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

HS 0705+6700 (= GSC 4123 − 265) was listed as a dwarf candidate from the Hamburg Schmidt survey (Hagen et al. 1995). Follow-up spectroscopy by Heber et al. (1999) and Edelmann et al. (2001) revealed that its effective temperature lies in the predicted pulsational instability. Therefore, in order to search for pulsations, this star was included in a photometric monitoring programme at the Nordic Optical Telescope (see Ostensen et al. 2001a, 2001b). The observations indicated that it was an eclipsing binary (Drechsel et al. 2001). A detailed photometric and spectroscopic investigation was carried out by Drechsel et al. (2001) who discovered that HS 0705+6700 is a detached, short-period eclipsing binary. Absolute parameters of both components were determined suggesting the primary is a subluminous B (sdB) star, while the secondary is a cool stellar object that does not contribute to the total optical light apart from a strong reflection effect. These detections reveal that HS 0705+6700 is the third one of a small group of HW Vir-like eclipsing binary stars that consist of a very hot sdB type primary component and a fully convective M-type secondary with a period between 2 and 3 hr. Up to now, only six of this type of binaries have been discovered (e.g., Menzies & Marang 1986; Kilkenny et al. 1998; Drechsel et al. 2001; Ostensen et al. 2007; Polubek et al. 2007; Wils et al. 2007). The hot sdB components in this group of binaries are on the extreme horizontal branch of the Hertzsprung–Russell diagram, they burn helium in their cores, and have very thin hydrogen envelopes. They are formed through a common-envelope evolution (e.g., Han et al. 2003) and will evolve into normal cataclysmic variables (CV) (e.g., Shimansky et al. 2006).

As pointed out by Qian et al. (2008a), because of the compact structures and large temperature differences between the components, light curves of this group of binaries show a strong reflection effect with very sharp primary and shallow secondary minima. Therefore, eclipse times can be determined with a high precision (e.g., Kilkenny et al. 1994, 2000), and very small-amplitude orbital period variations could be detected by analyzing the observed–calculated (O–C) diagram. Orbital period variations of HW Vir, the prototype of this group of systems, were discovered (e.g., Kilkenny et al. 1994; Qian et al. 2008a; Lee et al. 2009), which show a combination of a cyclic variation and a long-term period decrease. The cyclic variation suggests the presence of a brown dwarf tertiary companion in the system, while the continuous decrease can be explained as secular angular momentum loss via magnetic braking of the fully convective component star or as a part of another long-period cyclic change via the existence of another companion. To search for the variations in the orbital period of HS 0705+6700, it has been monitored since 2006. Here we report the discovery of a cyclic change in the orbital period of HS 0705+6700 that reveals the presence of a tertiary, most likely a brown dwarf companion in this system.

Drechsel et al. (2001) published 13 times of light minimum of HS 0705+6700 and obtained the first linear ephemeris,

$$\begin{equation} {\rm Min}. I = {\hbox{HJD}}\,2451822.75982+0.09564665\times {E}, \end{equation} \tag{ 1 }$$

where HJD 2451822.75982 is the initial epoch and 0.09564665 is the orbital period. Later, some eclipse times were derived by Niarchos et al. (2003), Németh et al. (2005), and Kruspe et al. (2007). To search for the variations in the orbital period of HS 0705+6700, it was monitored from 2006 December to 2008 December by using four telescopes in China and Poland (the 85 cm and the 60 cm telescopes in Xinglong station of National Astronomical Observatories (NAO), and the 60 cm Mt. Suhora and the 50 cm Krakow telescopes in Poland). 38 eclipse times were obtained and they are listed in Table 1. The (O–C)₁ values of all available times of minima were calculated by using the ephemeris from Equation (1). The corresponding (O–C)₁ diagram is shown in Figure 1, where our 38 new minima times and the other 31 eclipse times collected from the sources noted above.

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As shown in Figure 1, the linear component of the orbital period of HS 0705+6700 needs revision and it appears that there is a cyclic variation as well. To describe the general (O–C)₁ trend satisfactorily, a new linear ephemeris is required (dashed line in Figure 1) with additional cyclic variations superimposed. Using the least-squares method, we determined

$$\begin{eqnarray} {\rm Min}. I&=&2451822.76090(\pm 0.00007)\nonumber \\ && +0.095646625(\pm 0.000000003)\times {E}\nonumber \\ && +0.00107(\pm 0.00007)\sin [0\hspace{1pt}\fdg 0132(\pm 0.0001)\nonumber\\ && \times {E}+237\fdg 2(\pm 3\fdg 6)]. \end{eqnarray} \tag{ 2 }$$

The derived orbital period is slightly shorter than that determined by Drechsel et al. (2001). The cyclic oscillation has an amplitude of 92.4 s and a period of 7.15 years. During the analysis, two timings of light minima, HJD 2451957.5274 and HJD 2454706.50400, were not used because their (O–C)₁ values show large scatter when compared with the general trend formed by the other data points. Actually, the eclipse minimum, HJD 2451957.5274, has been deleted by Drechsel et al. (2001) too, in their analysis.

The (O–C)₂ values calculated with the new linear ephemeris are plotted in the upper panel of Figure 2 where the cyclic change is seen more clearly. After the periodic change was subtracted from the (O–C)₂ curve, the residuals are displayed in the lower panel where no variations can be found indicating that Equation (2) gives a good fit to the (O–C)₁ curve.

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One cause of cyclic period change could be the magnetic activity cycles of the fully convective component (i.e., the Applegate mechanism; Applegate 1992). It is assumed in the mechanism that a certain amount of angular momentum is periodically exchanged between the inner and the outer parts of the convection zone, and therefore the rotational oblateness and thus the orbital period will vary when the cool component goes through its activity cycles. As in the cases of HW Vir and NN Ser (Qian et al. 2008a; Brinkworth et al. 2006), the fully convective secondary in HS 0705+6700 rotates mainly as a rigid body, and lacks the thin interface layer between a radiative core and a convective envelope, where dynamo processes are thought to concentrate at for solar-type stars (e.g., Barnes et al. 2005). The analyses for HW Vir and NN Ser indicated that the required energies are much larger than the total radiant energy of the M-type components, suggesting, that the mechanism of Applegate cannot interpret the cyclic period variations of the two systems. Moreover, as discussed by Qian et al. (2008a, 2008b), a more general explanation of the cyclic period changes in close binaries would be the light-travel time effect via the presence of a third body.

Therefore, we analyzed HS 0705+6700 for the light-time effect that arises from the gravitational influence of a third companion. The presence of a tertiary body produces the relative distance changes of the eclipsing pair as it orbits the barycenter of the triple system. Since the sine fit seems quite good, we assumed the orbit of the third body to be circular. With the absolute parameters determined by Drechsel et al. (2001), we derived the mass function and the mass of the tertiary companion as: f(m) = 1.25(±0.24) × 10⁻⁵ M_☉ and M₃sin i' = 0.0377(±0.0043) M_☉, respectively. The relations between the mass M₃ and the orbital radius d₃ of the tertiary component and its orbital inclination i' are displayed in Figure 3. When the orbital inclination of the third body is larger than 328, the mass of the tertiary component corresponds to 0.0377 M_☉ ⩽ M₃ ⩽ 0.072 M_☉. In this case, the tertiary component cannot undergo a stable hydrogen burning in the core, and it should be a brown dwarf. Therefore, with 63.6% probability, the third body is a substellar object (by assuming a random distribution of orbital plane inclination). However, depending on the unknown orbital inclination of the third body, a low-mass, stellar companion cannot be totally excluded but with a lower possibility of 36.4%.

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HS 0705+6700 has passed through the phase of a common envelope (CE) after the more massive component star in the original system evolves into a red giant. The ejection of CE removed a large amount of the angular momentum, and the present, short-period sdB-type binary has been formed. As it is shown in Figure 3, the orbital radius d₃ of the tertiary component is smaller than 3.6 AU. The detection of a brown dwarf or a very low-mass stellar companion in HS 0705+6700 at this distance, could give some constraints on the stellar evolution and the interaction between red giants and their companions.

Apart from cyclic period changes, a long-term period decrease was discovered in HW Vir which can be plausibly explained by secular angular momentum loss via magnetic braking of its fully convective component (Qian et al. 2008a; Lee et al. 2009). If this is true, a long-term period decrease could be discovered in HS 0705+6700. Actually, as displayed in Figure 1, the (O–C)₁ diagram can also be described by a combination of a cyclic change and a long-term period decrease. To check whether a long-term period decrease exists or not, more times of light minimum are required in the future.

This work is partly supported by Chinese Natural Science Foundation (No.10778718 and No.10778707), the National Key Fundamental Research Project through grant 2007CB815406, the Yunnan Natural Science Foundation (No. 2008CD157), and by the Special Foundation of President and West Light Foundation of the Chinese Academy of Sciences. New CCD photometric observations of the system were obtained with several small telescopes in China and in Poland. The authors thank the referee for useful comments and suggestions that helped to improve the original manuscript.