The High Chromospheres of the Late A Stars

We report the detection of N V 1239Å transition region emission in HST/GHRS spectra of the A7 V stars, α Aql and α Cep. Our observations provide the first direct evidence of 1− 3 × 10 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 1206Å 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 > 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 the 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. Subject headings: stars: activity — stars: chromospheres — stars: individual (α Aql, α Cep) — ultraviolet: spectra Based on observations with the NASA/ESA Hubble Space Telescope (HST), which is operated by the Space Telescope Science Institute under NASA contract NAS5-26555 to the Association of Universities for Research in Astronomy, Inc.