183 Acta Polytechnica CTU Proceedings 2(1): 183–187, 2015 183 doi: 10.14311/APP.2015.02.0183 Close-In Substellar Companions and the Formation of sdB-Type Close Binary Stars L. Y. Zhu1,2,3, S. B. Qian1,2,3, E.-G. Zhao1,2, E. Fernández Lajús4,5, Z.-T. Han1,2,3 1Yunnan Observatories, Chinese Academy of Sciences, P.O. Box 110, 650011 Kunming, P.R. China 2Key Laboratory for the Structure and Evolution of Celestial Bodies, Chinese Academy of Sciences, P.O. Box 110, 650011 Kunming, P. R. China. 3Physical Sciences Department, Graduate University of the Chinese Academy of Sciences, Yuquan Road 19, Sijingshang Block, 100049 Beijing, China. 4Facultad de Ciencias Astronómicasy Geof́ısicas, Universidad Nacional de La Plata, 1900 La Plata, Buenos Aires, Argentina. 5Instituto de Astrofisica de La Plata (CCT La plata - CONICET/UNLP), Argentina. Corresponding author: zhuly@ynao.ac.cn Abstract The sdB-type close binaries are believed to have experienced a common-envelope phase and may evolve into cataclysmic binaries (CVs). About 10 % of all known sdB binaries are eclipsing binaries consisting of very hot subdwarf primaries and low-mass companions with short orbital periods. The eclipse profiles of these systems are very narrow and deep, which benefits the determination of high precise eclipsing times and makes the detection of small and close-in tertiary bodies possible. Since 2006 we have monitored some sdB-type eclipsing binaries to search for the close-in substellar companions by analyzing the light travel time effect. Here some progresses of the program are reviewed and the formation of sdB-type binary is discussed. Keywords: hot subdwarf binary - photometry - individual: NY Vir, HS 0705+6700, NSVS 07826147. 1 Introduction Hot subdwarf stars (sdB) lie in the extreme horizontal branch (EHB). They are burning helium in their cores and have very thin hydrogen envelopes. After leaving the main sequence, the progenitors of sdBs would evolve to red giant, ignite helium and settle down on the EHB (Heber 2009). They have high temperatures and gravi- ties. To explain these characters, their progenitors must have experienced enhanced mass loss at the tip of red gi- ant branch (Castellani and Castellani, 1993; D’Cruz et al., 1996). However, what causes this extreme mass loss remains an open question. Many authors suggested that the mass loss is triggered by the presence of a compan- ion. In 2007, Silvotti et al., found a giant planet orbiting around a sdB star, V391 Pegasi, at a distance of 1.7 AU. After that, an even more extreme case was published by Charpinet et al., (2011). They reported the presence of two nearly Earth-sized bodies orbiting the sdB star KIC 05807616 at distances of 0.0060 AU and 0.0076 AU, with orbital periods of 5.7625 and 8.2293 hours, respec- tively. These detections support the way to form single sdB stars that these bodies were originally giant planets immersed in the red-giant envelope and massive enough to survive engulfment and triggered the enhanced mass loss necessary for the formation of an sdB star. So the close-in substellar companions paly an important role to form single sdB stars. Actually, about half (or may be more) of the sdBs reside in close binaries (Morales-Rueda et al., 2006). Are there any substellar objects around sdB close bi- naries just like the substellars founded around the sin- gle sdB stars? Recently, some circumbinary substel- lars orbiting around sdB eclipsing binaries were in- deed detected, such as HW Vir (Lee et al., 2009), HS 0705+6700 (Qian et al, 2009), NY Vir (Qian et al., 2012) and SDSSJ0820+0008 (Geier et al, 2012). The sdB close binaries are believed to be formed from binary systems through common envelope(CE) ejection (Han et al., 2002, 2003). Because the separation of the two components in sdB close binaries is much less than the size of the subdwarf progenitor in its red-giant phase, these systems must have experienced a CE phase. Af- ter spiral in and envelope ejection, the present binary system consisting of a sdB star and a low-mass compan- ion star was formed with short periods. The presence of substellar tertiary, especially the close-in objects, orbit- ing sdB close binaries provides some clues to the inter- 183 http://dx.doi.org/10.14311/APP.2015.02.0183 L. Y. Zhu et al. active effect between them, and may give evidences of the formation of the planets and the sdB close binaries. 2 Targets and Method For searching for close-in substellar companions of the sdB close binaries, we chose eclipsing sdB close binaries (i.e. HW Vir binaries) as our targets to monitor be- cause their eclipsing profiles provide us with very use- ful information about their orbital period variations, which may be caused by the presence of third bodies. HW Vir binaries have been known for more than four decades. They are detached eclipsing binaries composed of a sdB star and a low-mass companion star. Till now, not more than fifteen of such objects were discovered. Their well-detached configuration indicates that their eclipses might not be influenced by the accretion-driven radiations from other parts (e.g., the accretion discs and hot spots etc.) observed in CVs. And the com- pact structures of both components make the eclipses in the light curves very narrow and deep. These charac- teristics favour a highly precise determination of their times of light minimum, generally with errors less than 0.0001days. Thus very small amplitude orbital period variations due to the substellar tertiaries can be de- tected by analyzing the observed-calculated (O-C) di- agrams. Therefore they are the most hopeful targets for detecting substellar objects around them with light time effect. Using 2.4-m, 1-m and 60-cm telescopes in Yunnan Observatories, 2.16-m and 85-cm telescopes in Xinglong station of National Observatories and 2.15-m Jorge Sahade telescope in Argentina, we began to mon- itor HW Vir binaries since 2006. Our targets are listed in Table 1. Table 1: Summary of our targets Name Coordinate Period Mag. (J2000) h (mag) HW Vir 12 44 20.24 -08 40 16.8 2.801 V=11.2 NY Vir 13 38 48.14 -02 01 49.2 2.426 V=14.2 HS 0705+6700 07 10 42.09 +66 55 44.0 2.296 B=14.1 HS 2231+2441 22 34 20.89 +24 57 00.4 2.654 V=14.1 NSVS 14256825 20 20 00.51 +04 37 55.6 2.649 V=14.3 BULSC 16-335 14 45 20.21 -06 44 03.2 3.001 V=16.3 NSVS 07826147 15 33 49.40 +37 59 28.6 3.883 V=13.6 SDSSJ 0820+0008 08 20 53.5 +00 08 53.4 2.304 g=14.9 2M 1938+4603 19 38 32.61 +46 03 59.1 3.024 g=11.9 EC 10246-2707 10 26 56.47 -27 22 57.1 2.844 B=14.2 ASAS 102322-3737 10 23 21.89 -37 36 59.9 3.343 V=11.6 Supposing that there is a third body existing in this system, under the mutual gravitation force, the compo- nents of eclipsing pair would rotate around the barycen- ter of this triple system. Seen by a distant observer, the light-travel time of this system will change because of the change in the orbital movement caused by the ad- ditional body, which can result in the observed cyclic change in the O-C diagram constructed by the eclipsing timings. This light time effect is very useful to detect tertiaries within AU distance level in evolved eclipsing binaries compared with the radial velocity and transit methods, which have been extensively used to search for planets around solar-type main-sequence stars. That is because the high surface gravity of the evolved stars prevents us to achieve the high radial velocity precision required to detect planets, and the small size of the components (for example, R1 = 0.166 and R2 = 0.152 for NSVS 07826147 derived by For et al., 2010.) make the low probability of transits, see Silvotti (2009) for more information. 3 New Results Here we present new results of our three targets (HS 0705+6700, NY Vir and NSVS 07826147) from our ongoing search for circumbinary substellars orbiting around sdB eclipsing binaries. For HS 0705+6700 and NY Vir, new observations confirmed our previous study (Qian et al., 2009, 2012). For NSVS 07826147, prelim- inary analysis of the O-C diagram constructed by our five years data implied the presence of a close-in planet orbiting around it at a distance about 0.64AU. 3.1 HS 0705+6700 HS0705+6700 ( = GSC 4123-265) was listed as a dwarf candidate from the Hamburg Schmidt survey (Hagen et al. 1995 ). The observations indicated that it is an eclipsing binary. A detailed photometric and spectro- scopic investigation was carried out by Drechsel et al. (2001). Absolute parameters of both components were determined suggesting that the primary is an sdB star, while the secondary is a cool stellar object. A cyclic change in the O-C curve of HS 0705+6700 was discovered by Qian et al. (2009), which indicated the presence of a possible brown dwarf companion. It was later confirmed by Camurdan et al. (2012), Beuer- mann et al. (2012), and Qian et al. (2013) who revised the parameters of the brown dwarf. The (O − C)1 di- agram of the sdB-type binary HS 0705+6700 with re- spect to the following linear ephemeris, Min.I = BJD 2451822.760549 + 0.09564665 × E, (1) was displayed in Fig. 1, where dots refer to our new data obtained after Qian et al. (2009). It is shown that, apart from the cyclic change reported by previous authors, there is an upward parabolic variation in the (O −C)1 curve. This reveals a period increase at a rate of Ṗ = +9.8×10−9 days/year. The angular momentum loss via gravitational radiation or/and magnetic brak- ing should cause a decrease in the orbital period rather than increase. Moreover, the period increase cannot be 184 Close-In Substellar Companions and the Formation of sdB-Type Close Binary Stars 0 10000 20000 30000 40000 50000 -0.003 -0.002 -0.001 0.000 0.001 0.002 0.003 E Figure 1: O-C diagram of HS 0705+6700. Dots re- fer to our new data obtained after Qian et al. (2009), and open circles to the other data. Dashed line refers to the continuous increase in the period, while the solid line to a combination of the period increase and a cyclic change. explained by mass transfer between the components because of the detached configuration of the system. Therefore, the observed upward parabolic variation may be only a part of another cyclic change with very long- period that is caused by the light-travel time effect due to the presence of another substellar object in a wider orbit. 3.2 NY Vir NY Vir (PG 1336-018) was discovered as a HW Vir bi- nary by Kilkenny et al., (1998) which is composed of 2.580 2.585 2.590 2.595 2.600 2.605 0.4 0.2 0.0 -0.2 -0.4 6.750 6.755 6.760 6.765 0.4 0.2 0.0 -0.2 -0.4 Figure 2: Eclipse profiles of NY Vir observed in November 2012 and April 2013 with the 2.15-m ”Jorge Sahade” telescope. Dots represent the magnitude dif- ferences between the NY Vir and the comparison star. a rapid pulsator and an M5-type star. Kilkenny et al., (2011) found the orbital period of the binary is decreasing at a rate of Ṗ = −11.2 × 10−13 d per orbit. A cyclic change was discovered to be su- perimposed on the long-term decrease by Qian et al. (2012), which was explained by the presence of a cir- cumbinary planet. Qian et al. (2012) pointed out that the long-term period decrease can not be explained by angular momentum loss via gravitational radiation or/and magnetic braking and proposed that there is another substellar object in a wider orbit as in the case of HS 0705+6700. New observations of the sdB binary were obtained. The eclipse profiles obtained by using the 2.15-m ”Jorge Sahade” telescope in Argentina were shown in Fig. 2. The new determined eclipse times confirm the presence of the circumbinary planet and the parameters will be revised by including more times of light minimum. 3.3 NSVS 07826147 NSVS07826147 (2M 1533+3759) has been suspected as a possible eclipsing sdB binary system by Kelley and Shaw (2007) using the data from the Northern Sky Variability survey and the Two Micron All Sky Survey. They derived the period of this system as 0.16177 days. Zhu and Qian (2009) improved its period as 0.16177046 days based on all available times of light minimum un- til that time. The detailed absolute parameters derived by the combination of photometric and spectroscopic observations were published by For et al., (2010). Till now, we have monitored this target for more than 5 years. New light curves obtained with the 85-cm tele- scope and the 2.4m telescope are displayed in Fig. 3. The constructed O-C diagram shows the trend of the periodic variation, which implies the presence of a close- in circumbinary planet in NSVS 07826147 at a distance about 0.64AU. The detailed analysis is in progress. 72.34 72.35 72.36 72.37 0.9 0.0 -0.9 m HJD2455900+ 74.256 74.260 74.264 74.268 74.272 74.276 2 1 0 -1 Figure 3: Light curves of NSVS07826147. Left panel: Eclipsing profiles of NSVS07826147 obtained with 85- cm telescope on May 26, 2012 in R band. Right panel: Obtained with 2.4-m telescope on Feb. 14, 2012 without filters. Dots refer to the magnitude differences between the target and the comparison star, and open circles to the magnitude differences between the comparison and check star. 185 L. Y. Zhu et al. 4 Summary One formation method of sdB-type binaries in theory is through a common envelope (CE) phase after the more massive star in the initially detached system evolves into a red giant. The ejection of the CE results in a large amount of angular momentum loss, and then a short- period sdB-type binary is formed. In previous section, new results of the three sdB-type eclipsing binaries are introduced. The presence of substellar companions es- pecially the one orbiting NSVS 07826147 with the clos- est distance will give some constrains on the formation of this type of objects as well as on the interaction be- tween red giants and their companions. To improve the results, more observations are required in the future. Acknowledgement This work is supported by Chinese Natural Science Foundation (No.11133007 and 11325315) and Yunnan Natural Science Foundation (No. 2013FB084). New CCD photometric observations were obtained with the 2.16m and 85-cm telescope at Xinglong station, the 2.4- m, 1.0-m, and 60-cm telescopes at Yunnan observatories in China, and the 2.15-m ”Jorge Sahade” telescope in Argentina. References [1] Beuermann, K., Breitenstein, P., Debski, B., Diese, J., Dubovsky, P. A., Dreizler, S., et al., 2012, A&A 540, 8 [2] Castellani, M., Castellani, V., 1993, ApJ, 407, 649 [3] Camurdan, C. M., Zengin Camurdan, D., Ibanoǧlu, C., 2012, NewA 17, 325 doi:10.1016/j.newast.2011.08.004 [4] Charpinet, S., Fontaine, G., Brassard, P., et al., 2011, Natur, 480, 496 doi:10.1038/nature10631 [5] D’Cruz, N. L., Dorman, B., Rood, Robert, T., O’Connell, R. W., 1996, ApJ, 466, 359 doi:10.1086/177515 [6] Drechsel, H., Heber, U., Napiwotzki, R., Ostensen, R., Solheim, J.-E., Johannessen, F., Schuh, S. 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G., 2009, ApJ 706, L96 [19] Qian, S.-B., Zhu, L.-Y., Dai, Z.-B., Fernández Lajús, E., Xiang, F.-Y., and He, J.-J., 2012, ApJ 745, L23 [20] Qian, S.-B., Shi, G., Zola, S., 2013, MNRAS, in print [21] Silvotti, R., 2009, International Journal of Astrobiology 8, 241 [22] Silvotti, R., Schuh, S., Janulis, R. et al., 2007, Natur, 449, 189 [23] Zhu, L. Y., Qian, S. B., 2010, Ap&SS, 329, 107 DISCUSSION RAYMUNDO BAPTISTA: Light time travel ef- fect is not the only interpretation for physical period changes in binary stars. An alternative is the magnetic activity cycle of the late-type star modulating the bi- nary period. It is hard to believe a planet at 1 AU or- bit would survive commom envelope phase of a binary which was at comparable separation before. LIYING ZHU: Firstly, the cyclic period change caused by light time effect is strictly periodic change in O-C diagram not only quasi-periodic variation aroused 186 http://dx.doi.org/10.1016/j.newast.2011.08.004 http://dx.doi.org/10.1038/nature10631 http://dx.doi.org/10.1086/177515 http://dx.doi.org/10.1088/0004-637X/708/1/253 http://dx.doi.org/10.1046/j.1365-8711.2003.06451.x http://dx.doi.org/10.1146/annurev-astro-082708-101836 http://dx.doi.org/10.1111/j.1365-2966.2010.17919.x http://dx.doi.org/10.1046/j.1365-8711.1998.01432.x Close-In Substellar Companions and the Formation of sdB-Type Close Binary Stars by the magnetic activity cycle. Secondly, in our five- years monitor, no asymmetric light curves caused by the magnetic activity for these HW Vir binaries were found. Their light curves are very stable. Thirdly, One of our target SDSS J0820+0008 is composed of an sdB star and a brown dwarf, the cyclic change can not be explained by magnetic activity cycles. The detected planet at 1 AU orbit may be protected by the engulf- ment of the low mass stellar companion during the CE phase. And the other possibility is that the planet is the second-generation one. 187 Introduction Targets and Method New Results HS0705+6700 NY Vir NSVS07826147 Summary