The Special Case of VV Cephei

Abstract

VV Cephei (M2 Iab + B0–2 V) is the only known ζ Aur binary that has an M-type supergiant. Studies of its atmospheric eclipses provide a valuable extension to what can be learned by eclipse mapping, but are fraught with many challenges. The space-UV absorption spectrum is extremely rich, and line blending is severe. Wind interaction and accretion effects are prominent, and the spectrum of the B star companion is heavily veiled by overlying Balmer continuum emission, which usually exceeds that of the B-star’s continuum flux in the UV. An accretion region immediately around the B star gives rise to a rich spectrum of lines, some of which sometimes show prominent inverse P Cygni profiles. The chromospheric absorption spectrum shows structure in the line profiles that persists throughout chromospheric eclipse, and implies the existence of large-scale supersonic, complex velocity flows across the whole extended chromosphere of the M supergiant. Spectrum formation in the atmosphere of VV Cep is more complex than in the other ζ Aur binaries owing to the much higher rate of mass loss from the primary. This chapter summarizes the fundamental stellar and orbit parameters of VV Cep, and presents key results found from three decades of UV astronomy. It also presents a simple model of the chromosphere and wind.