Spot Activity in the RS Cvn Binary UX Arietis
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J. Astrophys. Astr. (1984) 5, 159–168 Spot Activity in the RS CVn Binary UX Arietis M. B. K. Sarma & B. V. N. S. Prakasa Rao Nizamiah Observatory, Centre of Advanced Study in Astronomy, Osmania University, Hyderabad 500007 Received 1983 October 19; accepted 1984 January 25 Abstract. Photoelectric observations of the RS CVn type non-eclipsing binary UX Arietis obtained at Nizamiah Observatory during the observing seasons of 1975–76, 1981–82 and 1982–83 are presented. The light curve of UX Ari showed a distortion wave with an amplitude in V varying from 0.02 mag during 1975–76 to 0.15 mag during 1982–83. An analysis of the available data shows that the light maximum is almost constant. It is also evident that the light-curve minimum decreases as the wave amplitude increases. The constant light at maximum, V = 6.51 ± 0.03 indicates the unspotted photospheric brightness. It is also suggested that the variation in mean V brightness is mainly due to spot activity and not due to intrinsic variation. Key words: photoelectric photometry—RS CVn variables—stars, individual 1. Introduction UX Arietis (HD 21242), an RS CVn type system, is a double-lined spectroscopic binary with an orbital period of 6.44 d. Carlos & Popper (1971) have classified the components as G5 V and K0 IV. This system displays quasi-sinusoidal light variations modulated with the orbital period. A series of light curves have been obtained at a number of observatories (Guinan et al. 1981 and the references therein). Guinan et al. (1981) have showed that the wave minimum had retrograde motion during 1972–77 while it appeared to have direct motion during early and late 1980. Zeilik et al. (1982) have reported that the minimum phase of the distortion wave has remained steady for a year (early to late 1981) and will perhaps have retrograde motion again. The amplitude of the wave varied non-uniformly having the range 0.09 to 0.05 mag between 1974.9 to 1980.0 and increasing from 0.06 to 0.17 mag during late 1980 (Guinan et al. 1981). The non-uniform variation of the amplitude of the wave has been attributed to the variation in the total spot area or to the change in spot location or both. On the other hand, as we have already reported (Sarma, Prakasa Rao & Ausekar 1983, hereafter SPA) the behaviour of the phase of light minimum and amplitude of the distortion wave is systematic and not erratic as reported by Zeilik et al. (1982). It is evident from Fig. 2 of SPA that the motion of the phase of wave minimum is direct in the beginning of a cycle and becomes retrograde after the spots migrate below the co- rotating latitude (the latitude that is rotating in synchronism with the orbital period of the system). The amplitude of the wave is constant during one spot cycle. The period of this cycle is estimated to be ~ 5–6 yr. 160 M. B. K. Sarma & B. V.N. S. Prakasa Rao In this communication we present photometry of UX Ari in Β and V bands for three observing seasons and discuss the wave migration, the nature of spot activity and intrinsic variability of this system. 2. Observations UX Ari was observed during 1975–76, 1981–82, and 1982–83 seasons using the 38-cm Grubb refractor of the Nizamiah Observatory with an unrefrigerated RCA 1P 21 & EMI 9502B photomultipliers. The photocurrent was amplified with a GR 1230A DC amplifier and recorded on a Honeywell-Brown strip-chart recorder. Standard Johnson Β, V filters were used. 62 Ari and HR 999 were used as comparison and check stars respectively. The observations of the comparison star (62Ari) were used for determining the nightly extinction coefficients. The observations were made differentially with respect to the comparison star 62 Ari and were transformed to the standard Johnson system. The constancy of ∆m (HR 999–62 Ari) values suggests that the comparison star remained constant during the period of our observations within ± 0.02 mag in Β and V passbands. The transformation coefficients, the magnitude and colour of the com- parison star for the three years of observations are given in Table 1. The phases were calculated using the following ephemeris given by Landis et al (1978) (1) where zero phase corresponds to conjunction with the cooler component in front and the period is the orbital period determined spectroscopically by Carlos & Popper (1971). All the observations i.e. ∆m (variable – comparison) are grouped according to phases and normal points are formed for each passband. These are given in Tables 2 and 3 for yellow and blue passbands. The plot of ∆V versus phase is shown in Fig. 1. In order to determine the amplitude of the wave and the phase of its light minimum, individual observations are fitted to the truncated Fourier series (Sarma & Ausekar 1980), (2) The values of Am adopted to normalize l to 1 for each light curve are given in Table 4. The coefficients A0, A1, A2 and B1 are determined for each year of observation using the least-squares method. These coefficients, along with the full amplitude Table 1. Transformation coefficients. Spot activity in UX Arietis 161 Table 2. UX Arietis: Normal points in V band. 162 M. B. K. Sarma & B. V.N. S. Prakasa Rao Table 3. UX Arietis: Normal points in Β band. Spot activity in UX Arietis 163 164 M. B. K. Sarma & B. V.N. S. Prakasa Rao Figure 1. UX Ari: Plot of ∆m (normal points) versus phase in V band. 2 2 1/2 2(A1 +B1) in magnitude units and phase of light minimum, are also given in the table. 3. Discussion The data available in the literature on the epoch, phase of light minimum, full amplitude, ∆V max, ∆Vmin and Vmean for UX Ari are listed in Table 5. Table 5. UX Arietis: Photometric properties. References: 1. Guinan et al. (1981) 2. Present study 3. Zeilik et al. (1982) Spot activity in UX Arietis 165 3.1 Period of Wave Migration Zeilik et al. (1982) have reported that the phase of light minimum and amplitude of UX Ari showed an erratic behaviour during the past ten years. However, it is evident from Fig. 2 of SPA that the migration of wave minimum on the light curve was first direct while it was later moving towards decreasing orbital phases. A complete cycle is estimated to last about 5 to 6 years. At the end of the 1975–80 cycle, it appears that there has been a break in the smooth retrograde motion and a new cycle with a direct motion appears to have started around 1981. SPA have grouped the spots formed before 1975, between 1975–80 and 1981 onwards into three different cycles. Applying the analogy of the sunspots, it is assumed that spots first form on the star at a latitude higher than the co-rotating latitude. During their migration towards the equator the spots will have a direct motion before crossing the co-rotating latitude and a retrograde motion after crossing it. The spots of one cycle disappear after reaching the vicinity of the equator and the spots of the next cycle form above the co-rotating latitude. If the assumption of the solar analogy is applicable, two factors regarding the spots have to be considered. First, if the spots are migrating towards the equator, the rate of wave migration should increase along the cycle; secondly, as the spot cycle nears its end, its activity should reduce resulting in the decrease of the wave amplitude, provided there is no overlapping of spot cycles. We estimate from the data given in Table 5, for the cycle of 1975–80, that the wave migration has a rate of about 0.12 phase per year during its direct motion (1974.9–1975.3) and about 0.23 phase per year during its retrograde motion (1979.2–1979.9), in accordance with the idea that the spots are migrating from higher to lower latitudes. From Fig. 2 of SPA it appears that there is not much variation in the amplitude of the wave during the cycle 1975–80, suggesting that the nature of the spot or spots has not varied much in this cycle. If the solar analogy holds for UX Ari, this constancy in amplitude may be attributed to the overlap of spot cycles. In the absence of more direct evidence—such as photometric complications—indicating the coexistence of different spot cycles, we cannot be positive that the analogy of sunspot cycle holds for the spot cycles in UX Ari, as it does in RS CVn itself (Blanco et al. 1982). However, further observations over many cycles are necessary for a number of RS CVn systems before the final conclusion on the applicability of solar analogy to star spots is reached. 3.2 Colour Dependence In order to understand the colour changes of UX Ari, a plot of ∆ (B – V) versus phase is shown in Fig. 2 for the years 1976.0, 1982.0 and 1983.0. We infer from the figure that even though there is some scatter, the ∆(B – V) colour curve has remained flat during the three years. Also, it can be seen from Table 4 that the amplitude in Β and V passbands is fairly constant during each year suggesting that the amplitude is colour- independent. Because of this, any information regarding the physical nature of the spots cannot be obtained immediately. 3.3 Nature of the Spot Activity The wave amplitude showed a remarkable change from ∆V = 0.04 mag in 1979.9 to 0.21 166 M.