NEW CCD OBSERVATIONS AND THE FIRST PHOTOMETRIC STUDY OF THE CONTACT BINARY AP UMI

Abstract

We obtain the first complete CCD light curves (LCs) of the contact binary AP UMi in the VRI bands and analyzed them by means of the PHOEBE code. A spotted model is applied to treat the asymmetry in the LCs. The LC morphology clearly shows the O'Connell effect and the solution shows an influence of star spots on both components. Such effect of star spots is common between the RS CVn and W UMa chromospherically active stars. Based on the obtained solution of the LCs we investigate the evolutionary state of the components and conclude that the system is a pre-intermediate contact binary (f = 0.29) with mass ratio q = 0.38, and it is an A-type W UMa system where the less massive secondary component is cooler than the more massive primary one.

1. INTRODUCTION

Photometric and spectroscopic studies of eclipsing binaries (EBs) are important to determine their physical and geometrical properties. EBs in turn are important for testing stellar evolutionary models and can be used to determine mass and radius for both components of the binary system (e.g., Guinan et al. 2000; Torres & Ribas 2002).

One specific class of EBs is the W Ursae Majoris (W UMa) type systems whose light curves show continuous brightness variations and strongly curved maxima and minima with nearly equal depths. Binnendijk (1970) classified the W UMa binaries into two subclasses: A-type and W-type. The A-type binaries show moderate light curve variation (in comparison to W-type) or none at all and show deeper primary minima due to transit of the larger hotter component while the W-type binaries show primary minima due to occultation of the smaller less massive component. In general, orbital periods of A-type binaries are less than 0.3 day while for the W-type they are typically between 0.3 day and less than a day. In the case of W-type stars, which are contact or over-contact binaries, both components fill or over-fill their critical Roche lobes and are enclosed by a common convective envelope (e.g., El-Sadek 2012, and the references therein).

The evolution of the W-UMa type binaries has been discussed with more details in several studies (e.g., Vilhu 1982; Eggen & Iben 1989; Bradstreet & Guinan 1994). Most light curves of the W UMa binaries show the O’Connell effect, where a difference between the primary and secondary maxima is observed despite both components having nearly the same effective temperatures (O’Connell 1951). This asymmetry in the light curve maxima is often associated with the presence of star spot(s) on one or both components which is quite important in understanding the light variations of the W UMa binaries.

We aim at following several newly discovered W UMa systems observationally in different bands, analyze the obtained light curves and obtain new minima times, to enlarge the data base of these interesting short period eclipsing binaries which usually represent contact binaries that exhibiting a fairly sharp lower limit to their observed orbital periods, of around 0.22 day (Rucinski 2007; Lohr et al. 2012).

We study the variability of the newly discovered faint eclipsing binary system GSC 4405-00129 (=USNOA2.0 1575-03371434 = USNOB1.0 1600-0101666 = ALLWISE J134253.35+700149.6) with V = 14.4 – 15.1 (Khrusiov 2012), recently named AP UMi by Kazarovets et al. (2015). Khrusiov classified the system as an eclipsing W UMa type (EW) based on the light curve shape, as seen in Figure 1.

Figure 1.The light curve of the binary AP UMi without filter given by Khrusiov (2012).

 

2. OBSERVATIONS

New observations in VRI standard Johnson filters for the faint EW-type binary AP UMi were carried out using an EEV CCD 42-40 camera attached to the Newtonian focus of the 74-inch reflector telescope of the Kottamia observatory in Egypt. The CCD camera has a format 2048 × 2048 pixels with a scale of 0′′.308 pixel−1 that was cooled by liquid nitrogen down to about −125℃. The package C-Muniwin was used to reduce the CCD images.

We observed, in R and I bands, the variable (V), the comparison (C1) and the check (C2) stars during a very clear night of June 26, 2014. The identification field of the variable and comparison stars is shown in Figure 2. We used the following ephemeris given by Khrusiov (2012):

Figure 2.One of the CCD images of AP UMi (V), obtained using the 74 inch Telescope of the Kottamia observatory in Egypt. C1 and C2 are the comparison and check stars, respectively. North is up and east is to the left.

The coordinates and magnitudes of the variable and comparison stars are listed in Table 1 according to the catalogues noted at the end of the table. We again observed the system in the V -band during a clear night of July 5, 2014. The exposure time taken in V , R and I filters were 230, 120 and 120 seconds, respectively. A total of 84 observations in V, 79 observations in R and 77 observations in I were obtained and listed in Table 7, where ΔV , ΔR and ΔI in the table denote the magnitude differences in the sense, variable minus comparison. Figure 3 represents the V,R and I LCs of the system AP UMi. The figure shows the differential magnitude versus the corresponding calculated phase obtained by our new ephemeris:

Table 11 2MASS Catalogue (Cutri 2003): yCat 22460C. 2 NOMAD Catalogue (Zacharias et al. 2005), (2004): AAS 205, 4815. 3 The USNO-A2.0 Catalogue. (Monet et al. 1998).

Figure 3.The best match between the synthetic light curves and the observed light curves of binary AP UMi.

The new timings of one primary and one secondary minimum for each LC have been determined by using the paper tracing method. They are listed in Table 2.

Table 2Times of minima (in days)

In order to confirm that the comparison star C1 did not show any peculiar light variation, the magnitude differences between the comparison and check star Δmag. (C1-C2) for each filter are plotted on Figure 4, during the observational nights. They are linearly fitted giving standard deviations 0.012, 0.008 and 0.005 for V, R and I filter, respectively.

Figure 4.Differential magnitude of comparison stars C1 and check star C2 as function of time.

Also, the magnitudes of the comparison and check stars inside the atmosphere for Kottamia observatory, during the nights of observations, against the air mass were considered and plotted in Figure 5. Linear fits were applied and the results summarized in Table 3. From the values of the extinction coefficients given in Table 3, the atmospheric transparencies (P) have been calculated to be 73% and 74% (in the V filter) for the comparison and check stars, respectively. While for R and I, the transparency reached to 86% and 89-90% (Table 3), which indicate that the extinction coefficients in all filters were photometrically reasonable during all the nights of observations.

Figure 5.The relation between air mass vs the magnitude of C1 (lower panel) and C2 (upper panel) inside the atmosphere for different filters.

Table 3Extinction coefficients

 

3. LIGHT CURVE ANALYSIS

The observed light curves of AP UMi indicate typical short period W UMa eclipsing binary with narrow minima and broad maxima. To obtain the physical parameters of the system and to understand its geometrical structure, the new light curves were solved using the software PHOEBE (Prša & Zwitter 2005) and extensive q-search procedure. One of the facilities of PHOEBE program is that, we can analyze LC observed with different filters simultaneously. However, we observed the system AP UMi in R and I filters in one night, so we analyzed them simultaneously and the results are shown in Table 4; While the light curve of the V-filter was observed on another night, so we analyzed it separately and obtained nearly the same results as in case of R and I filters except l1 and l2 due to missing data in the secondary maximum and the presence of little scatter in the primary maximum.

Table 4The light curve fit parameters by PHOEBE for AP UMi

We have applied a spotted model to treat the asymmetry in the LCs. and proceeded the solution by fixing the surface temperature of the primary star at 4500 K according to its spectral type K2.5 from Cox (2002, chapter 7, Table 6). Also, we used the “overcontact binary not in thermal contact” mode based on the general shape of the light curves.

Since no photometric solution for AP UMi was obtained till now, we first used a q-search procedure to determine the mass ratio. We searched for solutions with mass ratios from 0.1 to 0.68. The relation between the resulted sum ∑ of the weight square deviation (O−C)2 and q is shown in Figure 6. The q-search of PHOEBE converged and showed acceptable photometric solution for a contact configuration at about qph = 0.386. By using the van Hamme (1993) tables, the corresponding bolometric coefficients x1 = −0.1599 & x2 = 0.74426, were interpolated. Following Lucy (1967) and Rucinski (1973) the gravity-darkening exponent g1 = g2 = 0.32 and the bolometric albedo A1 = A2 = 0.5 were assumed for both components with a convective envelope. With the assumed initial parameters, we continued the program process till the solution converged. Finally, we included the qph as an adjustable parameter and after some differential corrections the solution gave the final mass ratio of qph = 0.386 ± 0.012. Solution parameters including standard errors of the obtained adjusted parameters are presented in Table 4. The theoretical light curves computed with these parameters are plotted in Figure 3 as solid lines. The fill-out factors for both components imply that AP UMi is a contact binary system.

Figure 6.Relation between ∑ and q.

 

4. DISCUSSION

4.1. Light Curves Morphology

In order to follow the light curve variation for the system, the light curve levels at maxima and minima have been measured directly from Figure 3. The magnitude difference between both maxima (Dmax.) and minima (Dmin.) and the depths of the primary (Ap) and secondary (As) minima for the observed light curves in the VRI bands are calculated using the following equations:

and the results are listed in Table 5.

Table 5Magnitude differences and minima depths of AP UMi LCs.

The morphological studies for the LCs (Figure 3) are as follows:

The obtained light curves fit parameters of the V, R, and I filters are listed in Table 4. From Table 4, T1 > T2 (i.e., the primary minimum is deeper than the secondary one, see also Figure 3), the obtained mass ratio q is smaller than 0.54, and taking into consideration the Roche geometry configuration (Figure 7), one can deduce that AP UMi is likely to be an A-type W UMa system (see, Rucinski 1974).

Figure 7.Roche lobe configuration of AP UMi.

The LCs analysis also shows two spots, cool spot on the primary component and a hot one on the secondary. The parameters of the two spots are given in Table 6, and the lobe configuration by PHOEBE is shown in Figure 8.

Table 6Two spot parameters on the primary and secondary stars of AP UMi.

Figure 8.A schematic Spots modelling for AP UMi.

4.2. Color Indices Study

Pribulla et al. (2003) reported that the components of contact binaries are formed from almost normal, hydrogen-core-burning stars and the surface temperature of the contact binary depends on the orbital period. This so called period-color relation was first noticed by Eggen (1967) and empirically formulated by Wang (1994) as:

where, (B-V)o is the observed color index and P is the orbital period in days. For the contact binary AP UMi, the (B-V)o value equals to 0.832. If we consider the well known equation of the color excess E (B-V) as follows:

where (B − V )i is the intrinsic color excess; and on applying the catalogued value of E (B-V) = 0.013 from the All Sky Image Survey Catalogue, then we find that the spectral type for the system AP UMi is quite near K1.

To crosscheck this result we considered the magnitude values of J (=12.846), H (=12.319) and K (=12.177) from the 2MASS catalogue and calculated the color indices J-H, H-K and J-K. Also, the intrinsic color indices can be obtained by using the relative absorption values (Aλ) from Schlafly & Finkbeiner (2011) and Schlegel et al. (1998). Consequently we found nearly the same result that the spectral type of the primary is very close to K2. Of course this preliminary result should be confirmed spectroscopically.

Table 7VRI-Observations of AP UMi

 

5. CONCLUSIONS

Spectroscopic observations for the binary system AP UMi are strongly recommended in order to determine its physical parameters to verify the obtained results.