High-Excitation Emission Lines in the Far-Ultraviolet Spectrum of the Late A Star α Cephei

Abstract

The A7 V star α Cephei lies in a region of the Hertzsprung-Russell diagram that is generally thought to be devoid of solar-like magnetic activity. The far-ultraviolet spectrum of this star was observed with the Berkeley spectrograph during the 1996 ORFEUS-SPAS II mission. We detected emission lines of Si III, C III, and O VI in the 900-1200 Å interval, spanning formation temperatures of 2×10⁴-3×10⁵ K. The normalized strengths of these lines, \Rscr_L≡f_L/f_bol, are within a factor of 2 of solar values. Lines of two C III multiplets in the ORFEUS spectrum yield an electron density estimate, n_e~10^9.4+/-0.3, at a temperature of ~6×10⁴ K. The corresponding electron pressure, p≡n_eT~10^14.2+/-0.3, is similar to that of the average Sun, but several times smaller than previous estimates made for other late-type G-K stars. At higher temperatures, the normalized flux ratio for coronal soft X-rays is 20 times less for α Cep than it is for the Sun. This greatly reduced X-ray brightness suggests that the outer atmosphere of α Cep differs strikingly from that of the average Sun, being more akin to a low-density ``coronal hole.''