The highest rate orbital period increasing contact binary LP UMa revisited
Introduction
LP UMa (GSC 3822-1056) was first discovered to be variable by Martin (2000) and Bíró (2000) when observing the novalike variable DW UMa. At first, LP UMa was regarded as a δ Scuti variable by Martin (2000), based on single band photometry. Later in the same year, Bíró (2000) concluded that LP UMa is a β Lyr-type binary star by analysing VR bands photometric observations of 4 consecutive nights.
The first result of VRc photometry for LP UMa together with the light curve solution and the period analysis were presented by Csizmadia et al. (2003). Their light curve showed a positive type of O’Connell effect, which was explained by a dark spot on the secondary component. Their photometric solutions showed that LP UMa was a deep over-contact binary system with a high mass ratio of 0.886, and the temperature difference between the components was up to 1045 K. Vinod et al. (2014) derived some large different photometric results by using the same mass ratio with Csizmadia et al. (2003), such as a much lower fill-out factor (). However, no explanations were given by Vinod et al. (2014). Recently, Liao et al. (2015) reanalysed LP UMa using more smooth and continuous VRI bands light curves which were obtained on 2013 January 11. The photometric analysis by Liao et al. (2015) indicated that LP UMa is a deep, unusual over-contact binary system with a high mass ratio of 0.823. Different authors obtain different results, making LP UMa a very interesting target to be investigated.
In this paper, complete BV(RI)c light curves of LP UMa were presented and analyzed. At the same time, orbital period variation was also investigated by all available times of light minimum.
Section snippets
CCD photometric observations for LP UMa
Complete BV(RI)c photometric observations of LP UMa were carried out on 2015 February 23 using an Andor DZ936 CCD camera attached to the 1.0 m Cassegrain telescope (Hu et al. 2014) at Weihai Observatory of Shandong University. The Andor DZ936 CCD camera has 2048 × 2048 square pixels (13.5 × 13.5μm pixel), resulting an effective field of view about 11.8′ × 11.8′. The filter system is a standard Johnson and Cousins filter system (B, V, Rc, and Ic). The exposure times for each image were 60s,
Orbital period analysis
Csizmadia et al. (2003) firstly investigated the orbital period of LP UMa using a total of 45 times of minimum and found an extremely high rate of period increase which was explained by light-time effect or mass transfer between the components. Recently, Vinod et al. (2014) found that the period has increased by 10.8 s century by using 118 timing of minimum over a span of 14 years. Liao et al. (2015) confirmed that the highest rate of long-term period increase by computing 130 times of light
Reanalyzing light curves
As shown in Fig. 2, the shape of light curves changed significantly among different epochs. The depths of the two minima became much smaller than previous observations, revealing that the effective temperatures of both components were becoming similar. We analyze the light curve with the assumption of the presence of cool/hot spot models due to the asymmetric light curve using the fourth version of the W-D program (Wilson, Devinney, 1971, Wilson, 1990, Wilson, 1994, Wilson, Van Hamme). In the
Results and discussions
Complete photometric data were obtained in the BV(RI)c bands on a single night. It shows that the light curves are asymmetric. Photometric solutions based on four-band CCD light curves have been derived with the Wilson–Devinney code. The results showed that LP UMa is an A-subtype contact binary system with a mass ratio of q = 0.331, and a contact degree of . The asymmetric light curve could be explained by the presence of a hot spot on the more massive component. As
Acknowledgments
This work is partly supported by the National Natural Science Foundation of China (Nos. 11203016, 11333002, U1431105), and by the Basic Research Program of Shandong University at Weihai (No. 2015ZQXM013), and by the Open Research Program of Key Laboratory for the Structure and Evolution of Celestial Objects (No. OP201303). We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this study
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