1998A&A...334..633V

We present high-quality HST/GHRS spectra in the Hydrogen Lα spectral region of Vega and Sirius-A. Thanks to the signal-to-noise ratio achieved in these observations and to the similarity of the two spectra, we found clear evidence of emission features in the low flux region, λλ1190-1222Å. These emission lines can be attributed unambiguously to FeII and CrII transitions. In this spectral range, silicon lines are observed in absorption. We built a series of non-LTE model atmospheres with different, prescribed temperature stratification in the upper atmosphere and treating FeII with various degrees of sophistication in non-LTE. Emission lines are produced by the combined effect of the Schuster mechanism and radiative interlocking, and can be explained without the presence of a chromosphere. Silicon absorption lines and the Lα profile set constraints on the presence of a chromosphere, excluding a strong temperature rise in layers deeper than τ_R≃10^–4.