New Photometric Investigation of the Low-Mass-Ratio Contact Binary Star V1853 Orionis

Four-color charge-coupled device (CCD) light curves in $B$, $V$, $Rc$ and $Ic$ bands of the total-eclipsing binary system, V1853 Ori, are presented. By comparing our light curves with those published by previous investigators, it is detected that the O'Connell effect on the light curves is disappeared. By analyzing those multi-color light curves with the Wilson-Devinney code (W-D code), it is discovered that V1853 Ori is an A-type intermediate-contact binary with a degree of contact factor of $f=33.3\%(3.7\%)$ and a mass ratio of $q=0.1896(0.0013)$. Combining our 10 new determined times of light minima together with the others published in the literature, the period changes of the system is investigated. We found that the general trend of the observed-calculated $(O-C)$ curve shows a downward parabolic variation that corresponds to a long-term decrease in the orbital period with a rate of $dP/dt=-1.96(0.46)\times{10^{-7}}$ d yr$^{-1}$. The long-term period decrease could be explained by mass transfer from the more-massive component to the less-massive one. By combining our photometric solutions with the Gaia DR 2 data, absolute parameters were derived as $M_{1}$ = 1.20 M$_{\odot}$, $M_{2}$ = 0.23 M$_{\odot}$, $R_{1}$ = 1.36 R$_{\odot}$, and $R_{2}$ = 0.66 R$_{\odot}$. The long-term period decrease and the intermediate-contact configuration suggest that V1853 Ori will evolve into a high fill-out overcontact binary.


INTRODUCTION
V1853 Ori (GSC 01283-00053 = NSVS 9553026 = ASAS 051305+155812) was discovered as a variable star by the Robotic Optical Transient Search Experiment (ROTSE)-I telescope by Gettel et al. (2006). They pointed out it was a contact binary candidate by using the observed period-color relation and the binary nature was confirmed by visual examination of the light curves. Later, a total of 236 measurements in V and R bands were obtained by Blaettler & Diethelm (2007). They classified V1853 Ori as an EW-type binary and gave the first linear ephemeris, and Rc bands but negative O'Connell effect in U and Ic bands. The asymmetry light curves were explained by two dark spots on the primary star. Their photometric analysis with W-D code showed that this system is an extreme mass ratio W-type overcontact binary (q = 0.20). The derived fill-out factor 50% revealed that it approaches its final stages of binary star evolution. These properties indicate that V1853 Ori is an interesting target for further investigations. Both components in the binary may merge into a rapid-rotating single star when it meets the more familiar criterion that the orbital angular momentum is less than 3 times the total spin angular momentum, i.e., Jorb < 3Jrot (Hut 1980). Therefore, it is a progenitor candidate of luminous red novae (e.g., Zhu et al. 2016).
They showed that the period of V1853 Ori is not variable. Recently, V1853 Ori was observed by LAMOST (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope) spectroscopic survey on October 15, 2014. Stellar atmospheric parameters of the binary published by Qian et al. (2017a) and are listed in Table   1. The distributions of the metallicity ([Fe/H]) and the gravitational acceleration log g for EWs were given by Qian et al. (2018). It is show that the values of V1853 Ori are close to the peaks of the distributions.
Moreover the period (0.38 d) of this system is also close to the peak of the period distribution given by Qian et al. (2017a). These reveal that V1853 Ori is a typical EW-type binary. This eclipsing binary was also observed by Gaia DR 2 (Gaia Collaboration et al. 2018) and the parallax (π = 2.6856(0.0465) mas) was Photometric Investigation of V1853 Ori 3  Table 3, Table 4, Table   5 and are shown in Figure 2. The magnitude difference between the comparison and the check stars are also shown in the low pane of this figure. The epoch in the equation (3) is obtained by us and the period is from Samec et al. (2011).
Apart from the 1-m reflecting telescope at the Yunnan Observatories, we also used the 0.4-m telescope at Naresuan University in Thailand and the 60-cm R-C reflect telescope at the Yunnan Observatories in China. By using least-squares parabolic fitting method, 30 CCD times of light minimum averaged into 10 mean eclipse times are listed in Table 7.

ORBITAL PERIOD INVESTIGATION
Since the variable was discovered by Gettel et al. (2006), the period changes were study only by Samec et al. (2011), and their study shows no variation. Diethelm (2011) and Diethelm (2012) (3) are listed in the third column of Table 8    With the quadratic term of this ephemeris, a continuous period decrease, at a rate of dP/dt = −1.96(0.46)× 10 −7 d yr −1 is determined. The residuals from equation (4) are showed in the lower panel of Figure 3 and listed in the column 4 of Table 8.

PHOTOMETRIC SOLUTIONS
To understand its geometrical structure and evolutionary state, the B, V , Rc and Ic light curves shown in the Figure     are nearly the same indicating the nearly same temperature of both components. Therefore, we take the same values of the gravity-darkening coefficients and the bolometric albedo for both components, i.e., g 1 = g 2 = 0.32 (Lucy 1967) and A 1 = A 2 = 0.5 (Ruciński 1969). The limb-darkening coefficients were used according to Claret & Gimenez (1990) (x and y are the bolometric and bandpass limb-darkening coefficients.). The adjustable parameters include: the orbital inclination (i); the mean temperature of star 2 (T 2 ); the monochromatic luminosity of star 1 (L 1B , L 1V , L 1Rc , L 1Ic ); and the dimensionless potential of star 1 (Ω 1 = Ω 2 , mode 3 for overcontact configuration).
A q-search (q = M 2 /M 1 ) method was used to determine the mass ratio of the system. Solutions were carried out for a series of values of the mass ratio. For each value of q, the calculation started at mode 2 (detached mode) and we found that the solutions usually converged to mode 3. The relation between the resulting sum ΣW i (O − C) 2 i of weighted square deviations and q is plotted in Figure 4. As shown in the figure, two minima are found at q = 0.18 and q = 5.6. They are the inverses of each other. The 18 is less than that at q = 5.6. Therefore, we chose the initial value of q as 0.18 and made it as an adjustable parameter. Then we performed a differential correction until it converged and final in column 2 Table 9 Figure 6.

DISCUSSIONS AND CONCLUSIONS
The light curves of V1853 Ori published by Samec et al. (2011)    mass ratio (q = 0.2), W-type contact binary with a fill-out factor of ∼50%. By analyzing our light curves obtained by the 1.0-m reflecting telescope at the Yunnan Observatories with the W-D code, we found that V1853 Ori is an A-type contact binary with a mass ratio q=0.1896 that is slightly smaller than the value q=0.20 obtained by Samec et al. (2011). But the fill-out factor we derived is 33.3% that is much smaller than that 50% obtained by Samec et al. (2011). The photometric results of Samec et al. (2011) showed that the temperature of the less-massive component was about 61 K higher than the more-massive one, while our photometric results showed that the temperature of the less-massive is only 3 K lower than the moremassive one. We think that our results are more credible because they are based on the hight-precision B, V , Rc and Ic bands and symmetric CCD LCs. It is possible that the two cool magnetic spots on the  (2011) and Diethelm (2012) and green solid dots refer to the data obtained by us.  (1964). Such phenomena also encountered in other W UMa binaries such as RZ Com (He & Qian 2008) and FG Hya (Qian & Yang 2005).
The absolute magnitude of V1853 Ori in V band is estimated as 3.67 mag by using the relation  Radius of star 2 (relative to semimajor axis) in pole direction 0.2404 (31) Radius of star 2 (relative to semimajor axis) in side direction 0.2516 (38) Radius of star 2 (relative to semimajor axis) in back direction 0.2955 (83) Equal-volume radius of star 1 (relative to semimajor axis) R1 0.54201 (63) Equal-volume radius of star 2 (relative to semimajor axis) R2 0.2634 (30) Radius ratio R2/R1 0.4859 (56) Σω ( on our mass ratio. By using the kepler's third law, the semi-major axis of the binary is derived as 2.50 R ⊙ .
Finally, the radii of the components R 1 and R 2 are computed as 1.36 R ⊙ and 0.66 R ⊙ , respectively.
By monitoring V1853 Ori for about six year, 10 times of light minimum were obtained. A period analysis with all available eclipse times shows that the period of V1853 Ori is decreasing at a rate of dP/dt = −1.96(0.46)× 10 −7 d yr −1 . This is very common in contact binary, such as V524 Mon (dP/dt = Cyg (dP/dt = −2.24 × 10 −7 d yr −1 , Tian et al. 2018). The long-term decrease of the orbital period could be explained by mass transfer from the more-massive component to the less-massive one. As the orbital period is decreasing, V1853 Ori will evolve into a high fill-out overcontact binary. Finally, it will merge and produce a luminous red nova similar to that of V1309 Sco (e.g., Zhu et al. 2016).