A photometric study of the short-period eclipsing binary 1SWASP J204932.94-654025.8, showing strong third light

New Astronomy 76 (2020) 101324

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A photometric study of the short-period eclipsing binary 1SWASP J204932.94-654025.8, showing strong third light

T

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Bin Zhang ,a,b, Sheng-Bang Qiana,b,c,d,e,f, Miloslav Zejdag, Jing-Jing Wangh, Qi-Jun Zhia,b, Ai-Jun Donga,b, Wei Xiea,b, Li-Ying Zhuc,d,e, Lin-Qiao Jiangi a

School of Physics and Electronic Science, Guizhou Normal University, Guiyang 550001, China Guizhou Provincial Key Laboratory of Radio Astronomy and Data Processing, Guizhou Normal University, Guiyang 550001, China c Yunnan Observatories, Chinese Academy of Sciences (CAS), P. O. Box 110, Kunming 650216, China d Key Laboratory of the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, P. O. Box 110, Kunming 650216, China e University of Chinese Academy of Sciences, Yuquan Road 19#, Sijingshang Block, Beijing 100049, China f Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China g Department of Theoretical Physics and Astrophysics, Masaryk University, Kotlářská 2, Brno 611 37, Czech Republic h China University of Petroleum-Beijing at Karamay, Anding Road 355, Karamay 834000, China i School of Physics and Electronic Engineering, Sichuan University of Science and Engineering, Zigong 643000, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Binary Eclipsing binary Orbital period Light curves

1SWASP J204932.94-654025.8 (hereafter J2049) is a newly discovered eclipsing binary system with an orbital period of 0.2299103 days. BVRc light curves (LCs) are presented and analyzed by using the 2013 version of the Wilson–Devinney (W–D) program. Because the observed LCs are asymmetric, a hot star-spot was employed on the secondary component during our analysis. We found that J2049 is a W-subtype shallow contact eclipsing binary system with an orbit inclination of 62∘.69 ± 0∘.95 and a mass ratio of q =1.326 ± 0.056. More importantly, we found the presence of a strong third light, with an average luminosity contribution of 31.3% of the total light. Based on times of the light minima, the orbital period changes of J2049 are studied for the first time, and there is no evidence for any significant dp/dt now. Considering the presence of the third light and the short time span of the eclipse times, more observations are needed in the future.

1. Introduction The W UMa-type eclipsing binaries(EBs) are generally composed of two late-type stars with a common convective envelope (Qian et al., 2013; 2015b; Zhang et al., 2017a). Because of that, the temperature of two components are similar and their LCs always show nearly equal eclipsing minima (Li et al., 2019). Generally, for these EBs, mass transfer between two stars is ongoing with the angular momentum loss (AML) at the same time (Zhang et al., 2018a). In addition, it was discovered that the orbital period distribution of some of contact EBs shows a very sharp cut-off at 0.22 days (Rucinski, 1992). A recent statistical study using the data released by the Large Sky Area MultiObject Fiber Spectroscopic Telescope (LAMOST) reveals that the new value of period limit of this kind of EBs is around 0.20 days (Qian, He, Zhang, et al., 2017; Zhang and Lu, 2018). Many researchers have tried to solve this issue and a series of explains have been suggested since 1992. For example, Rucinski (1992) thought that the contact eclipsing binaries (CEBs) with a period around this limit are fully convective and ⁎

these systems are dynamically unstable; Stepien (2006) suggested that the timescale of AML for CEBs is too long to form the EBs with a period shorter than 0.20 days; Jiang et al. (2012) thought that the main reason is the unstable mass transfer from the components of these systems; Qian et al. (2015a) indicated that the circumbinary companions play an important role in the origin and evolution of these CEBs and their report revealed that all M-dwarf binaries with ultrashort period are possibly triple systems. However, at present, it is still an open question. With the development of sky surveys, such as CSS (Drake et al., 2013; Zhang et al., 2019a), LAMOST (Wu et al., 2011; Luo et al., 2015) and Kepler Mission (Shan et al., 2015; Borkovits et al., 2016), more and more CEBs near the period limit with a third body have now been discovered. Recent statistical research reports suggest that some CEBs with high metallicity may be formed with the help of the third body, which can explain the existence of EBs below the period limit (Qian et al., 2015a; 2017; 2018). Just as Liu et al. (2015) discussed, multiplicity may be a common phenomenon among close binaries. These multi-body candidates can be detected via searching for third light by

Corresponding author. E-mail address: [email protected] (B. Zhang).

https://doi.org/10.1016/j.newast.2019.101324 Received 26 July 2019; Received in revised form 13 October 2019; Accepted 24 October 2019 Available online 06 November 2019 1384-1076/ © 2019 Elsevier B.V. All rights reserved.

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using the W-D code, such as FV CVn (Liao and Sarotsakulchai, 2019), V776 Cas (Zhou et al., 2016b) and 1SWASP J050904.45-074144.4 (Li et al., 2019). 1SWASP J204932.94-654025.8 was first reported by Butters et al. (2010) as a CEB candidate with an orbital period about 0.23 days. The Two Micron All Sky Survey (2MASS, Cutri et al., 2003) offers its magnitudes as J = 12.55, H = 11.916 and K = 11.812, V = 14.22 (from Super WASP, Lohr et al., 2013), with color indices of V-K = 2.408, J-H = 0.634 and H-K = 0.104 for the star, implying that it could be a red dwarf with an average spectral type about K4. In this paper, the first photometric solutions of the target are obtained by using the (W-D) program. And then, we analyze its period changes for the first time. Finally, based on the study results mentioned above, we discuss its formation and evolution. 2. Observations and data reductions Fig. 1. The light curves of J2049 in BVRc-bands observed in 2016 at SAAO. Different symbols refer to the observed data in different bands. The C − Ch means the magnitude difference between the Comparison Star and the Check Star, and offset for clarity, the C − Ch curves of V and Rc-bands are indicated in the legend by +0.1 and +0.2.

Photometric observations of J2049 in three bands(BVRc) were carried out on 2016 September 11, using the 1024 × 1024 PI1024 STE4 CCD camera attached to the SAAO 1.0 m telescope at Sutherland, South Africa. The field of view of the camera on the telescope is 5 × 5 arcmin2. Its filter system is a standard Bessel multicolor CCD photometric system. The coordinates of the stars are listed in Table 1. The integration times were 50 s for B-band, 40 s for V-band and 30 s for Rcband, respectively. More than 165 CCD images were obtained, and the observed data were reduced by Zejda using the C-Munipack software package (Motl, 2016, http://c-munipack.sourceforge.net/). This package offers the complete solution for reduction of images taken by CCD camera of variable stars (Zhang et al. 2017). Using the following linear ephemeris equation,

Min. I (HJD) = 2, 457, 643.32948(33) + 0d . 2299103(4) × E

Table 2 New CCD times of Light Minima for J2049. JD(Hel.) 2457643.32948 2457643.44732 2457643.21757 2457647.28272 2457647.28320 2457647.28278 2457821.90391

(1)

Error(d) ± ± ± ± ± ± ±

0.00033 0.00040 0.00047 0.00024 0.00020 0.00022 0.00013

Method

Filter

Telescope

CCD CCD CCD CCD CCD CCD CCD

BVRc BVRc V V Rc Ic Ic

SAAO SAAO SAAO SAAO SAAO SAAO SAAO

1.0 m 1.0 m 1.0 m 1.0 m 1.0 m 1.0 m 1.0 m

the LCs along with their magnitude difference are calculated, where Min.I is the primary minima times and E means the cycles of the system revolution. Then, the corresponding results are plotted in Fig. 1. 3. Analysis of the orbital period changes For studying the orbital period changes of J2049, we collected its eclipse times published previously. Many of the times of minima used in this paper were provided by Lohr et al. (2013, 2015). Also, as few of them are obtained though our observations, and these new data are listed in Table 2. Secondly, the O − C values of these data were calculated by using the Eq. (1). These data were fitted using a least-squares method. During our analysis, weights of 1/σ2 were assigned to the data, where σ is the error of eclipse times (Liu et al., 2018). It should be noted that only the mean values were adopted in our analysis for the minimum times with the same epoch from different bands (Zhou et al., 2016b). After analyzing these data, we did’t find any significant orbital period changes of the system at present. The final (O − C ) diagram of system is shown in Fig. 2.

Fig. 2. The (O − C ) diagram of J2049.

of the W-D code were used to analyse its LCs (Van Hamme, 1993; Wilson, 2012; Wilson and Devinney, 1971). In the light of the values of the average color index we calculated, the temperature of the primary component of the system was set as T1 = 4500 K (Cox, 2000). In addition, the bolometric albedo and the gravity-darkening coefficients for both components are fixed and taken the same value, i.e. A1 = A2 = 0.5 for late-type stars with a convective envelope (Rucinski, 1969,Liu et sal. 2015), and g1 = g2 = 0.32 according to the stellar temperatures given by M Lucy (1967). The adjustable parameters are: the mass ratio q ≡ M2 , the 1 mean temperature of the secondary component T2, the orbital inclination i, the monochromatic luminosity of the primary L1B, L1V and L1Rc , and the dimensionless potentials of the two components Ω1 and Ω2 (Zhang et al., 2018b).

4. Photometric solutions with W-D program In order to obtain the orbital parameters of J2049, the 2013 version Table 1 Coordinates of J2049, the Comparison Star, and the Check Star. Stars

αj2000 h

δj2000 m

s

J2049

20 49 32 .94

− 65∘40 ′25 ″. 8

Comparison

20h49m28s.45

− 65∘41′37 ″. 2

Check

20h49m30s.02

− 65∘40 ′18″. 8

2

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Table 3 Photometric solutions for J2049. Parameters

g1 = g2 A1 = A2 T1(K) q T2(K) i(∘) L1/(L1 + L2)(B ) L1/(L1 + L2)(V ) L1/(L1 + L2)(R c ) L3/(L1 + L2 + L3)(B ) L3/(L1 + L2 + L3)(V ) L3/(L1 + L2 + L3)(R c ) Ω1 = Ω2 r1(pole) r1(side) r1(back) r2(pole) r2(side) r2(back) θs(∘) ψs(∘) rs(∘) Ts

Fig. 3. The q-search curve of J2049. The ordinate (∑) represents the fitting residuals with each mass ratio fixed.

There is no mass ratio from spectroscopic observations, so, a qsearch method is used. The aim of this technique is to search for the best fitting values of mass ratio when it is fixed, and then treat it as an adjustable parameter to obtain the final photometric solutions for the binary system (Zhang et al., 2017b), and it is always adopted when fitting the LCs of the EBs (Wang et al., 2014; 2015). We found that the final photometric solutions of the target converged on a contact model. We searched for the primal mass ratio from 0.3 to 3.2. Finally, the best q-search curves are plotted in Fig. 3 with a lowest value at q = 1.20, and the ordinate (∑) represents the fitting residuals with each mass ratio fixed. After inputting the mass ratio value obtained by the q-search method and setting it as an adjustable parameter, we found that the fitting curves were still not good. Because the LCs we observed is asymmetric, star-spot model was adopted to get better results. We tried the cool and hot star-spot mode by adjusting four correlated parameters many times (Zhang et al., 2014), and after that, we found that when adding a hot star-spot on the secondary component of the system can get a better photometric solution than before. At the same time, in order to check the existence of the third light, we treated the third light as an adjusting parameter during our calculation. We found that the contribution of the third light for the whole luminosity is 29.4%( ± 4.1%) for B-band, 31.9%( ± 3.8%) for V-band and 32.3%( ± 3.8%) for Rcband, which implies that the tertiary companion maybe a late-type star (Zhu et al., 2008). The best photometric elements with star-spots are listed in Table 3, meanwhile, the theoretical LCs computed with a hot star-spot are displayed in Fig. 4. The geometrical structure of the system is shown in Fig. 5.

∑ (O − C )i2

With hot star-spot and third light Photometric elements 0.32 0.50 4500 1.326 4080 62.69 0.6582 0.6170 0.5879 0.2943 0.3189 0.3230 4.105 0.3494 0.3689 0.4120 0.4004 0.4264 0.4666 145.9 58.9 60.0 1.1 0.000776

Errors Assumed Assumed fixed ± 0.056 ± 20 ± 0.95 ± 0.0367 ± 0.0330 ± 0.0312 ± 0.0408 ± 0.0380 ± 0.0377 ± 0.086 ± 0.0043 ± 0.0049 ± 0.0061 ± 0.0143 ± 0.0190 ± 0.0304 ± 10.1 ± 4.2 ± 8.7 fixed

Note. The parameters marked with ’1’, ’2’ and ’3’ refer to the primary component, secondary component and third body of the system, respectively; the θs, ψs, rs and Ts represent the latitude, longitude, angular radius and temperature factor of the star-spot, respectively; the ri is the component of mean relative radius.

Fig. 4. Observed (open circles with different colours) and theoretical (black solid lines) light curves with hot star-spot for J2049. The fitting residuals in different bands are displayed in the bottom panel.

5. Discussion and conclusions The first photometric solutions of the short-period EB J2049 are obtained by using the 2013 version of (W-D) code. The final analysis results suggest that the target is a W-subtype CEB system with a mass ratio of q =1.32 and an orbital inclination about 63∘. The temperature difference between two components is 420 K, which means that they are in geometrical contact, and the contact degree is 27.8%( ± 15.2%). In order to explain the asymmetric light curves of the target, a hot starspot on the secondary component was employed, and this reveals that this system is active at present, as in other late-type EBs (Zhang et al., 2018a; 2019b). Besides, during our analysis, an obvious third light are detected. The luminosity contribution of the third light to the total light as 29.4%( ± 4.1%) for B-band, 31.9%( ± 3.8%) for V-band and 32.3%( ± 3.8%) for Rc-band, respectively. Using the three-dimensional correlations of physical parameters for contact binaries supplied by Gazeas (2009), we estimated the absolute parameters of the two

binaries as follows: M1=0.51 ± 0.03 M⊙, M2=0.67 ± 0.02 M⊙, R1=0.67 ± 0.04 R⊙, R2=0.75 ± 0.06 R⊙, L1=0.27 ± 0.04 L⊙ and L2=0.34 ± 0.05 L⊙for J2049. It should be noted that the standard errors obtained by using the W-D program will only the fitting errors, the real parameter uncertainties may be increase two to four times on that basis (Liu et al., 2015a; Popper, 1984). During our analysis, we noted that the observed LCs are asymmetric ′ (with an obvious O Connell effect), which can be explained by the starspot activity from the active late-type stars (Pi et al., 2019; Zhang et al., 2014; Zhou et al., 2015, 2016a, 2016b). After adjusting the four starspot parameters in the (W-D) code, we found that a hot star-spot on the more massive star can fit the LCs best (with the least residual). It is suggested that the target is active now like many other similar EBs 3

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work is partly supported by Chinese Natural Science Foundation (Nos. U1931101, 11933008, 11573063, U1731238, U1831120, 11565010), the Key Science Foundation of Yunnan Province (No. 2017FA001), the Special Funds for Theoretical Physics of the National Natural Science Foundation of China (No. 11847102), the Joint Research Fund in Astronomy (grant numbers U1631108, U1831109) under cooperative agreement between the National Natural Science Foundation of China (NSFC) and Chinese Academy of Sciences (CAS), the research fund of Sichuan University of Science and Engineering (grant number 2015RC42), the Science Foundation of China University of PetroleumBeijing At Karamay (No. RCYJ2016B-03-006), the Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences (No. OP201708), the Doctoral Starting up Foundation of Guizhou Normal University 2018 (GZNUD[2018] 12), and the Guizhou province’s innovation and entrepreneurial project for high-level overseas talents (Grant no. [2019] 02). This paper uses the observations made at the South African Astronomical Observatory (SAAO).

Fig. 5. Geometric structure of the contact eclipsing binary J2049 at 0.25 phase with a hot star-spot.

published before, such as, 1SWASP J200503.05–343726.5 (Zhang et al., 2017a), 1SWASP J140533.33+114639.1 (Zhang et al., 2018a) and 1SWASP J015100.23-100524.2 (Qian et al., 2015b). Generally, for low-mass EBs, the activity of the star-spots is seen as an evidence of the presence of the magnetic field. The reason is that these late-type EBs have a faster rotation and a deeper convective envelope, which will reproduce a strong magnetic field (Chabrier et al., 2007; Zhang et al., 2015; 2019a; Li et al., 2018; 2019). For some active EBs, the frequent flares and the obvious emission line from their chromosphere (Qian et al., 2012; 2014; Pi et al., 2014; 2019) can be observed as well. The O − C diagram of the system reveals an irregular change of dP/ dt at present. The main reason is the lack of the eclipses, for example, there is an obvious gap between SuperWASP data and ours. The scatter distribution of our data maybe caused by the magnetic activity from active components of the system partly. In fact, for these late EBs, a typical feature is their strong magnetic field, and many published papers have reported this (Li et al., 2014a; 2014b; 2019; Zhou et al., 2016b; 2018; Zhou and Soonthornthum, 2019). Similar late-type EBs include V1007 Cas (Li et al., 2018), V2284 Cyg (Wang et al., 2017) and V1104 Her (Liu et al., 2015b). It should be noted that the time span of our data is less than 10 years, so, we need more observational data with high accuracy in the next. The origin and evolution of the EBs near the period limit are still the open questions, and several hypothesis have been put forward since 1992 (Rucinski, 1992; 2007). It is found that these short-period EBs are usually in shallow contact with a late spectral type and a strong starspot activity (Kjurkchieva et al., 2018; Liu et al., 2018; Zhang et al., 2015; 2018a; Li et al., 2019). The formation of these systems could be the result of combined effect from several physical mechanisms, and one of them is the third body (Stepien, 2006; Jiang et al., 2012; 2015; Qian et al., 2015b; 2015a; Liu et al., 2015a). Statistic studies have suggested that the multiple systems are common for contact EBs (Tokovinin et al., 2006; Pribulla and Rucinski, 2006; Liao and Qian, 2010; Rappaport et al., 2013). The third body may play a very important role during the formation of the binary via removing the angular momentum from their host eclipsing systems, and this leads to very low angular momentum of the central eclipsing pair (Stepien, 2006; 2011; Qian et al., 2013; Liao and Sarotsakulchai, 2019; Li et al., 2019; Zhang et al., 2019b). Besides, the progenitor of J2049 may be a system similar to DV Psc (Robb et al., 1999; Pi et al., 2019), which is a RS CVn-type binary with a spectral type of K4-5 (Stephenson, 1986; Lu et al., 2001). In order to obtain the accurate physical parameters and understand the detailed evolutional state of J2049, high-quality RV curves and more times of light minima are needed in the future.

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Acknowledgments We thank the anonymous referee for useful comments and suggestions that have improved the quality of the manuscript. Many thanks to Dr.Marcus Lohr for his kindly sending us eclipse timings of J2049. This 4

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