Pulsation properties of the $\beta$ Cephei star SY Equ from combined photometric and spectroscopic data

Abstract

We present the analysis of simultaneous multicolour $uvyI_{\rm C}$ photometry and low-resolution spectroscopy for the rapidly rotating $\beta$ Cephei star SY Equ. From the photometric time series, we confirm the dominant pulsation frequency, $f_1$ = 6.029 d$^{-1}$, and we find an evidence for two additional modes. In spectroscopy, the highest peak occurs at $f_{\rm a}$ = 0.197 d$^{-1}$ or its alias 0.803 d$^{-1}$. It can be interpreted either in terms of a binary motion or as the $g$-mode pulsation. In addition, we reveal the pulsation mode with frequency of about 6.029 d$^{-1}$, i.e. the same which dominates photometric variations, and a few new candidates. For the dominant frequency we obtain mode identification from the combined photometric and spectroscopic observations. From non-adiabatic pulsation calculations, we show that the frequency of the dominant mode in SY Equ is consistent with the stellar models of much lower effective temperatures than used in many papers.