The High Chromospheres of the Late A Stars

We report the detection of N V 1239 Å transition region emission in HST /Goddard High Resolution Spectrograph spectra of the A7 V stars, α Aql and α Cep. Our observations provide the first direct evidence of 1-3 × 10 5 K material in the atmospheres of normal A-type stars. For both stars, and for the mid A-type star τ 3 Eri, we also report the detection of chromospheric emission in the Si III 1206 Å line. At a B - V color of 0.16 and an effective temperature of ~8200 K, τ 3 Eri becomes the hottest main-sequence star known 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 are any lower than those of moderately active, solar-type G and K stars. This contrasts sharply with their coronal X-ray emission, which is more 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 that the X-ray faintness of A stars is due to their forming very cool, ≤1 MK coronae. An alternative explanation in terms of mass loss in coronal winds remains a possibility, though we conclude from moderate resolution spectra of the Si III lines that such winds, if they exist, do not penetrate into the chromospheric Si III-forming layers of the star, since the profiles of these lines are not blueshifted and may well be redshifted with respect to the star.