New Light Curves and Orbital Period Investigations of the Interacting Binary System UV Piscium

Abstract

UV Psc is a typical RS CVn type system undergoing dynamic chromosphere activity. We performed photometric observations of the system in 2015 and secured new BVR light curves showing well-defined photometric waves. In this paper, we analyzed the light curves using Wilson-Devinney binary code and investigated the orbital period of the system. The combination of our light curve synthesis with the spectroscopic solution developed by previous investigators yielded the absolute parameters as: $M_1=1.104{\pm}0.042M_{\odot}$, $R_1=1.165{\pm}0.025R_{\odot}$, and $L_1=1.361{\pm} 0.041L_{\odot}$ for the primary star, and $M_2=0.809{\pm}0.082M_{\odot}$, $R_2=0.858{\pm}0.018R_{\odot}$, and $L_2=0.339 {\pm}0.010L_{\odot}$ for the secondary star. The eclipse timing diagram for accurate CCD and photoelectric timings showed that the orbital period may vary either in a downward parabolic manner or a quasi-sinusoidal pattern. If the latter is adopted as a probable pattern for the period change, a more plausible account for the cyclic variation may be the light time effect caused by a circumbinary object rather than an Applegate-mechanism occurring via variable surface magnetic field strengths.

1. INTRODUCTION

RS CVn type system is a detached binary showing very dynamic chromosphere activity (Hall 1976). These systems are composed of rapidly rotating late-type or giant stars (Kim et al. 2014). The light curves of RS CVn type systems show a distortion wave caused by stellar activity, particularly at outside eclipse phases. The wave changes continuously according to variations in cool spots. Moreover, angular momentum loss (AML) can occur via magnetized stellar winds induced by the chromospheric activity of the system, which finally decreases the orbital period. Therefore, long-term photometric observations of RS CVn type systems are important to more clearly understand these phenomena.

UV Psc, discovered by Huth (1959), is a representative RS CVn type system. The distortion wave was clearly observed in the previous light curves of this system (Oliver 1974; Sadik 1979; Vivekananda Rao & Sarma 1981, 1983a; Zeilik et al. 1982; Antonopoulou 1983, 1987; Han & Kim 1988). This phenomenon was interpreted as being caused by the existence of cool spots (Budding & Zeilik 1987; Han et al. 1996; Radhika & Vivekananda Rao 2001; Vivekananda Rao & Radhika 2002; Kjurkchieva et al. 2005). The radial velocity (RV) curves for both components were determined by Popper (1991, 1997), and Kjurkchieva et al. (2005) despite the fact that it is not easy to measure the spectral lines of the less massive component due to its fast rotation. The observed RV curves indicated that the mass ratio of this system falls within the range of 0.7 to 0.8.

Many investigations of variations in the orbital period have been performed to understand the dynamic properties of the UV Psc system, but the orbital period variation of this system remains controversial. Sadik (1979) and Kjurkchieva et al. (2005) suggested a decreasing orbital period, while Hall & Kreiner (1980) and Milano et al. (1986) proposed an increasing orbital period. Shengbang et al. (1999) suggested that the orbital period oscillates with a period of approximately 61 years and a semi-amplitude of 0.21 × 10^–5 days.

In this study, we investigated the physical properties of UV Psc using new multiband photometric light curves obtained during the observing seasons in 2015. An overview of our observations is provided in Section 2. Sections 3 and 4 contain the investigations of the light curves and orbital period variation, respectively. Finally, we summarize the results presented in Section 5.

2. NEW OBSERVATIONS

New photometric observations of UV Psc were performed at Jincheon station of the Chungbuk National University Observatory (CBNUOJ) over 12 nights from October to November 2015 using the 60 cm reflector installed by the Korea Astronomy and Space Science Institute and operated by CBNUOJ. The telescope was equipped with an electronically cooled SBIG STX16803 4K CCD camera at a f/2.92 prime focus. A field of view of 72’ × 72’, and an image scale is 1.05 arcsec pixel^–1. Standard BVR filters were used during the observations with different exposures from 30 to 90 s according to the weather conditions for each observation. Further details regarding the telescope and instrument systems can be found in Han at al. (2015). The brightness values of the field stars were measured using aperture photometry and all images were corrected with bias, dark, and flat-field image. All reduction processes were performed using the IRAF package. TYC 26-70-1 and TYC 26-1046-1 in the UV Psc field were chosen as the comparison (C) and check (K) stars, respectively; further information is listed in Table 1. During the entire observation period, observational errors were calculated using the standard deviations of the differential BVR magnitudes between the K and C stars, were 0.^m013, 0.^m014, and 0.^m015, respectively. A total of 2913 observations were obtained (973 in B, 972 in V, and 968 in R), and a representative sample data set is listed in Table 2.

3. Light Curve Synthesis

3.1 New Light Curves of UV Psc

Fig. 1 shows the V light curves calculated by the ephemeris (Kreiner 2004):

C = HJD 2452500.0411(±0.0007) + 0.^d 8610468(±0.0000003) E,     (1)

where the black dot and red cross represent the data measured before and after HJD2457315, respectively. The light curve shows light variation of approximately 0.^m05 at the primary eclipse where the more massive and hotter primary component is transited by the less massive and cooler secondary component. This value is larger than that of the observational error. In contrast, the brightness at the secondary eclipse remained unchanged. These facts may imply that the spot effect of the less massive component predominates in this light curve.

3.2 Binary Modeling and Absolute Dimensions

In this light curve investigation, we used the BVR data acquired before HJD2457315. To model the BVR light curves of UV Psc, Wilson-Devinney binary code (Wilson & Devinney 1971; WD) mode 2 for the detached binary systems was used. The temperature of the more massive star (T₁) and mass ratio (q = M₂/M₁) were fixed as 5780 K and 0.733, as determined by Popper (1997) and Kjurkchieva et al. (2005), respectively. Considering that RS CVn type system consists of two late-type main-sequence stars with convective envelopes, the gravity darkening coefficients and bolometric albedos for both components were assumed to be g₁ = g₂ = 0.32 and A₁ = A₂ = 0.5, respectively (Lucy 1967; Rucinski 1969). The limb-darkening coefficients obtained using the logarithmic law were adopted from the table developed by van Hamme (1993). The projected rotation velocities of the primary and secondary components measured by Kjurkchieva et al. (2005) were ν₁sini = 66 and ν₂sini = 54 km/s, respectively, while the synchronized rotation velocities calculated by the absolute parameters in Kjurkchieva et al. (2005) were ν_1,syn = 67.0 ± 2.3 km/s and ν_2,syn = 49.9 ± 5.3 km/s, respectively. Because the projected rotation velocities are in excellent agreement with the synchronized velocities within the errors, we assumed the rotation parameters to be F₁ = F₂ = 1. In addition, the adjustable parameters were the initial epoch (T₀), orbital period (P), inclination (i), temperature of the secondary component (T₂), surface potentials (Ω₁ and Ω₂), relative luminosity of the primary component (l₁), and third light (l₃).

A cool spot was proposed as an explanation to interpret the distortion wave of the light curve, as suggested by Budding & Zeilik (1987), Han et al. (1996), Radhika & Vivekananda Rao (2001), Vivekananda Rao & Radhika (2002), and Kjurkchieva et al. (2005). To interpret the distortion wave in our light curves, we adopted the cool spot on the secondary component, but an appropriate model was not calculated. Therefore, an additional cool spot on the primary component was adopted because the primary component also has a convective envelope. The final WD solution of UV Psc is listed in Table 3 and the synthetic BVR light curves are shown in the upper panel of Fig. 2 as solid lines. The spot radii of the primary and secondary components are 15.00 and 35.71 deg, indicating that both components are the sources of the distortion wave in UV Psc. The temperature of the secondary component is T₂ = 4756 ± 4 K. The orbital inclination was modelled as i = 90.10 ± 0.18 deg, implying that the line of sight is almost parallel to the orbital plane and UV Psc may have a retrograde orbit. From the i, the model light curves in Fig. 2 show the total eclipse at the secondary eclipse for approximately 27 min. The third light parameter l₃ was adjusted but not detected. To figure out the effect of the spots, theoretical light curves were calculated without considering the spots in Table 3 and are shown as dashed lines in the upper panel of Fig. 2. The residuals corresponding to presence and absence of the spots are plotted in the lower panel of Fig. 2 as the black and gray circles, respectively. Fig. 2 shows that the light depression from phase 0 to 0.3 was affected mainly by the cool spots, which existed actively on the surfaces of both components during the observation period.

To calculate the absolute parameters of UV Psc, we used the photometric solution in Table 3 and the semi-amplitude of the RV curves (K₁ and K₂) from Kjurkchieva et al. (2005). The masses, radii, and luminosities of both components were obtained as follows: M₁ = 1.104 ± 0.042 and M₂ = 0.809 ± 0.082 Mⵙ; R1 = 1.165 ± 0.025 and R2 = 0.858 ± 0.018 Rⵙ; and L₁ = 1.361 ± 0.041 and L₂ = 0.339 ± 0.010 Lⵙ, respectively. These values are similar to those reported by Popper (1997) and Kjurkchieva et al. (2005).

4. Orbital Period Variation Investigation

To investigate the orbital period variation of UV Psc, we determined 17 times of minimum light (primary: 10, secondary: 7) from measurements by Han & Kim (1988), Heckert (2012), and CBNUOJ using Kwee & van Woerden (1956) method. We also collected the times of minimum light from previous studies, the O – C gateway (http://var. astro.cz/ocgate/), and the database compiled by Kreiner et al. (2000). The same epoch data collected from the same literature were calculated as a weighted mean value. All data used in this investigation are listed in Table 4. The eclipse timing diagram of UV Psc was calculated using Eq. (1) and is shown in Fig. 3, where the timings are differentiated by assorted symbols according to the observational method. From the diagram, the CCD and photoelectric (O – C) residuals after 1966 clearly varied in a continuously decreasing pattern, although this pattern does not appear in the residuals for earlier plate timings because of their large short-term scattering. Therefore, we fitted all (O – C) residuals to a downward parabolic ephemeris using a leastsquare fitting method. The resultant ephemeris is as follows : 

(O – C) = –0.^d38(±3.65) × 10^–4 + 1.^d 33(±0.74) × 10^–7E –2.^d 16(±0.72) × 10^–11E².       (2)

In this calculation, the weights of plate (P), visual (VI), photoelectric (PE), and CCD measurements were assigned values of 0.01, 1, 10, and 10, respectively. The black dashed line in the first panel of Fig. 3 was drawn using Eq. (2), indicating that the equation is in good agreement with CCD and PE data. The second panel shows only the residuals ((O – C)_res,1) of PE and CCD timings from Eq. (2). The standard deviations from all timings and those from the PE and CCD timings are σALL,1 = 0.0144 and σCCD,PE,1 = 0.0016, respectively. The coefficient of quadratic term in Eq. (2) gives an orbital period decrease of 1.83(±0.06) × 10–8 days year–1.

As mentioned in Section 1, Shengbang et al. (1999) proposed a cyclic orbital period variation. If we assume that the continuously decreasing variation in the (O – C) residuals of the PE and CCD timings in Fig. (3) is a part of a sinusoidal variation, the residuals could be fitted to a sine curve ephemeris using an iterative least-squares method. The equation was obtained as follows: