THE HIGH CHROMOSPHERES OF THE LATE A-STAR

We report the detection of N v 1239 Angstrom transition region emissio n in HST/Goddard High Resolution Spectrograph spectra of the A7 V star s, alpha Aql and alpha Cep. Our observations provide the first direct evidence of 1-3 x 10(5) K material in the atmospheres of normal A-type stars. For both stars, and for the mid A-type star tau(3) Eri, we als o report the detection of chromospheric emission in the Si III 1206 An gstrom line. At a B-V color of 0.16 and an effective temperature of si milar to 8200 K, tau(3) Eri becomes the hottest main-sequence star kno wn to have a chromosphere and, thus, an outer convection zone. We see no firm evidence that the Si III line surface fluxes of the A stars ar e any lower than those of moderately active, solar-type G and K stars. This contrasts sharply with their coronal X-ray emission, which is mo re than 100 times weaker than that of the later type stars. Given the strength of the N v emission observed here, it now appears unlikely th at the X-ray faintness of A stars is due to their forming very cool, l ess than or equal to 1 Mg coronae. An alternative explanation in terms of mass loss in coronal winds remains a possibility, though we conclu de from moderate resolution spectra of the Si III lines that such wind s, if they exist, do not penetrate into the chromospheric Si III-formi ng layers of the star, since the profiles of these lines are not blues hifted and may well be redshifted with respect to the star.