The eclipsing binary HS0705+6700 binary and the search for circumbinary objects

2015 September 29

D. Pulley, G. Faillace, D. Smith, A. Watkins & C. Owen

Introduction

HS0705+6700 (hereafter HS0705) is a detached eclipsing binary system that was first investigated by Drechsel et al. in 2001. HS0705 is a member of the HW Vir family of short period binary systems that consist of a very hot subdwarf B type (sdB) star and a cool, low mass, main sequence star or brown dwarf. Their compact structure and the large temperature difference between the two components give rise to short and well defined primary eclipses, allowing times of minima to be determined with high precision. The sdB components of these systems have masses of ~0.5Mo and consist of a helium burning core with a thin hydrogen envelope, and are located at the left hand extremity of the horizontal branch in the H-R diagram.

Various evolutionary scenarios have been proposed for these stars, but a definitive mechanism remains to be established, in particular whether or not binary evolution, as outlined by Paczynski, Webbink & Zorotovic, is a requirement. These models suggest mass transfer from the primary to secondary component occurs at a rate that cannot be accommodated by the secondary component. This results in material filling first the Roche lobe of the secondary component and then the lobe of the primary, resulting in a common envelope enshrouding the binary system. Angular momentum is transferred from the binary system to the common envelope, bringing the binary pair closer together and resulting in a short binary period of typically between 2 and 3 hours. Eventually the common envelope has sufficient energy to be violently ejected from the system.

Zorotovic et al. provided an overview of thirteen of these systems in 2013. Interestingly in five of the 13 systems eclipse TTVs have been interpreted as showing the presence of low mass circumbinary objects, e.g. brown dwarfs, massive planets etc. While some of the cases remain unclear, the evidence provided for a third body, based upon the cyclical eclipse TTVs, increases. If such bodies do exist then they must either have survived the energetic common envelope ejection process or have formed during the short period since the common envelope was ejected.

In this paper we review the transit time variations exhibited by one such system, HS0705, over the last thirteen years and explore whether there is conclusive evidence that these TTVs can predict the presence of a third body. We first look at the documented historical measurements made on HS0705 and their subsequent analyses since 2001. We then discuss our 25 new measurements taken between late 2013 and early 2014, before completing a period and transit time variation analysis, using both unweighted and weighted data, on all known times of minimum. Finally we discuss the possible causes of the observed TTVs and the possibility of the presence of a third body, and potentially a fourth, before presenting our conclusions.

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