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The High Chromospheres of the Late A Stars*

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Theodore Simon1 and Wayne B. Landsman2

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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 × 105 K material in the atmospheres of normal A-type stars. For both stars, and for the mid A-type star τ3 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, τ3 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.


Footnote
*  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.
Subject headings

stars: activity; stars: chromospheres; stars: individual (α Aquilae, α Cephei); ultraviolet: stars


Dates

Issue 1 (1997 July 1)

Received 1996 November 26, accepted for publication 1997 January 30



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