Doppler Imaging of VW Cephei: Distribution and Evolution of Starspots on a Contact Binary

Abstract

We present maximum entropy Doppler images of the contact binary VW Cephei, produced from seven data sets of simultaneous spectroscopic and photometric data at epochs from 1991 March to 1993 May. The geometric and orbital parameters of the contact binary were fitted, along with third-light characteristics, at the same time as spots, using both the spectroscopic and photometric data to constrain the models. The Doppler images indicated the presence of large polar spots on both components. The polar spot on the primary was about 50° in diameter and slightly off-center, similar to polar spots on other unevolved systems, while the polar spot on the secondary was 30° in diameter. A number of lower latitude features were also present. Spots were found to migrate around the primary component in the same direction as the orbital motion of the system. Slow differential rotation of the primary component was observed, with spots at higher latitudes moving at higher angular rates. The spot distribution on the secondary appeared to be quite stable, with spots congregating at active longitudes, and with no organized pattern of migration. The spot coverage on both components was extremely high, though not unrealistic compared to spot coverages of some RS CVn systems. Spots covered 66% of the surface area of the primary and 55% of the secondary. The spot coverage was much greater than that suggested by asymmetry of the light curves. Our Doppler images were modeled in agreement with the Mullan starspot model. However, they also explained the success of the hot secondary model, since the bolometric flux-weighted mean surface temperature of the primary was less than that of the secondary, owing to the large numbers of nonblack spots. The models indicated the presence of a great number of unresolved spots. We discuss the implications of this to the field of Doppler imaging and argue in favor of the use of two-temperature photosphere models for the Doppler imaging of all rapidly rotating systems. We detected three flares at Hα and estimate that one such detectable flare occurs on VW Cep every 35 ± 20 hr. We also noted that the distribution of chromospheric emission at Hα over the primary of VW Cep may vary from epoch to epoch.