Hubble Space Telescope observations of SV Cam - II. First derivative light-curve modelling using PHOENIX and ATLAS model atmospheres

The variation of the specific intensity across the stellar disc is an essential input parameter in surface brightness reconstruction techniques such as Doppler imaging, where the relative intensity contributions of different surface elements are important in detecting star-spots. We use PHOENIX and ATLAS model atmospheres to model light curves derived from high precision (signal-to-noise ratio similar or equal to 5000) Hubble Space Telescope (HST) data of the eclipsing binary SV Cam (F9V+K4V), where the variation of specific intensity across the stellar disc will determine the contact points of the binary system light curve. For the first time, we use chi(2) comparison fits to the first derivative profiles to determine the best-fitting model atmosphere. We show the wavelength dependence of the limb darkening and that the first derivative profile is sensitive to the limb-darkening profile very close to the limb of the primary star. It is concluded that there is only a marginal difference (< 1 sigma) between the chi(2) comparison fits of the two model atmospheres to the HST light curve at all wavelengths. The usefulness of the second derivative of the light curve for measuring the sharpness of the primary's limb is investigated, but we find that the data are too noisy to permit a quantitative analysis.