Vol 17, No 5 (2017) / Kjurkchieva

Observations and light curve solutions of the W UMa binaries V796 Cep, V797 Cep, CSS J015341.9+381641 and NSVS 3853195

Observations and light curve solutions of the W UMa binaries V796 Cep, V797 Cep, CSS J015341.9+381641 and NSVS 3853195

Kjurkchieva Diana Petrova1, , Popov Velimir Angelov1, 2, Ibryamov Sunay Ibryamov1, Vasileva Doroteya Lyubenova1, Petrov Nikola Ivanov3

Department of Physics, Shumen University, 115 Universitetska Str., 9712 Shumen, Bulgaria
IRIDA Observatory, Rozhen NAO, Bulgaria
Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose Blvd., 1784 Sofia, Bulgaria

† Corresponding author. E-mail: d.kyurkchieva@shu.bg


Abstract: Abstract

Photometric observations in Sloan and bands of four W UMa binaries, V796 Cep, V797 Cep, CSS J015341.9+381641 and NSVS 3853195, are presented. Our observations showed that CSS J015404.1+382805 and NSVS 3853195 are the same star. We determined the initial epochs T 0 of all targets and found improved values for the period of NSVS 3853195. The light curve solutions of our data revealed that the components of each target are almost the same in terms of mass, temperature, radius and luminosity. The stellar components are G and K spectral types and undergo partial eclipses. All systems have barely-overcontact configurations and can be classified as H subtype W UMa binaries. We established that the relation between the luminosity ratio and mass ratio q of our targets is approximately .

Keywords: stars: binaries: eclipsing;stars: fundamental parameters;stars: individual (V796 Cep, V797 Cep, CSS J015341.9+381641, NSVS 3853195)



1 Introduction

The creation of a stellar evolutional scheme requires a knowledge of the fundamental parameters of stars in different stages of their evolution. Eclipsing binary systems, especially W UMa binaries, are the most important sources of such information. It is supposed that they result from the evolution of wide binaries by angular momentum loss and mass-ratio reversals (Stepien 2006 ; Qian 2003 ). Around 25% of main-sequence star binaries have separations small enough so that when their primaries ascend the giant branch, mass transfer via Roche-lobe overflow marks the beginning of a common envelope phase (Willems & Kolb 2004 ). At this stage the two stars orbit within a single envelope of material, quickly losing angular momentum and spiraling towards each other (Webbink 1984 ; Ivanova et al. 2013 ). The common envelope phase is probably a short-lived stage that ends by envelope ejection and a tighter binary or by a merger. However, understanding the common envelope stage remains one of the most important unsolved problems in stellar evolution (Ivanova et al. 2013 ).

The components in a W UMa system have nearly equal surface temperatures in spite of their often greatly different masses (Binnendijk 1965 ). The model of Lucy ( 1968b ), Lucy ( 1968a ) explains this effect by a common convective photosphere in which two main sequence stars are embedded. As a result one should expect the observable luminosities to have another dependence on the mass ratio than would be the case of two main sequence stars in detached configuration. The condition for equal surface temperatures leads to specific period-luminosity-color (PLC) relations of W UMa stars (Rucinski 1994 , Rucinski & Duerbeck 1997 ), allowing researchers to currently predict their absolute magnitudes M V to about 0.25 mag (Rucinski 2004 ). The PLC relations combined with easy detection make these binaries useful tracers of distance (Klagyivik & Csizmadia 2004 ; Gettel et al. 2006 ; Eker et al. 2009 ). Moreover, W UMa stars are important targets for modern astrophysics because they give information on the processes of tidal interactions, mass loss, mass transfer, angular momentum loss, and merging or fusion of the stars (Martin et al. 2011 ).

In this paper we present photometric observations and light curve solutions of four W UMa binaries: V796 Cep (GSC 04502–00138, TYC 4502–138–1), V797 Cep (GSC 04502–01040, 2MASS J01424764+8007522), CSS J015341.9+381641 and NSVS 3853195 (CSS J015404.1+382805). Table 1 presents the coordinates of our targets and available information on their light variability.

Target RA Dec Period V Amplitude Type
[d] [mag] [mag]
V796 Cep 01 41 36.39 +80 04 19.1 0.3929661 12.20 0.65 EW
V797 Cep 01 42 47.64 +80 07 52.3 0.270416 14.60 0.40 EW
CSS J015341.9+381641 01 53 41.95 +38 16 41.1 0.347518 13.47 0.40 EW
NSVS 3853195 01 54 04.05 +38 28 05.2 0.29253 13.52 0.39 EW

Table 1 Parameters of Our Targets from the VSX Database

2 Observations

Our CCD photometric observations of the targets in Sloan bands were carried out at Rozhen National Astronomical Observatory with the 30-cm Ritchey-Chrétien Astrograph (located in the IRIDA South dome) using CCD camera ATIK 4000M (2048 × 2048 pixels, , field of view 35 × 35 arcmin). Information on our observations is presented in Table 2. In fact, the pairs V796 Cep, V797 Cep and CSS J015341.9+381641, NSVS 3853195 fall in two observed fields (see coordinates in Table 1).

Target Date Exposure Exposure Number Number
V796 Cep, V797 Cep 2015 Oct 25 90 120 125 125
2015 Oct 26 90 120 84 82
2015 Oct 27 90 120 60 59
2015 Oct 28 90 120 146 146
CSS J015341.9+381641, NSVS 3853195 2015 Nov 07 60 90 85 84
2015 Nov 08 60 90 67 75
2015 Nov 11 60 90 84 84
2015 Nov 12 60 90 43 42
2015 Nov 13 60 90 84 85

Table 2 Journal of Our Photometric Observations

The data were obtained during photometric nights with seeing within and humidity below 70%. Twilight flat fields were obtained for each filter, and dark and bias frames were also taken throughout the run. The frames were combined respectively into single master bias, dark and flat frames. The standard procedure was used for reduction of the photometric data (debiasing, dark frame subtraction and flat-fielding) by software AIP4WIN2.0 (Berry & Burnell 2006 ).

We used aperture photometry with a radius of 1.5 FWHM of the star image, along with sky background measurements with annuli enclosing a comparable area. The light variability of the targets was estimated with respect to nearby comparison (constant) stars in the observed field of each target, so called ensemble photometry. A check star served to determine the observational accuracy and to check constancy of the comparison stars. The CCD ensemble photometry calculates the difference between the instrumental magnitude of the target and a comparison magnitude obtained from the mean of the intensities of the comparison stars. The use of numerous comparison stars scattered over the CCD field considerably increases the statistical accuracy of the comparison magnitude (Gilliland & Brown 1988 , Honeycutt 1992 ).

We performed ensemble aperture photometry with the software VPHOT. Table 3 presents coordinates of the comparison and check stars of our targets from the catalog UCAC4 (Zacharias et al. 2013 ) and their magnitudes from the catalog APASS DR9 (Henden 2016 ). The values in brackets correspond to standard deviations of the standard stars during the observational nights. The choice of comparison and check stars in the same field of view as the targets means practically equal extinctions for all stars. The transformation of the obtained instrumental magnitudes to standard ones was made manually. For this aim we used the mean color of the ensemble comparison star and transformation coefficients of our equipment (calculated earlier using standard star field M67). The calculated corrections of the instrumental magnitudes for our targets were from −0.0008 mag to 0.0003 mag in filter (within observational precision) and from −0.0258 mag to 0.0085 mag in filter.

Label Star ID RA Dec
Target 1 V0796 Cep 01 41 36.39 +80 04 19.10 12.320 11.789
Target 2 V0797 Cep 01 42 47.64 +80 07 52.30 14.966 14.025
Chk UCAC4 851–002007 01 41 16.52 +80 04 21.76 13.755 (0.010) 13.018 (0.010)
C1 UCAC4 851–002085 01 45 07.01 +80 10 45.03 13.238 (0.011) 12.534 (0.011)
C2 UCAC4 851–002011 01 41 28.03 +80 11 18.42 13.870 (0.010) 13.351 (0.012)
C3 UCAC4 851–002002 01 40 56.97 +80 04 14.51 13.448 (0.009) 12.844 (0.010)
C4 UCAC4 851–002062 01 43 56.73 +80 02 08.49 14.205 (0.011) 13.452 (0.013)
C5 UCAC4 850–002063 01 41 51.38 +79 56 58.55 13.257 (0.007) 12.651 (0.009)
C6 UCAC4 851–002028 01 42 25.41 +80 01 00.68 13.898 (0.009) 13.058 (0.009)
Target 3 CSS J015341.9+381641 01 53 41.95 +38 16 41.10 13.897 13.117
Target 4 NSVS 3853195 01 54 04.05 +38 28 05.26 14.054 13.178
Chk UCAC4 643–007188 01 54 30.68 +38 29 00.15 13.104 (0.014) 11.754 (0.009)
C1 UCAC4 644–007104 01 54 12.54 +38 36 49.06 13.975 (0.016) 13.902 (0.018)
C2 UCAC4 643–007165 01 54 04.61 +38 35 54.88 14.112 (0.011) 13.419 (0.015)
C3 UCAC4 643–007180 01 54 23.95 +38 35 42.60 14.061 (0.014) 13.369 (0.015)
C4 UCAC4 643–007204 01 54 50.23 +38 33 52.57 13.971 (0.019) 13.147 (0.015)
C5 UCAC4 643–007182 01 54 26.03 +38 29 38.62 13.720 (0.009) 13.012 (0.011)
C6 UCAC4 643–007126 01 53 26.47 +38 30 02.18 13.702 (0.016) 12.978 (0.013)
C7 UCAC4 642–006881 01 54 12.52 +38 23 56.10 13.937 (0.012) 12.907 (0.011)
C8 UCAC4 642–006842 01 53 30.38 +38 20 34.86 13.929 (0.024) 13.071 (0.018)
C9 UCAC4 642–006908 01 54 36.06 +38 20 04.67 14.068 (0.013) 11.491 (0.014)
C10 UCAC4 642–006921 01 54 46.30 +38 19 35.13 13.161 (0.021) 11.764 (0.014)
C11 UCAC4 643–007147 01 53 51.89 +38 28 10.11 12.849 (0.018) 12.191 (0.014)

Table 3 List of the Standard Stars

We determined the times of individual minima (Table 4) by the method of Kwee & van Woerden ( 1956 ).

Target Min. I Min. II IRIDA cycle
V0796 Cep 2457321.43582(9) 0.0
22457322.41776(13) 2.5
2457323.40045(19) 5.0
2457324.38263(8) 7.5
2457324.57929(1) 8.0
V0797 Cep 2457321.31715(74) 0.0
2457321.45201(25) 0.5
2457321.58629(22) 1.0
2457322.26135(33) 3.5
2457322.39815(24) 4.0
2457323.34511(22) 7.5
2457324.29034(18) 11.0
2457324.42626(3) 11.5
CSS J015341.9+381641 2457333.44320(11) –0.5
2457333.61496(31) 0.0
2457334.48599(14) 2.5
2457338.47982(18) 14.0
2457340.39396(31) 19.5
2457340.56578(15) 20.0
NSVS 3853195 2457333.44266(9) –0.5
2457333.59001(21) 0.0
2457334.46774(15) 3.0
2457338.41610(11) 16.5
2457338.56296(13) 17.0
2457339.44062(12) 20.0
2457340.46366(14) 23.5

Table 4 Times of Minima for Our Targets.

3 Light Curve Solutions

The light curves of our targets were solved by the code PHOEBE (Prša & Zwitter 2005 ). It is based on the Wilson–Devinney (WD) code (Wilson & Devinney 1971 , Wilson 1979 ) but also provides a graphical user interface and modeling in Sloan filters associated with our observations. We used the traditional convention that Min. I (phase 0.0) is the deeper light minimum and the star that is eclipsed at Min. I is the primary component.

Mean temperatures T m of the binaries were determined in advance (see Table 6) on the basis of their infrared color indices from the 2MASS catalog and the calibration color-temperature of Tokunaga ( 2000 ). The preliminary runs revealed that all targets are overcontact systems. Hence, we applied mode “Overcontact binary not in thermal contact” of the code. Firstly we fixed T 1 = T m and varied the initial epoch T 0 and period P to search for the best fitting for the phases of light minima and maxima. After that we fixed their values and simultaneously varied secondary temperature T 2, orbital inclination i, mass ratio q and potential Ω to search for an accurate reproduction of the whole light curves. The data in and bands were modeled simultaneously.

Target T m T 1 T 2 r 1 r 2 f
V796 Cep 6407 6410(19) 6403(19) 0.421(1) 0.412(1) 0.101 0.951
V797 Cep 4770 4833(44) 4688(42) 0.424(1) 0.403(1) 0.075 0.771
CSS J015341.9+381641 5715 5765(29) 5657(28) 0.434(1) 0.414(1) 0.166 0.867
NSVS 3853195 5688 5733(31) 5637(30) 0.425(1) 0.406(1) 0.089 0.865

Table 6 Calculated Parameters

We adopted coefficients of gravity brightening and reflection effect appropriate for late-type stars while the linear limb-darkening coefficients for each component and each color were updated according to the tables of van Hamme ( 1993 ). Solar metallicity was assumed for the targets because they consist of late stars from the solar vicinity. In order to reproduce the light curve distortions we used cool spots whose parameters (longitude λ, angular size α and temperature factor κ) were adjusted.

After reaching the best solution we varied together all parameters (T 2, i, q, Ω, T 0 and P) around the values from the last run and obtained the final model. In order to determine stellar temperatures T 1 and T 2 around the mean value T m we used the formulae (Kjurkchieva et al. 2016b )

where (luminosity ratio) and were taken from the last PHOEBE fitting.

Although PHOEBE (as WD) works with potentials, it provides the possibility to calculate all values (polar, point, side and back) of relative radius of each component (R i is linear radius and a is orbital separation). In the absence of radial velocity curves we set as default because from photometry only we cannot determine binary separation. Moreover, PHOEBE yields bolometric magnitudes of the two components as output parameters in conditional units (when radial velocity data are not available). However, their difference determines the true luminosity ratio . Fillout factor can be also calculated from the output parameters of the PHOEBE solution.

In order to take into account the effect of expected correlation between the mass ratio and orbital inclination, we carried out q-search analysis as described in Kjurkchieva et al. ( 2016b ). For this aim we fixed the component temperatures and radii as well as the spot parameters and calculated the normalized χ 2 for a two-dimensional grid along i and q. Figure 1 illustrates the result from this q-search procedure for the target V796 Cep.

Fig. 1 Illustration of the q-search analysis for V796 Cep: the different isolines circumscribe the areas whose normalized χ 2 are smaller than the marked values; the empty circle corresponds to the final value of the mass ratio and orbital inclination given in Table 5.

Table 5 contains final values of the fitted stellar parameters and their PHOEBE uncertainties: initial epoch T 0; period P; mass ratio q; inclination i; potential Ω; secondary temperature .

Star T 0 P q i Ω
V796 Cep 2457321.43582(9) 0.392966 0.948(2) 70.7(1) 3.612(7) 6400(19)
V797 Cep 2457321.31715(74) 0.270416 0.886(2) 64.7(1) 3.525(2) 4625(42)
CSS J015341.9+381641 2457333.61496(31) 0.347518 0.892(2) 70.0(2) 3.490(1) 5607(28)
NSVS 3853195 2457333.59001(21) 0.292524(4) 0.899(2) 69.8(1) 3.539(3) 5592(30)

Table 5 Values of the Fitted Parameters

Table 6 displays the calculated parameters: stellar temperatures ; stellar radii (back values); fillout factor f; ratio of relative stellar luminosities . Their errors are determined from the uncertainties of output parameters used for their calculation. Table 7 gives information on the spot parameters. The synthetic light curves corresponding to our solutions are shown in Figure 2 as continuous lines.

Star β λ α k
V796 Cep 90(5) 35(1) 5.0(1) 0.90(1)
CSS J015341.9+381641 90(5) 90(1) 20.0(1) 0.80(1)
NSVS 3853195 80(5) 120(1) 25.0(2) 0.95(1)

Table 7 Parameters of the Cool Spots of the Targets

Fig. 2 The folded light curves of the targets with their fits and the corresponding residuals (shifted vertically by different amounts to save space).

The mean ( ) residuals for the final fittings are: (0.005, 0.007) for V796 Cep; (0.021, 0.022) for V797 Cep; (0.009, 0.012) for CSS J015341.9+381641; (0.009, 0.013) for NSVS 3853195. The mean ( ) residuals of the standard stars (Table 3) for the first and second pairs of targets are correspondingly (0.010, 0.011) and (0.017, 0.015). Hence, our fittings are excellent for the three targets and very good for the faint star V797 Cep (Fig. 2). The small imperfectness of our modeling may be due to inadequate treatment of the overcontact binaries (Prša et al. 2016 ) or to long exposures (Kipping 2010 ).

4 Conclusions

The main results from the light curve solutions of our data are as follows:

We determined the initial epochs T 0 of the four targets (Table 5).

We improved the period of NSVS 3853195 (Table 5) based on all photometric data: CRTS, NSVS, SWASP and IRIDA. The previous period values of the other three targets fitted our data well.

Our observations revealed that CSS J015404.1+382805 and NSVS 3853195 are the same star (but the International Variable Star Index (VSX) database identified two stars).

The components of each target are almost the same in terms of mass, temperature, radius and luminosity (Tables 5 and 6).

The stellar components of all targets are G and K spectral types and they undergo partial eclipses.

All targets have overcontact configurations with a small fillout factor (Fig. 3, Table 6). This means that they are probably newly formed contact binaries (Qian et al. 2014 ).

3D configurations of the targets.

Three binaries exhibited the O’Connell effect, which was reproduced by cool spots (Table 7) on their primary components. They indicate magnetic activity on these targets.

All our targets have mass ratio (Table 5), i.e. they can be classified as H subtype W UMa systems (with ). Csizmadia & Klagyivik ( 2004 ) revealed that the different subtypes of W UMas are located in different regions on the mass ratio – luminosity ratio diagram (their fig. 1) but above the line representing the mass-luminosity relation for MS detached stars. Our targets support this conclusion and the relation between their mass ratio and luminosity ratio is , i.e. close to that of Lucy ( 1968b ).

The investigation of shallow-contact binary stars with high mass ratios is important for modern astrophysics because they are considered to be newly formed contact configurations, at the beginning of contact evolution (Qian et al. 2014 ). The most detailed studies of this type refer to the binaries AD Cnc (Qian et al. 2007 ), BI Vul (Qian et al. 2013 ) and CSTAR 038663 (Qian et al. 2014 ). They revealed that these cool, short-period (0.25–0.28 d), shallow-contact binaries exhibit strong magnetic activity (including optical 0.2 mag flares of CSTAR 038663) and multiple period changes. Recently we observed and modeled (in the same way) shallow-contact W UMas of H subtype (Table 8). On the fillout factor–mass ratio diagram (Fig. 4), the targets from Table 8 fall in the bottom right (red triangles) due to their small fillout factors (0.0–0.25) and high mass ratios (0.7–1.0). On the same diagram, the contact binaries with decreasing periods from the sample of Yang et al. ( 2013 ) cluster (black circles) in the upper left from our sample because they have intermediate fillout factors (0.05–0.30) and moderate mass ratios (0.3–0.6). One could speculate that deep-contact W UMas would form an additional third cluster left and upwards from the first two clusters on the diagram. So, the fillout factor–mass ratio diagram can be interpreted with an evolutional meaning: through the common envelope phase the position of a given star will describe a trace starting from the bottom right and ending in the upper left side of the diagram.

Parameters of Cool Spots on the Targets

Star q f Reference
AD Cnc 0.77 1.00 0.08 Qian et al. ( 2007 )
BI Vul 0.97 1.22 0.04 Qian et al. ( 2013 )
CSTAR 038663 0.89 1.13 0.10 Qian et al. ( 2014 )
1SWASP J174310.98+432709.6 1.00 0.65 0.23 Kjurkchieva et al. ( 2015a )
NSVS 11234970 0.99 0.55 0.21 Kjurkchieva et al. ( 2015a )
NSVS 11504202 0.98 0.71 0.00 Kjurkchieva et al. ( 2015a )
NSVS 11534299 0.87 0.77 0.00 Kjurkchieva et al. ( 2015a )
NSVS 1776195 0.83 0.96 0.00 Kjurkchieva et al. ( 2015b )
NSVS 113026 0.79 1.00 0.07 Kjurkchieva et al. ( 2015b )
NSVS 2244206 0.73 0.53 0.26 Kjurkchieva et al. ( 2016a )
NSVS 908513 0.71 0.60 0.15 Kjurkchieva et al. ( 2016a )
VSX J062624.4+570907 0.77 0.63 0.16 Kjurkchieva et al. ( 2016a )
CSS J171508.5+350658 0.89 0.64 0.00 Kjurkchieva et al. ( 2016b )
USNO-B1.0-1395-0370184 0.97 0.90 0.01 Kjurkchieva et al. ( 2016c )
USNO-B1.0-1395-0370731 0.85 0.83 0.25 Kjurkchieva et al. ( 2016c )
NSVS 2459652 0.786 0.73 0.17 Kjurkchieva et al. ( 2016d )
NSVS 7377875 0.898 0.84 0.11 Kjurkchieva et al. ( 2016d )
V796 Cep 0.95 0.95 0.10 this paper
V797 Cep 0.89 0.77 0.07 this paper
CSS J015341.9+381641 0.89 0.87 0.17 this paper
NSVS 3853195 0.90 0.86 0.09 this paper

Distribution of fillout factor–mass ratio for W UMa stars: red triangles are for shallow contact high mass ratio targets; black circles are for targets with decreasing periods from the sample of Yang (2013).

More investigations of shallow-contact binary stars with high mass ratios will provide more statistics on their global parameters and opportunity for further study of the rapid evolution of binary stars that have reached the contact stage. The present study is only a step in that direction.


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Cite this article: Kjurkchieva Diana Petrova, Popov Velimir Angelov, Ibryamov Sunay Ibryamov, Vasileva Doroteya Lyubenova, Petrov Nikola Ivanov. Observations and light curve solutions of the W UMa binaries V796 Cep, V797 Cep, CSS J015341.9+381641 and NSVS 3853195. Res. Astron. Astrophys. 2017; 5:042.

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