HIGH-EXCITATION EMISSION-LINES IN THE FAR-ULTRAVIOLET SPECTRUM OF THELATE A-STAR ALPHA-CEPHEI

The A7 V star alpha 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. Vil e detected emission lines of Si III, C III, and O VI in the 900-1200 A ngstrom interval, spanning formation temperatures of 2 x 10(4)-3 x 10( 5) K. The normalized strengths of these lines, R-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) approxim ate to 10(9.4+/-0.3) at a temperature of similar to 6 x 10(4) K. The c orresponding electron pressure, p = n(e)T similar to 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 tempe ratures, the normalized flux ratio for coronal soft X-rays is 20 times less for alpha Cep than it is for the Sun. This greatly reduced X-ray brightness suggests that the outer atmosphere of alpha Cep differs st rikingly from that of the average Sun, being more akin to a low-densit y ''coronal hole.''