UBVRI Photometric Study of the Totally Eclipsing Very Short Period W Ursae Majoris Binary GSC 2765-0348

The variability of GSC 2765-0348 (α[2000] = 23^h26^m29.3067^s, δ[2000] = +31°20^'40.920'' V = 11.90 Guide 9) was discovered by the Semi-automatic Variability Search (SAVS) (Maciejewski et al. 2004). They gave the following ephemeris:

Other times of minimum light are given in Gürol et al. (2007). In addition, this star was listed in the Catalog of 1022 Bright Contact Binary Stars (Gettel et al. 2006). This system was observed as a part of our student/professional collaborative studies of interacting binaries from data taken from NURO observations (National Undergraduate Research Observatory). The observations were taken by Samec, Faulkner, Jaso, Oliver, and Rehn. Reduction and analyses were mostly done by Samec and Flaaten. Our UBVRI light curves were taken with the Lowell Observatory 0.81-m reflector on Anderson Mesa, AZ on 2009 September 24–26. The CRYOTIGER-cooled (−100°C) 2048 × 2048 NASACAM CCD used standard Bessel UBVRI filters.

Individual observations included 206 in B, 209 in V, 208 in R, and 206 in I. The standard error of a single observation was ± 3 mmag in R and I, ± 1.7 mmag in B, ± 4 mmag in V. Nightly images were calibrated with 25 bias frames, five flat frames in each filter, and ten 300 s dark frames . Exposure times were 300 s in U, 50 s in B, 25 s in V, 10 s in R, and 15 s in I. Our observations are given in Table 1 in delta magnitudes, in the sense of variable minus comparison star. The comparison star C (GSC 2765 0304, α[2000] = 23^h25^m56.536^s, δ[2000] = 31°17^'14.53'', V = 12.46, B - V = 0.310, Guide 9) and check star K (GSC 2765 1028, α[2000] = 23^h26^m35.584^s, δ[2000] = 31°18^'37.04'', V = 12.26, B - V = 0.388, Guide 9) were chosen in the same field with Δ(B - V) ≈ 0.2, and designated on the finding chart included for the convenience of future observers as Figure 1. Figure 2 shows sample observations of B, V, and ΔB - V color curves on the night of 25 September. Tables 1 pasp_124_920_1025tb2pasp_124_920_1025tb3pasp_124_920_1025tb45 list the observations in ΔU, ΔB, ΔV, ΔR, and ΔI.

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Four mean times of minimum light were determined from the UBVRI observations in each of the bands. The timings of two primary and two secondary eclipses are presented here:

HJD I = 2455099.6673 ± 0.0002, 2455099.9506 ± 0.0002

HJD II = 2455098.6729 ± 0.0007, 2455099.8066 ± 0.0001

These timings, along with all others, were combined to calculate equation (2) below. The second ephemeris is from the Wilson code fitting to our recent modeled curves only. The difference in these has to do with time averaging. The first is from the fit to all the data over 7740 cycles, while equation (3) has to do with the ephemeris over the time interval of our observations, some five cycles. This might show that the period is increasing, but further observations are needed to confirm this tentative result.

The O - C residuals are shown, graphically, and the Figure 3 and tabled residuals are given in Table 6. The equation above (2) represents a high-precision, improved linear ephemeris for the variable.

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The phased light curves determined from equation (2) of GSC 2765-0348, delta-mag versus phase, are shown in Figures 4a, 4b, and 4c. The light curves are seen to have asymmetries with a large difference in maxima and depths on eclipses that are fairly equal. Actual light-curve characteristics at quadratures are given in Table 7. The primary and secondary amplitudes are given in the table, along with the O'Connell (1951) effects (difference in the maxima) which amount to 3–5% in each filter. This tells us that the spot activity is quite strong on the surface. We note also from Figure 4a, that the U - B color index curves increase in the minima, instead of decrease as in the B - V and R - I curves, which may indicate hot coronal activity or possibly a flare occurring at the time of observation. We note that the UBVRI Wilson code models do not fit this portion either (Fig. 5a) even though a hot spot is modeled near that position. This means the shallow hump is due to something not treated by the synthetic code, or averaged out and not as well detected at the other wavelength bands. This all reveals that GSC 2765-0348 is an active, very short period (high rotational velocities with a strong dynamo), W-type, W UMa system. Through our UBVRI modeled curves, we find that the system has a rather long 33.2 ± 0.5 minute total eclipse (in its secondary minimum) as compared to the binary's orbital period, 408 minute. Also, we note in the color curves that there are low-level ripples, probably caused by asteroseismological vibrations due to all the activity.

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Terrell et al. (2012) determined a mean B - V = 0.746 (0.006) from several CCD observations. His reddening calculations gave E(B - V) = 0.098 (0.022) and result in a corrected (B - V)₀ = 0.648 (0.028). This indicates the system has a G3.5 spectral type. From these calculations and from the fact that there is no spectral type versus orbital phase information, we assumed a value of 5750 ± 200 K as the temperature of the primary component for the light-curve synthesis calculations that follow.

The U, B, V, R, and I curves were pre-modeled with Binary Maker 3.0 (Bradstreet and Steelman 2002) fits with its one-dimensional limb darkening coefficients and black body atmospheres. Convective parameters g = 0.32 and A = 0.5 were used throughout the modeling process. These initial fits gave a mass ratio of 0.32. The parameters from these fits were then averaged and input into a 5-color simultaneous light-curve calculation using the Wilson Code (Wilson & Devinney 1971; Wilson 1990, 1994; Van Hamme 1998). The solution was computed in the Mode 3 contact mode with two-dimensional limb darkening coefficients and Kurucz atmospheres. Our first solution was calculated with a primary temperature of 5000 K from estimates of temperatures of binaries with similar periods. This was adjusted to the values given by the observations of Terrell et al. (2012) and the model was recalculated. Modeled results included two spot regions, one hot and one cool, both at the equatorial positions of the primary, more massive, component. A 34% fill-out and a mass ratio of 0.48 was determined. The mass ratio was refined to ∼0.31. The temperature difference in the components, T₁ - T₂ ∼ 300 K makes both components of G-type, although the main sequence masses would be that of G and M types if the stars were separated. A high inclination of ∼89° was determined. The full solution is given in Table 8. The UBVRI normalized flux light curves and the U - B, B - V, and R - I color curves of the variable are shown as Figures 5a, 5b, and 5c as calculated from the differential magnitudes ( V - C) versus phase. These are shown with UBVRI solution curves overlaying the data. Roche lobe surfaces are given at quadratures in Figures 6a–6d. GSC 2765-0348 is a W-type (less massive star is hotter), W UMa variable, as previously indicated by the light-curve appearances.

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GSC 2765-0348 is a solar type, magnetically active, contact binary. The activity is somewhat extreme, but not unusually so for a binary of its class. The 34% fill-out shows that the system is in firm contact. This along with the very short period and mass ratio may indicate that the system is fairly well evolved. A steady coalescence (increase in fill-out, decrease in mass ratio and period) in such binaries is expected due to torques provided by stellar winds leaving the star along stiff rotating magnetic field lines resulting in magnetic braking angular momentum loss.

This system should be patrolled for the next 10 years or more to determine the long-term period behavior of the system. Radial velocity curves are needed to obtain absolute (not relative) system parameters, although our mass ratio should be accurate due to the emphatic total eclipses displayed by the system.

We wish to acknowledge an Arizona Space Grant supported for partial flight support to our students this observing run. We acknowledge NURO for their observing assistance for our undergraduate students and the TAC committee for the allocation of observing time and American Astronomical Society Small Research grants which have helped support travel expenses. Also, this research was supported in part by a grant from the R. M. Santilli Foundation.