A-Type and Chemically Peculiar Stars

Abstract

Conventional wisdom holds that early-type and late-type stars should have very different outer atmospheres, because the early-type stars lack deep convective zones and thus a mechanical energy source to heat their outer layers. I will show that the magnetic chemically peculiar (CP) stars hotter than about spectral type A2 display many of the phenomena seen in the most active late-type stars. In particular, many CP stars are luminous nonthermal radio sources and ROSAT confirms that many are also luminous coronal x-ray sources like the RS CVn systems. A wind-fed magnetosphere model has been proposed to explain both the nonthermal radio and the x-ray emission. In this model the stellar wind plays the role of a mechanical energy source analogous to the role played by convection in the active late-type stars. By comparison, the chemically normal single stars in the spectral range B2-A5 typically show neither nonthermal radio emission nor x-ray emission, and thus appear to be very different from the CP stars of similar spectral type and from the active late-type stars.