DETECTION OF A TERTIARY BROWN DWARF COMPANION IN THE sdB-TYPE ECLIPSING BINARY HS 0705+6700

Abstract

HS 0705+6700 is a short-period (P=2.3 hours), close binary containing a hot sdB-type primary and a fully convective secondary. We have monitored this eclipsing binary for more than 2 years and as a result, 32 times of light minimum were obtained. Based on our new eclipse times together with these compiled from the literature, it is discovered that the O-C curve of HS 0705+6700 shows a cyclic variation with a period of 7.15 years and a semiamplitude of 92.4 s. The periodic change was analyzed for the light-travel time effect that may be due to the presence of a tertiary companion. The mass of the third body is determined to be M 3 sin i ′ = 0.0377(±0.0043) M ⊙ when a total mass of 0.617 M ⊙ for HS 0705+6700 is adopted. For orbital inclinations i ′ ≥ 32.8 • , the mass of the tertiary component would be below the stable hydrogen-burning limit of M 3 ∼ 0.072 M ⊙ , and thus it would be a brown dwarf. The third body is orbiting the sdB-type binary at a distance shorter than 3.6 astronomical units (AU). HS 0705+6700 was formed through the evolution of a common envelope after the primary becomes a red giant. The detection of a sub-stellar companion in HS 0705+6700 system at this distance from the binary could give some constraints on stellar evolution in such systems and the interactions between red giants and their companions.

Origin of apparent period variations in eclipsing post-common-envelope binaries

“…Qian et al (2009b) (Drechsel et al 2001;Niarchos et al 2003;Németh et al 2005;Kruspe et al 2007). They detected cyclic variations that were attributed to the light-travel time effect produced by the presence of a third object.…”

The quest for companions to post-common envelope binaries

“…The source was subsequently monitored by Qian et al (2009Qian et al ( , 2010a, and references therein). For NSVS1425, on the other hand, there is a dearth of data between 1999 and 2007, when Wils et al (2007) noted its eclipsing nature.…”

“…A large body of mid-eclipse times of HS0705 has been collected by Qian et al (2009Qian et al ( , 2010a and interpreted in terms of a third body with a mass of about 30 M Jup , orbiting the binary with a period of 7.15 yr. Çamurdan et al (2012) added a few timings and found a similar period. We obtained 18 more primary and three secondary eclipses of HS0705, significantly extending the coverage.…”

“…It was also realized that the detached post-common envelope binaries (PCEBs) represent more suitable laboratories for studying eclipse time variations than the accreting cataclysmic variables, in particular, the variety to which UX UMa belongs. With this in mind, the interpretation of eclipse time variations in terms of the light-travel time effect produced by a third body was revived in a series of papers by Qian et al (2009Qian et al ( , 2010a, and references therein).…”

“…Qian et al (2009Qian et al ( , 2010a summarized all the then available eclipse times and, fitting a sinusoidal to the data, suggested the presence of a brown dwarf orbiting the binary with a period of 7.15 yr.…”