DY\ Pegasi -- a SX Phoenicis Star in a Binary System with an Evolved Companion

Hui-Fang Xue, Jia-Shu Niu

Published 2020-08-06Version 1

In this work, the photometric data from AAVSO are collected and analyzed on the SX Phoenicis star DY Pegasi (DY Peg). From the frequency analysis, we get 3 independent frequencies: $f_0 = 13.71249\ \rm{c\ days^{-1}}$, $f_1 = 17.7000\ \rm{c\ days^{-1}}$, and $f_2 =18.138\ \rm{c\ days^{-1}}$, in which $f_0$ and $f_1$ are the radial fundamental and first overtone mode respectively, while $f_2$ is detected for the first time and should belong to a non-radial mode. The $O-C$ diagram of the times of maximum light shows that DY Peg has a period change rate $(1/P_0)(\mathrm{d} P_0/\mathrm{d} t) = -(5.87 \pm 0.03) \times 10^{-8} \ \mathrm{yr^{-1}}$ for its fundamental pulsation mode, and should belong to a binary system which has a orbital period $P_{\mathrm{orb}} = 15425.0 \pm 205.7 \ \mathrm{days}$. Based on the spectroscopic information, single star evolutionary models are constructed to fit the observed frequencies. However, some important parameters of the fitted models are not consistent with that from observations. Combing with the information from observation and theoretical calculation, we conclude that DY Peg should be a SX Phoenicis star in a binary system and accreting mass from a dust disk, which is the residue of its evolved companion (most probability a white dwarf at the present stage) in the AGB phase. Further observations are needed to confirm this inference, and it might be potentially a universal formation mechanism and evolutionary history for SX Phoenicis stars.