New evidence of the binary model for the $\delta$ Scuti star BS Aqr

Jian-ning FU¹ and Shi-yang JIANG^2,3 1. Observatoire Midi-Pyrénées, 14 avenue Édouard Belin, F-31400 Toulouse, France
2. Beijing Astronomical Observatory,Chinese Academy of Sciences,Beijing 100080,China
3. Bohyunsan Optical Astronomical Observatory, Kyungpook, 770-820, Korea

BS Aqr is a high amplitude $\delta$ Scuti star with long-term observations. There are different interpretations on the period change for this star in the literature based on the (O-C) calculations: the period is continuously decreasing or the period is actually decreasing due to both the intrinsic continuous decreasing combined with the light-time effect caused as part of a binary system (Fu et al. 1997, IBVS, No.4518). However, more new times of light maxima determined by the observations are needed to favor one of the present hypotheses. With the 6 new times of light maxima provided by Rodríguez et al. (1998, A&A, 336, 911) and the times collected in the bibliography, a new (O-C) calculation was performed.

Figure: The (O-C)_l diagram and the fit curve by using both the quadratic and the trigonometric functions for BS Aqr. $\triangle$ points based on the new times of light maxima provided by Rodríguez et al. (1998)

The quadratic and trigonometric functions, concerning the formula of $C_{orb}=T+PE+ 0.5\beta E^2+Asin\phi +Bcos\phi$, is used to fit all the times of light maxima, where $\phi$ is the solution of $\phi -esin\phi =2\pi f (PE-\tau)$. The related parameters are determined as: T=HJD2428095.3320, P=0.197822690 days, $\beta= -0.67\times 10^{-12}$ days/cycle, A=-0.0040 days, B=0.0005 days, P_orb=31.68 years, e=0.5, with the residual of 0.0035 days. The (O-C)_l diagram and the fit curve are shown in Figure 4. From Figure 4, one may find that the fitting is reasonable and favoring the binary hypothesis of Fu et al. (1997).