Physical Parameters of the Overcontact Binary AH ...

Astrophysics of Variable Stars ASP Conference Series, Vol. 349, 2006 C. Sterken & C. Aerts

Physical Parameters of the Overcontact Binary AH Cnc in the Old Open Cluster M 67 Kadri Yakut1,2 and Conny Aerts1,3 1 Instituut

voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, B-3001 Leuven, Belgium 2 Ege

University, Science Faculty, Department of Astronomy & Space ˙ Sciences, 35100 Izmir, Turkey

3 Department

of Astrophysics, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands Abstract. We present a photometric study of the overcontact binary AH Cnc. The CCD observations were done with the Russian-Turkish 1.5 m telescope and the light-curve was solved with the Wilson-Devinney code. The physical parameters of the components have been deduced as M1 = 1.22 M⊙ , M2 = 0.20 M⊙ , R1 = 1.37 R⊙ , R2 = 0.66 R⊙ , L1 = 2.65 L⊙ and L2 = 0.63 L⊙ from the obtained orbital parameters and the distance modules of M 67. AH Cnc has been compared with similar systems in the Hertzsprung-Russell diagram.

1.

Introduction

AH Cnc (V = 13.d 33, F 7V ) is an A-type low-temperature contact binary (LTCB) system which is a member of the old open cluster M 67. The open clusters host numerous close binary stars. Studying binaries which are members of a cluster is an advantage. Indeed, if we combine some of the parameters of the cluster (i.e. distance module), with the binary light curve, we can estimate the absolute parameters of the binary. M 67 is one of the well-studied old open clusters. The age of the cluster is about 4 Gyr and its distance is about 0.9 kpc. This cluster contains many close binary stars. The variability AH Cnc has been reported by Kurochin (1960) who classified the star as an RR Lyrae type variable. Later on, the system was classified as LTCB by Efremov et al. (1964) and Kurockin (1965). The radial velocity curve of the system was obtained only by Whelan et al. (1979), despite the system had been studied photometrically by numerous researcher previously. Since the system is faint, the obtained results have a large uncertainty. The mass ratio obtained spectroscopically displaysa great deal of difference from those obtained by photometric studies. In the current work we deduce the physical parameters of the system combining the light curve solution of the system with the known parameters of the cluster.

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376 2.

Yakut and Aerts Observations

CCD photometry of the system was obtained using the Russian-Turkish 1.5¨ ITAK ˙ m telescope (RTT150) at TUG (TUB National Observatory, Antalya) on February 23 – 24 and April 28, 2005. All the images were obtained using the ANDOR CCD. The CCD uses a DW436 chip and operates stably at −60 degC with electrical cooling. The CCD chip has 2048 × 2048 pixels. The gain corresponds to 1.4 e− /ADU with readout noise of 8 electrons rms. The comparison and check stars were GSC 814 1803 and GSC 814 1685, respectively. All the CCD reductions were done with IRAF1 . The orbital phases were calculated by using the following light elements given by Zhang et al. (2005): HJD (Min I) = 2453 004.1144 + 0.d 36045754 × E.

(1)

The differential magnitudes are plotted in Fig. 1.

Figure 1. A comparison of the observed (dots) and computed (solid line) light curve of AH Cnc.

3.

Light-curve Solution

The 2003 version of the Wilson-Devinney code (Wilson & Devinney 1971; Wilson 1994; Wilson & van Hamme 2003) was employed for our light-curves analysis. Mode 3, which assumes that both of the components fill their Roche lobes, was used. The following values were adopted: the temperature of the primary component was set at T1 = 6300 K according to the spectral type (Drilling & Landolt 2000), the values of the gravity darkening coefficients and albedos were 1

http://iraf.noao.edu

The Overcontact Binary AH Cancri

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taken as g1 = g2 = 0.32 (Lucy 1967) and A1 = A2 = 0.5 (Rucinski 1969). The adjustable parameters are the inclination i, the temperature of the secondary component T2 , the luminosity L1 , the surface potential Ω, the limb darkening coefficients (x1 , x2 ). The differential correction (DC) code was run until the corrections to the input parameters were lower than their errors. The results are given in Table 1. The light-curve computed using the parameters given in Table 1 is shown by the solid line and compared with all the observations in Fig. 1. The LC program was used to create the synthetic light curves. The filling factor, which varies from zero to unity from the inner to the outer critical surface, amounts to f = (Ωin - Ω) / (Ωin - Ωout ) = 0.79. Table 1. Photometric elements of AH Cnc. WWR: Whelan et al. (1979), MMR: Maceroni, Milano & Russo (1984), SS:Sandquist & Shetrone (2003), ZZD: Zhang, Zhang & Deng (2005). Parameter ◦

i( ) T2 (K) Ω1,2 q f (%) 1 ( L1L+L ) 2 r¯1 r¯2

4.

WWR

MMR

SS

ZZD

Present study

65 0.75 85 -

63 6416 4.494 0.64 41 0.62 0.453 0.369

86 0.157-0.165 73-62 -

82.8 6354 2.0370 0.149 65 0.83 0.572 0.260

87.7(1.1) 6330(20) 2.0489(67) 0.160(1) 79 0.82 0.572(2) 0.276(7)

Results and Conclusions

A radial velocity study of AH Cnc was made by Whelan et al. (1979). Because of the faintness of the system, the obtained parameters were not accurately determined. Therefore it is useful to deduce the mass ratio from photometry. M 67 has been studied by different researchers, and its distance modulus is known accurately: m − M = 9.72 (Sandquist, 2004). The maximum brightness of the system is mmax = 13.m 33 (Zhang et al. 2005). From these results, we deduced the absolute physical parameters of the components as given in Table 2. The mass of the primary star has been estimated from the mass-radius relation of LTCBs (Yakut, 2005). These parameters are in good agreement with A-type LTCBs (Fig. 2). AH Cnc is one of the unique contact binary systems with a low mass ratio. Studying the evolution of binaries, especially of the contact binaries with low mass ratio (q