Research Paper J. Astron. Space Sci. 28(3), 163-172 (2011) http://dx.doi.org/10.5140/JASS.2011.28.3.163 WZ Cephei: A Dynamically Active W UMa-Type Binary Star Jang Hae Jeong1, 2 and Chun-Hwey Kim1, 2† 1Department of Astronomy and Space Science, Chungbuk National University, Cheongju 361-763, Korea 2Chungbuk National University Observatory, Chungbuk National University, Cheongju 361-763, Korea An intensive analysis of 185 timings of WZ Cep, including our new three timings, was made to understand the dynami- cal picture of this active W UMa-type binary. It was found that the orbital period of the system has complexly varied in two cyclical components superposed on a secularly downward parabola over about 80y. The downward parabola, cor- responding to a secular period decrease of -9.d97 × 10-8 y-1, is most probably produced by the action of both angular mo- mentum loss (AML) due to magnetic braking and mass-transfer from the massive primary component to the secondary. The period decrease rate of -6.d72 × 10-8 y-1 due to AML contributes about 67% to the observed period decrease. The mass -8 -1 flow of about 5.16× 10 M⊙ y from the primary to the secondary results the remaining 33% period decrease. Two cycli- cal components have an 11.y8 period with amplitude of 0.d0054 and a 41.y3 period with amplitude of 0.d0178. It is very interesting that there seems to be exactly in a commensurable 7:2 relation between their mean motions. As the possible causes, two rival interpretations (i.e., light-time effects (LTE) by additional bodies and the Applegate model) were con- sidered. In the LTE interpretation, the minimum masses of 0.30 M⊙ for the shorter period and 0.49 M⊙ for the longer one were calculated. Their contributions to the total light were at most within 2%, if they were assumed to be main-sequence stars. If the LTE explanation is true for the WZ Cep system, the 7:2 relation found between their mean motions would be interpreted as a stable 7:2 orbit resonance produced by a long-term gravitational interaction between two tertiary bodies. In the Applegate model interpretation, the deduced model parameters indicate that the mechanism could work only in the primary star for both of the two period modulations, but could not in the secondary. However, we couldn't find any meaningful relation between the light variation and the period variability from the historical light curve data. At present, we prefer the interpretation of the mechanical perturbation from the third and fourth stars as the possible cause of two cycling period changes. Keywords: W UMa-type star, WZ Cep, period change, light-time effects, magnetic activities 1. INTRODUCTION analysis of his light curve using Russell's classical model. The first photoelectricBV light curves were secured by The W UMa-type binary star, WZ Cep (AN 244.1928, Hoffmann (1984) who detected a strong O'Connell ef- 2MASS J23222421+7254566, 1RXS J232216.6+725505) fect in his light curves, as the brightness in the 0.25 phase was discovered by Schneller (1928), as an RR Lyr type (Max I) was significantly higher than that in the 0.75 phase variable star with a period of 0.d260984. Nine years later, (Max II). The spectral type was assigned as F5 in the Gen- a photographic light curve was obtained by Balázs (1937), eral Catalogue of Variable Stars (Kholopov et al. 1987). who correctly classified WZ Cep as a W UMa binary star Hoffmann's light curves were reanalyzed by Kałużny with an orbital period of 0.d41745. Detre (1940) also made (1986) with the Wilson Devinney binary model (WD; photographic observations of the system, confirmed Ba- Wilson & Devinney 1971). His solutions showed that WZ lázs' result, and gave basic system parameters from the Cep is a contact binary with the components of unequal This is an Open Access article distributed under the terms of the Received Jun 27, 2011 Revised Jun 29, 2011 Accepted Jun 30, 2011 Creative Commons Attribution Non-Commercial License (http://cre- †Corresponding Author ativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, E-mail: [email protected] provided the original work is properly cited. Tel: +82-43-261-3139 Fax: +82-43-274-2312 Copyright © The Korean Space Science Society 163 http://janss.kr pISSN: 2093-5587 eISSN: 2093-1409 J. Astron. Space Sci. 28(3), 163-172 (2011) surface temperatures (ΔT ≈ 1,000 K). The O'Connell ef- Camera exposure time ranged between 40 seconds and fect in the light curve was ascribed to a small hot spot on 60 seconds according to the quality of the night. the less massive component near the neck between stars. The instrumentation used and the reduction method These interpretations were questioned by Djurašević et for the raw CCD frames have been described in detail al. (1998), who obtained new BVR light curves that were by Kim et al. (2006). The resultant standard errors of our almost symmetrical. Their analysis of both their own observations in terms of comparison minus check star light curves and Hoffmann's using their own Roche com- were about ±0.m02. From our observations, three primary puter model (Djurašević 1992a, b) showed that the WZ times of minimum light were determined using the con- Cep system is a shallow contact binary (f = 9~14%) with ventional Kwee & van Woerden (1956) method, and are the components of slightly different temperatures (ΔT listed in Table 1. ≈ 140 K) and with two large dark spots on the surface of the massive, hotter primary star. Recently, WZ Cep was revisited by Lee et al. (2008) and Zhu & Qian (2009). Lee 3. PERIOD STUDY et al. (2008) presented symmetrical BVR light curves in the observing season in 2005, while Zhu & Qian (2009) To investigate the period variation of WZ Cep, a total of obtained an asymmetrical V light curve with a strong 185 (91 visual, 4 photographic, and 90 photoelectric and O'Connell effect. Both authors analyzed their light curves CCD) times of minimum light, including ours, have been using the WD method and confirmed the photometric re- collected from a modern database (Kreiner et al. 2001) sults of Djurašević et al. (1998). At the same time, Zhu & and from the recent literature. Table 1 lists only photo- Qian (2009) made the first intensive period study of WZ electric and CCD minima, which were not compiled by Cep with all timings available to them, and found that Zhu & Qian (2009) or published after their study. the period variation consists of both a periodic and secu- The (O-C ) residuals of all timings were calculated with larly decreasing terms. The periodic term has a period of the linear ephemeris of Kreiner et al. (2001), as follows: 34.y2 with amplitude of 0.d013, while the secular period -8 d decrease rate is about -8.8 × 10 day/year. C1 = HJD 2449890.3582 + 0. 41744670 E. (1) Numerous timings just before and after the period study of Zhu & Qian (2009) have been published in the The (O-C1) diagram is shown in Fig. 1, where the times of literature. In this paper, all published and newly ob- minimum are marked by assorted symbols that differ in served timings have been reanalyzed. It is noticed that size and shape according to observational method and the ephemeris suggested by Zhu & Qian (2009) is not type of eclipse. The (O-C1) residuals were listed in the compatible with recent timing residuals. New ephemeris sixth column of Table 1. As can be seen in the figure, it is derived with a detection of a secondary cyclic modula- tion of the O-C residuals by a period of about 11.8 year. A discussion of the implications on the existing working hypotheses follows. 2. OBSERVATIONS Charge-coupled device (CCD) photometric observa- tions of WZ Cep for the purpose of timing determination were made on the three nights of May 28 and October 9, 2005 and September 23, 2006 with the 35-cm reflector of the campus station of Chungbuk National University Observatory in Korea. The telescope was equipped with an SBIG ST-8 CCD imaging system, electrically cooled, with a 19' × 12' field of view. No filters were used in our The (O-C ) diagram of WZ Cep drawn together with the non- observations. GSC 4486-1402 and GSC 4486-1352 were Fig. 1. 1 linear terms (solid and dashed lines) of Eq. (2). The recent residuals from chosen as the comparison and check stars, respectively. Eq. (2) in the bottom are remarkably deviating from the non-linear terms Our comparison is the same one used by Lee et al. (2008). of Eq. (2). http://dx.doi.org/10.5140/JASS.2011.28.3.163 164 Jang Hae Jeong & Chun-Hwey Kim WZ Cephei, A dynamically active binary star is clear that the variation patterns of the residuals before although there is a large gap of about 25 years between and after 1995y are quite different from each other. The 1939y and 1964y. However, the period from 1995y to the orbital period before 1995y seemed to be nearly constant, present has dramatically decreased in a continuous way. Table 1. Photoeletric and CCD times of minima of WZ Cep not listed in Zhu & Qian (2009) or published after their study. HJD Error MEa Ty b Cycle (O-C ) (O-C ) Reference (2400000+) 1 2 50,642.7978 0.0002 CC II 1,802.5 -0.0081 0.0003 Baldwin & Samolyk (2004) 51,088.6282 0.0002 CC II 2,870.5 -0.0108 0.0028 Baldwin & Samolyk (2004) 51,280.8540 CC I 3,331.0 -0.0192 -0.0029 In O-C gatewaya 51,452.6349 0.0003 CC II 3,742.5 -0.0176 0.0013 Baldwin & Samolyk (2004) 51,576.6148 0.0003 CC II 4,039.5 -0.0193 0.0014 Baldwin & Samolyk (2004) 52,146.8363 0.0003 CC II 5,405.5 -0.0300 -0.0003 Baldwin & Samolyk (2004) 52,223.6456 0.0002 CC II 5,589.5 -0.0309 0.0000