Four color light curves and period changes investigation of the contact binary BX Peg

Highlights

• B, V, R_c and I_c light curves of BX Peg have been observed by the 1m telescope at Weihai Observatory of Shandong University.

• Photometric analysis and orbital period variation investigation are presented.

• BX Peg is a shallow contact W-type binary and may be a triply system.

Abstract

We presented BVR_cI_c light curves of BX Peg obtained by the Weihai Observatory 1.0-m telescope of Shandong University. The W-D code was used to analyze the four color light curves, it is shown that BX Peg is a W-subtype shallow contact binary with a fill-out factor of $f=14.6\%$. The asymmetric light curves were explained by a dark spot on the less massive hot component. A total of 328 times of minimum light were used to study the orbital period variation of BX Peg. We found that the orbital period of BX Peg was continuous decrease with a rate of $dp/dt=-2.07\times {10}^{-7}$ d yr${}^{-1}$ superimposed on a cyclic oscillation with a period of 57.8 yr. The secular period decrease should be caused by the angular momentum loss via magnetic stellar wind. The cyclic modulation is very likely to be produced by the light travel time effect due to a tertiary companion.

Introduction

BX Peg was identified to be a variable by Shapley and Hughes (1934). The light variability of this star exhibits W UMa type eclipsing binary variation. This type of eclipsing binary usually contains two main-sequence stars. These two component stars are in contact and sharing a common convective envelope. Though W UMa type eclipsing binaries have been analyzed more than forty years, they are still a challenge for testing stellar evolution models. Most W UMa type eclipsing binaries show O’Connell effect, the brightness at two maxima is different. Such systems usually have one or two F to K spectral type components with deep convective envelopes. The orbital period change investigation of W UMa type binaries is very helpful for the study of their structure and evolution. Many W UMa type binary stars exhibits cyclic orbital period variation, which can be explained either by the magnetic activity of one or both components (e.g. Applegate, 1992) or by the light travel time effect (LTTE) due to the presence of a third companion. A statistical study by Liao and Qian (2010) suggested that the existence of a tertiary companion is very often in W UMa type binary stars.

Recently, the detailed study of the light curves and period variation of BX Peg was carried out by Lee et al. (2009). They found that BX Peg is a W-subtype W UMa type system and displays very obvious O’Connell effect. A secular period decrease and two additional cyclic oscillation are found in the period changes of BX Peg. They though that the angular momentum loss cause the orbital period decrease, the long period modulation is due to the LTTE of a third body and the short term oscillation could be produced either by a second LTTE of a forth body or by the effect of magnetic activity. Therefore, BX Peg is a very interesting target to be studied. In this paper, we present a new light curve synthesis of BX Peg together with the orbital period investigation.