THE PHOTOSPHERIC FILLING FACTOR OF THE ACTIVE BINARY II PEGASI

Abstract. UBVand JHKphotometry of the active single-lined binary II Peg, we performed in 1995, is presented.A method to determine the fraction of the photosphere cov-ered by spots (filling factor) and to check the accuracy of gen-erally assumed values of photospheric parameters has been de-veloped. The procedure is based on the comparison betweenmultiband fluxes and low resolution synthetic spectra weightedon the base of the spot filling factor and scaled with the ratiobetween the star radius and distance (R/d), so that we can alsoestimatetheR/dratio.Aχ 2 fithasbeenperformedforIIPegob-servations close to the light maximum and minimum by assum-ing reliable values of the photospheric parameters. Although aunique solution cannot be reached, we found clear indicationfor a spot filling factor at light maximum ≥ 40%. We find thatthe same set of parameters that gives us the best fit solutions atlight maximum also provides the best fit at light minimum. Theresulting solutions are consistent with the observed amplitudeofthephotometricwave,andwiththecommonlyacceptedvalueof R, unspotted V magnitude and spectral classification for IIPegasi.Key words: stars: activity – stars: binaries: spectroscopic –stars: starspots – stars: individual: II Peg1. IntroductionTo explain the photometric variability of a large sample of latetype stars, surface inhomogeneities are invoked. Several otherfeatures, such as the presence of C IV emission (Gunn et al.1998), CaIIHK 1993; Byrne et al. 1998) and Hα lines (Bopp & Talcott1978, 1980; Huenemoerder & Ramsey 1984; Liu & Tan 1986,FrascaCLetoetal.1997),leadtotheconclusionthatthephotometricandspectroscopic behaviour of these active late type stars is due tothe manifestation of solar type magnetic activity.