Vol 17, No 12 (2017) / Zhang

Reanalysis of the orbital period variations of two DLMR overcontact binaries: FG Hya and GR Vir

Reanalysis of the orbital period variations of two DLMR overcontact binaries: FG Hya and GR Vir

Zhang Xu-Dong , Yu Yun-Xia , Xiang Fu-Yuan , Hu Ke

Department of Physics, Xiangtan University, 411105 Xiangtan, China

† Corresponding author. E-mail: fyxiang@xtu.edu.cn


Abstract: Abstract

We investigate orbital period changes of two deep, low mass ratio (DLMR) overcontact W UMa-type binaries, FG Hya and GR Vir. It is found that the orbital period of FG Hya shows a cyclic change with a period of . The cyclic oscillation may be due to a third body in an eccentric orbit, while the orbital period of GR Vir shows a periodic variation with a period of and an amplitude of A = 0.0352 d. The periodic variation of GR Vir can be interpreted as a result of either the light-time effect of an unseen third body or the magnetic activity cycle.

Keywords: binaries: close;binaries: eclipsing;stars: individual (FG Hya, GR Vir)



1 Introduction

FG Hya (AN 1934.0334, BD +03°1979, GSC 0201.01844, Hip 041437) is a W UMa-type binary with a very low mass ratio and very high degree of overcontact (Qian & Yang 2005 ). It was discovered by Hoffmeister ( 1934 ). After its discovery, this system has been observed by many authors. Firstly, Tsesevich ( 1949 ) made visual observations and classified it as a cluster type. Photoelectric and spectroscopic observations were subsequently performed by Smith (Smith 1955 , Smith 1963 ), who suggested that FG Hya is a W UMa-type system with the spectral type of G0. Binnendijk ( 1963 ) published complete light curves in the B and V bands, from which Lafta & Grainger ( 1986 ) determined the mass ratio to be . Yang et al. ( 1991 ) performed a photometric study in which FG Hya was further identified as an A-type contact system with a fill-out factor of 90%. Lu & Rucinski ( 1999 ) presented spectroscopic observations. They derived a spectroscopic mass ratio of and the absolute parameters , and . Yang & Liu ( 2000 ) acquired CCD photometric observations. By comparing their observations with the previous photometric data obtained by Smith ( 1955 ), Binnendijk ( 1963 ), Mahdy et al. ( 1985 ), Yang et al. ( 1991 ), they found that the shape of the light curves of FG Hya shows long-term changes in the light level changes. Qian & Yang ( 2005 ) reported that the light curves showed asymmetries. They interpreted such variations in the light curves as a dark spot on the primary component. Combining their own CCD photometric observations with the spectroscopic elements of Lu & Rucinski ( 1999 ), Qian & Yang ( 2005 ) improved the absolute parameters (in solar units) which are summarized in Table 1.

Star M 1 M 2 R 1 R 2 a L 1 L 2 T 1 T 2 Reference
FG Hya 1.44 0.16 1.41 0.59 2.34 2.16 0.41 5900 6012 Qian & Yang ( 2005 )
GR Vir 1.37 0.17 1.42 0.61 2.40 2.87 0.48 6300 6163 Qian & Yang ( 2004 )

Table 1 Summary of Absolute Parameters for FG Hya and GR Vir

The orbital period changes of FG Hya were first noted and investigated by Qian et al. ( 1999 ). They revealed that its orbital period had undergone five sudden changes from 1950 to 1999, and suggested that these sudden changes are related to asymmetries in the light curves. However, subsequent analysis of its orbital period performed by Yang & Liu ( 2000 ) revealed a secular decrease at a rate of . Qian & Yang ( 2005 ) studied the orbital period changes again. They found that the orbital period of FG Hya shows a sinusoidal variation with a period of superimposed on a secular period decrease at a rate of .

GR Vir (BD −06°4068, GSC 4998.00885, HD 129903, Hip 072138, SAO 140120) is another W UMa-type binary with very low mass ratio and very high degree of overcontact. It was discovered by Strohmeier et al. ( 1965 ). Its eclipsing nature was identified by Harris ( 1979 ). The complete photoelectric light curves of GR Vir were reported almost at the same time by Cereda et al. ( 1988 ) & Halbedel ( 1988 ). The radial velocity curves of the system were obtained by Rucinski & Lu ( 1999 ), who suggested a spectral type of F7/F8. Qian & Yang ( 2004 ) made the CCD photometric observations, from which they revealed that GR Vir is an A-type overcontact system with a degree of overcontact of f = 78.6%. Combining their own CCD photometric solutions with the spectroscopic elements of Rucinski & Lu ( 1999 ), Qian & Yang ( 2004 ) determined the absolute parameters of GR Vir which are compiled in Table 1. Also, Qian & Yang ( 2004 ) analyzed the orbital period changes of GR Vir, where a secular decrease in its orbital period has revealed the orbital period of GR Vir varies with a cyclic period of superimposed on a secular period decrease at a rate of .

In the most recent decade, a large number of times of light minimum for FG Hya and GR Vir have been published. Unfortunately, these new times cannot be predicted well and even significantly deviate from the nonlinear ephemeris obtained in previous studies. Therefore, it is necessary to revisit the orbital period changes for these two systems, aiming to uncover the underlying physical processes and provide a useful clue for understanding their evolutionary status.

2 Orbital Period Analyses

2.1 FG Hya

In order to reveal the orbital period changes of FG Hya, we have performed a careful search for all available photoelectric and CCD times of light minimum. Some of them have been compiled by Qian & Yang ( 2005 ). Others are listed in Table 2. With the linear ephemeris given by Kreiner ( 2004 ),

the values are calculated and displayed in Figure 1, where solid dots represent the photoelectric and CCD observations. From Figure 1, we can see that the orbital period change of FG Hya is continuous, and the trend shows obvious cyclic variation, which may be caused by the light-time effect of a third body. As shown in the figure, the shape of the the oscillation is not strictly sinusoidal, meaning that the third body is moving in an elliptical orbit. By using the least-squares method, the following equation is obtained,
where b i , c i and ω are well-known Fourier constants. The values of the fitted parameters are listed in Table 3. The residuals that have no other variations are displayed in the lower panel of Figure 1, which means our fitting is sufficient. With , the orbital period of the third body rotating around the eclipsing pair was determined to be . The orbital parameters of the tertiary component were computed with the formulae given by Vinko ( 1989 ),
where and . The results are listed in Table 4.

Fig. 1 curve of FG Hya. The dots refer to photoelectric and CCD data. The solid line in the upper panel represents our fitting curve (Eq. (2)). Residuals of FG Hya with respect to Equation (2) are displayed in the lower panel.
HJD.2400000+ Method E Type Ref. HJD.2400000+ Method E Type Ref.
48271.4960 CCD −12899 I 0.0675 [1] 53402.0060 CCD 2751 I 0.00362 [1]
48290.5064 CCD −12841 I 0.06368 [1] 53402.1700 CCD 2751.5 II 0.0037 [1]
48358.3682 CCD −12634 I 0.06416 [1] 53404.6285 CCD 2759 I 0.00346 [1]
48500.3210 CCD −12201 I 0.06567 [1] 53404.7926 CCD 2759.5 II 0.00364 [1]
48625.5493 CCD −11819 I 0.06207 [1] 53405.6126 CCD 2762 I 0.00406 [1]
48683.4109 CCD −11642.5 II 0.06129 [1] 53405.7800 CCD 2762.5 II 0.00755 [1]
49004.5203 CCD −10663 I 0.0591 [1] 53406.7616 CCD 2765.5 II 0.00565 [1]
49004.6796 CCD −10662.5 II 0.05443 [1] 53409.7104 CCD 2774.5 II 0.00396 [1]
49393.3247 CCD 37899 I 0.02473 [1] 53410.3672 CCD 2776.5 II 0.0051 [1]
49416.4349 CCD −9406.5 II 0.05249 [1] 53410.5292 CCD 2777 I 0.00318 [1]
49772.4535 CCD −8320.5 II 0.04532 [2] 53445.4428 CCD 2883.5 II 0.00265 [1]
51192.1180 CCD −3990 I 0.03248 [1] 53764.0943 CCD 3855.5 II 0.00125 [1]
51216.0490 CCD −3917 I 0.03173 [1] 53774.0928 CCD 3886 I 0.00087 [1]
51485.8480 CCD −3094 I 0.02483 [1] 53775.0773 CCD 3889 I 0.00187 [1]
51908.2566 CCD −1805.5 II 0.02164 [1] 53799.0056 CCD 3962 I −0.00158 [1]
51950.0564 CCD −1678 I 0.02283 [1] 53829.0034 CCD 4053.5 II −0.00042 [1]
51958.0953 CCD −1653.5 II 0.02984 [1] 54499.7477 CCD 6099.5 II −0.0008 [1]
52297.0621 CCD −619.5 II 0.01815 [1] 54529.5808 CCD 6190.5 II −0.00043 [1]
52299.0260 CCD −613.5 II 0.01505 [1] 54554.9858 CCD 6268 I −0.00243 [1]
52300.0136 CCD −610.5 II 0.01916 [1] 54829.8831 CCD 7106.5 II 0.00757 [1]
52341.9745 CCD −482.5 II 0.01754 [1] 54889.7014 CCD 7289 I −0.00351 [1]
52343.1236 CCD −479 I 0.01922 [1] 55593.0691 CCD 9434.5 II 0.00021 [1]
52347.5530 CCD −465.5 II 0.02289 [3] 55623.3952 CCD 9527 I 0.00183 [1]
52629.8087 CCD 395.5 II 0.01506 [3] 55632.7353 CCD 9555.5 II −0.00129 [1]
52657.1851 CCD 479 I 0.01748 [1] 55979.5835 CCD 10613.5 II 0.00045 [1]
52660.7860 CCD 490 I 0.01222 [1] 56000.7286 CCD 10678 I 0.00037 [1]
52696.0303 CCD 597.5 II 0.01456 [1] 56003.3548 CCD 10686 I 0.00391 [1]
53018.1229 CCD 1580 I 0.01202 [1] 56298.8936 CCD 11587.5 II 0.00198 [1]
53025.8222 CCD 1603.5 II 0.00727 [3] 56743.4426 CCD 12943.5 II 0.01052 [1]
53055.0030 CCD 1692.5 II 0.011 [1] 56745.4071 CCD 12949.5 II 0.00803 [1]
53088.6048 CCD 1795 I 0.01 [3] 57010.1411 CCD 13757 I 0.01752 [1]
53094.3444 CCD 1812.5 II 0.01254 [1] 57048.3330 CCD 13873.5 II 0.01697 [1]
53105.1573 CCD 1845.5 II 0.00697 [1] 57048.4950 CCD 13874 I 0.01506 [1]
53387.4199 CCD 2706.5 II 0.00605 [1] 57097.3480 CCD 14023 I 0.02106 [1]
53387.5812 CCD 2707 I 0.00343 [1] 57415.0261 CCD 14992 I 0.02976 [1]
53387.7496 CCD 2707.5 II 0.00792 [1] 57415.0262 CCD 14992 I 0.02986 [1]
53388.7329 CCD 2710.5 II 0.00772 [1] 57415.0278 CCD 14992 I 0.03146 [1]
53389.7165 CCD 2713.5 II 0.00783 [1] 57415.0281 CCD 14992 I 0.03176 [1]
53400.6934 CCD 2747 I 0.00235 [1] 57498.4590 CCD 15246.5 II 0.02936 [1]

Notes: References [1] http://var2.astro.cz/ocgate/; [2] Agerer & Hubscher ( 1996 ); [3] Samolyk ( 2012 ).

Table 2 Times of Light Minimum for FG Hya

Parameter Value Error
a 1 0.0577
a 2
b 1 –0.0520
b 2 0.0079
b 3 0.0015
c 1 –0.0374
c 2 –0.0069
c 3 0.0028
ω (rad) (rad)

Table 3 Parameters in the Period Changes of FG Hya

Parameter Value Unit
T 0 2452500.1937 (d)
54.44 (yr)
e 0.40
113.7 (°)
13.00 (AU)
τ 2444804 (d)
0.7023 ( )

Table 4 Orbital Parameters of the Tertiary Component Star in FG Hya

2.2 GR Vir

To study the orbital period change of GR Vir in detail, we have collected all available photoelectric and CCD times of light minimum from the literature. Since some of the data have been tabulated by Qian & Yang ( 2004 ), here we only list the others in Table 5. The values computed with the linear ephemeris given by Kreiner ( 2004 ),

are displayed in Figure 2. By using the least-squares method, the following equation is obtained,
The sinusoidal term in Equation (8) suggests a cyclic oscillation with a period of and an amplitude of A = 0.0352 d. The orbital parameter of the third body was computed to be with the well-know equation
where a 12 is the orbital radius of the binary rotating around the common center of mass, i is the inclination of the orbit of the third component and c is the speed of light.

Fig. 2 Same as Fig. 1 but for GR Vir. In the upper panel, the dots refer to photoelectric and CCD data computed by the linear ephemeris (i.e., Equation (7)). The solid line represents our fitting curve. Residuals coming from Equation (8) are displayed in the lower panel.
HJD.2400000+ Method E Type Ref. HJD.2400000+ Method E Type Ref.
48500.069 CCD −11528.5 II 0.02516 [1] 54961.0042 CCD 7092.5 II 0.02938 [1]
50594.002 CCD −5493.5 II −0.00664 [1] 54966.0339 CCD 7107 I 0.02802 [1]
52782.511 CCD 814 I −0.01179 [1] 54976.9647 CCD 7138.5 II 0.02926 [1]
53489.1241 CCD 2850.5 II −0.00338 [1] 54978.0003 CCD 7141.5 II 0.02395 [1]
53490.6846 CCD 2855 I −0.00425 [2] 55004.375 CCD 7217.5 II 0.02891 [1]
53492.594 CCD 2860.5 II −0.00319 [2] 55384.316 CCD 8312.5 II 0.03761 [1]
53492.7665 CCD 2861 I −0.00417 [2] 55616.6148 CCD 8982 I 0.0399 [1]
53496.7582 CCD 2872.5 II −0.00263 [2] 55632.5836 CCD 9028 I 0.04808 [4]
53497.624 CCD 2875 I −0.00425 [2] 56100.481 CCD 10376.5 II 0.05624 [1]
53497.7983 CCD 2875.5 II −0.00344 [2] 56404.4268 CCD 11252.5 II 0.05622 [1]
53846.1555 CCD 3879.5 II −0.00426 [1] 56419.0047 CCD 11294.5 II 0.06135 [1]
53854.1363 CCD 3902.5 II −0.00377 [1] 56421.0878 CCD 11300.5 II 0.06251 [1]
53861.0808 CCD 3922.5 II 0.00133 [1] 56426.9863 CCD 11317.5 II 0.06267 [1]
53877.0415 CCD 3968.5 II 0.0014 [1] 56455.435 CCD 11399.5 II 0.05979 [1]
53877.2173 CCD 3969 I 0.00371 [1] 56456.479 CCD 11402.5 II 0.06288 [1]
54587.1354 CCD 6015 I 0.02091 [1] 56805.5413 CCD 12408.5 II 0.07324 [1]
54647.3362 CCD 6188.5 II 0.02239 [3] 57140.0262 CCD 13372.5 II 0.0789 [1]
54951.1149 CCD 7064 I 0.02873 [1] 57187.7353 CCD 13510 I 0.07961 [1]
54954.0645 CCD 7072.5 II 0.02908 [1]

Notes: References [1] http://var2.astro.cz/ocgate/; [2] Ogloza et al. ( 2008 ); [3] Yilmaz et al. ( 2009 ); [4] Hubscher et al. ( 2012 ).

Table 5 Times of Light Minimum for GR Vir

In Figure 2, one can clearly see that our fitting is reasonable, and the sum of squares of the residuals is very acceptable. The residuals from Equation (8) are displayed in the lower panel of Figure 2.

3 Mechanisms of the Orbital Period Variations

3.1 Light-Time Effect of a Third Companion

The orbital period oscillation of FG Hya and GR Vir may be caused by the light-time effect of a third companion. If this is true, the orbital period of the third body rotating around the eclipsing pair is about 54.44 yr and 28.56 yr for FG Hya and GR Vir, respectively. For GR Vir, considering the third body is moving in a circular orbit and inserting the absolute parameters compiled in Table 1 into the well-known equations

we can compute the lower limit on the third body’s mass and the upper limit on orbital radius to be and respectively.

For FG Hya, the third body is moving in an elliptical orbit. By inserting the absolute parameters of FG Hya (Table 1) and the orbital parameters of the tertiary component star (Table 4) into Equation (10), the minimum mass of the third body can be computed to be .

3.2 Magnetic Activity Mechanism

Since the spectral type of GR Vir is G0, it is a late-type binary, and its curve shows normal sinusoidal variation. This means that the magnetic activity mechanism is also a possibility for explaining cyclic variations of the period in this system. Using the following formula (Lanza et al. 1998 )

where
we can calculate variation of the quadruple moment for the primary star and for the secondary star.

4 Discussion and Conclusions

We have analyzed the orbital period of FG Hya and GR Vir. It is found that both FG Hya and GR Vir show period oscillations. We can compare our results with the works of Qian & Yang ( 2004 ; 2005 ). They found that orbital periods of both FG Hya and GR Vir show sinusoidal variations superimposed on secular period decreases (see Sect. 1). In our results, we found that each of the orbital periods of these two systems only shows a periodic oscillation. No secular period changes were discovered in either overcontact binary, meaning that FG Hya and GR Vir may be in the transition between thermal relaxation oscillation (TRO)-controlled and variable angular momentum loss (AML)-controlled stages (Qian et al. 2005 ).

In Section 3, we interpreted the orbital period oscillations of FG Hya and GR Vir by the light-time effect of third bodies, and obtained the mass of the third body for FG Hya and for GR Vir. If such large third bodies really exist, they should be found photometrically. So far, no third lights have been observed in either system, suggesting that the third bodies may be unseen components, e.g., a small black hole or a white dwarf.

Since the spectral type of GR Vir is G0, the orbital period oscillation can also be explained by magnetic activity on both components of the system. This mechanism is based on the hypothesis that a hydromagnetic dynamo can produce changes in the gravitational quadrupole moment of an active star through a redistribution of the internal angular momentum and/or action of the Lorentz force in the stellar convective zone (Applegate 1992 ; Lanza et al. 1998 ). If energy for transferring angular momentum is provided by luminosity variation of the active star, there must be . Based on the equation given by Yu et al. ( 2015 ),

we can obtain and , implying that the mechanism of magnetic activity can be used to explain cyclic variations of GR Vir. In this equation, G, σ and T are the gravitational constant, Stefan-Boltzman constant and surface temperature of the active star, respectively. All these physical elements are in the international system of units.

In summary, the orbital periods of FG Hya and GR Vir show periodic oscillations with periods of and , respectively. Our study demonstrates that periodic variations in FG Hya and GR Vir can be explained by the light-time effect of third bodies. For GR Vir, the orbital period oscillation can be also explained by magnetic activity on both components of the system. In order to check these conclusions, more observations are needed.

Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 11703020) and Joint Research Funds in Astronomy (U1531108, U1731106 and U1731110) under cooperative agreement between the National Natural Science Foundation of China and Chinese Academy of Sciences.


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Cite this article: Zhang Xu-Dong, Yu Yun-Xia, Xiang Fu-Yuan, Hu Ke. Reanalysis of the orbital period variations of two DLMR overcontact binaries: FG Hya and GR Vir. Res. Astron. Astrophys. 2017; 12:128.

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