UBV(RI)_C and UVBY photometry of HD 81410 and starspot distribution in RS CVn objects

Abstract

We present UBVRI and uvby photometry of HD 81410 obtained on a total of 40 nights during 1987-90, and an analysis of its long-term photometric behaviour. The star is found to show two well-separated minima in its light curve most of the time; probably there are two preferred effective longitudes about which spots are mainly formed. The migration of the phase of the light minimum usually seen in RS CVn stars is absent in HD 81410. The rather small spread in the maximum V amplitudes observed in active RS CVn objects seen at different inclinations of rotational axis implies that the longitudinal asymmetry in the distribution of spots, which causes the light modulation, is largely restricted to within around +/-40(deg) latitudes. The total ranges of rotational periods which are quoted in the literature for several spotted stars based on long-term photometry also imply a similar latitudinal extent of spots about the equator. We find that the light and colour curves produced by an equatorial band of spots limited by latitudes and covering the full range of longitudes across the hemisphere visible at light minimum can be approximated reasonably well by two well-separated circular spots. Further, we find that the net effect in the colours produced by limb-darkening depends on the exact distribution of spots on the stellar surface; it could be even negligible for certain spot distributions. From the spot modeling of the light and colour curves of HD 81410 we find that the maximum temperature difference between the photosphere and spots is around 1400 K. We also find that the light modulation is caused by several small individual spots, and during the epochs of shallow minimum spots are spread out, both latitudinally and longitudinally, over a wider region, as indicated by a higher temperature for the equivalent circular spots at those epochs. It seems that in RS CVn stars the spots occur largely distributed about the equator as in the case of the Sun and the solutions which indicate polar spots result from limiting the number of spots in the modeling to a few. Based on the observations collected at the European Southern Observatory, La Silla