Tau and V483 Tau

Abstract

In this study, pulsational and physical characteristics of two $\delta$ Scuti stars, V775 Tau and V483 Tau, are analysed by using four-year high-precision photometric data of STEREO satellite. Thus, it is aimed to gain new insights into behaviours of these pulsators and evolution of $\delta$ Scuti, $\gamma$ Dor and Am type stars. The data are taken between 2007--2011 and examined with the help of the Lomb-Scargle method. The detection precision in the four-year combined data is around $10^{-5}$ c/d in frequency and $10^{-5}$ mag in amplitude. It is revealed that V775 Tau exhibits weak pulsation characteristic which is interpreted as the existence of the interaction between the helium loss in the partial ionization zone and pulsation intensities. It is also considered that the absence of strong pulsations is also related to the evolution status of the star. Further, its periodogram shows low-frequency peaks. If these oscillations are g-modes, V775 Tau can be thought to be one of the rare stars that show all $\gamma$ Dor, $\delta$ Scuti and Am type variations. V483 Tau is comparatively more luminous, hotter and has higher rotational velocity. Therefore, although it shares the same region with V775 Tau in the H-R diagram, it is not considered to be an Am star. Yet, it exactly overlaps with the $\gamma\,$ Dor stars. These clues as well as g-modes detected in its periodogram indicate that V483\,Tau is a hybrid star. Finally, both V775 Tau and V483 Tau display period changes whose rates are between $10^{-3}$ and $10^{-4}$ $yr^{-1}$. Considering the $\delta$ Scuti nature, it may be speculated that these changes are non-evolutionary.