THE HIGH CHROMOSPHERES OF THE LATE A STARS

摘要

We report the detection of N Ⅴ 1239 A 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 x 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 A 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.