6. Photometric solution

The B and V-band light curves of V 839 Oph of 1992, 1993 and 1994 have been used simultaneously for the determination of the geometric and photometric elements using the Wilson-Devinney (WD) code of the light variations of eclipsing variables. First, the observed points ordered in phase were combined into 50 normalised binned points in each colour so that each bin equals 0.02 of an orbital period with weights equal to the number of original points. The solutions were computed on the personal computers of Ankara University Observatory using WD code's PC version that was implemented by Müyesseroğlu (1994). Based on the General Catalogue of Variable Stars's spectral classification of the primary component, F8 V, the temperature was adopted to be T₁= . The other adopted parameters are the gravity-darkening coefficients g₁=g₂, the albedoes A₁=A₂ and the linear limb-darkening coefficients, which were determined from tables by Al-Naimy (1978) for both x₁ and x₂. Model 3 of the WD-differential correction program was used to adjust the following parameters: the orbital inclination i, the mass-ratio q=M₂/M₁, the temperature of secondary component T₂, the potential function (or ) and the luminosities of the two components L₁, L₂. These adjustable parameters were varied until the solution converged.

The subcategories of the W UMa systems, called A-type and W-type by Binnendijk (1970), divided into two spectral groups A9-F8 and F7-M5. There is no certain limit between the two subclasses. Eaton (1983) suggested that the two types of W UMa systems are distinguished by the presence (W-type) or absence (A-type) of magnetic spots. V 839 Oph was classified as an A-type of W UMa systems (Binnendijk 1970). Because of its late spectral type, F8 V, the WD-differential correction program was applied under both the radiative envelope and the convective envelope assumptions, seperately. Both assumptions give similar quality fits, which means that we cannot determine easily in this way what kind of envelope is present here. In the theory of Anderson et al. (1983) the radiative photosphere of stars with spectral type later than F5 develop convective "continents'', and these "continents'' grow for later types until the entire star is enveloped in convection. They were the first to suggest the convective "continents'' idea for AW UMa, an A-type W UMa system with a late spectral type (F0-F2V). For V 839 Oph, the light curves show asymmetric structure, usually caused by surface brightness anomalies. For that reason, we have the suspicion that it has convective "continents'' on the common envelope of the system. In this case, the gravity-darkening coefficients and the albedoes should be different from the case of a completely radiative assumption. To distinguish between the two cases, we reanalysed the light curve of V 839 Oph for several values of the gravity-darkening coefficients (g) in the range 0.32-1.00 and albedoes (A) in the range 0.50-1.00. For each set of parameters (A, g) we present the sum of the squared residuals of the simultaneous fit to the light curves in B and V () in Table 7 (click here). There is one minimum of at A=0.80 and g=0.40. The final solution is given in Table 8 (click here), and the computed light curves based on these elements are shown in Fig. 6 (click here). The geometrical representation of V 839 Oph at phase is displayed in Fig. 7 (click here). The Roche lobe surface was produced by the PC version of Binary Maker (Bradstreet 1990). The fits confirm that the convective "continents'' on the envelope of V 839 Oph exist, and that they are thin enough to create surface brightness anomalies, spots, and are able to generate the magnetohydrodynamic energy needed to heat a chromosphere and corona. V 839 Oph is not the only late A-type W UMa system for which these more realistic assumptions produce better fits. The same is the case for some systems presented in Twigg (1979).

  
Figure 6: B and V intensity light curves and B-V colour curve as defined by the individual observations and theoretical light curves for V 839 Oph

  
Figure 7: Geometrical representation of V 839 Oph at phase

There is one more piece of evidence for convective structure in V 839 Oph. Although our own observations were not taken on subsequent days, we could investigate the short time light variations from the compilation of all other published light curves.

 

A g

0.50 0.32 0.0075223 0.0104486

0.50 0.40 0.0075682 0.0108482

0.50 0.50 0.0073665 0.0102710

0.50 0.70 0.0083371 0.0110518

0.50 1.00 0.0098089 0.0100897

0.70 0.32 0.0090161 0.0132735

0.70 0.40 0.0085283 0.0121798

0.70 0.50 0.0083241 0.0114845

0.70 0.70 0.0087596 0.0122712

0.70 1.00 0.0110986 0.0126784

0.80 0.32 0.0082005 0.0121450

0.80 0.40 0.0065233 0.0095031

0.80 0.50 0.0075505 0.0108820

0.80 0.70 0.0080520 0.0104879

0.80 1.00 0.0136256 0.0158817

1.00 0.32 0.0085341 0.0124114

1.00 0.40 0.0082158 0.0117043

1.00 0.50 0.0076007 0.0110721

1.00 0.70 0.0082611 0.0115917

1.00 1.00 0.0090303 0.0110089

 

 

Parameter Value 4300 5500

x_1B = x_2B 0.766

x_1V = x_2V 0.612

g₁ = g₂ 0.40

A₁ = A₂ 0.80 i 6250 q L₁/(L₁+L₂)B L₁/(L₁+L₂)V

l₃ 0.00

fill out 38.8%

 

The night-to-night variations in the light curves could be defined at phases of each quadrature, especially between the phases of 0.17 and 0.25 and between the phases of 0.75 and 0.83. Niarchos (1989) reported that the magnitude difference of the outer and the inner envelopes of have a width of in yellow and mag in blue. Until we have some spectra to better understand the short-time light variations, we suggest that they could be explained by the active convective "continents'', and/or by the mass transfer through the connecting neck of their Roche lobes, which will change the surface brightness in short periods.

There are many result in literature about the chromospheric and coronal activities on W UMa systems which explain observational results taken by the HEAO 1, IUE, HEAO 2 (Einstein) and ROSAT satellities (for example, Dupree et al. 1980; Carroll et al. 1980; Eaton 1983; Cruddace & Dupree 1984; Vilhu & Heise 1986; Mcgale et al. 1996). V 839 Oph was placed in only some of X-ray observation lists, and the last X-ray detection of the system has been undertaken with the ROSAT satellite. Mcgale et al. (1996) showed that the ROSAT X-ray fluxes showed that the system was 70 per cent brighter in soft X-rays at the ROSAT epoch with constant light curve than in the epoch of the 1979-1981 Einstein survey done by Cruddace & Dupree (1984). Another A-type W UMa systems, AK Her (F2+F6V), also was shown to have a constant light curve in soft X-rays as V 839 Oph in the X-ray spectral survey.