Fe II emission lines in the UV spectrum of Sirius-A and Vega

Abstract

We present high-quality HST/GHRS spectra in the Hydrogen L alpha 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, 1190-1222 A. These emission lines can be attributed unambiguously to Fe II and Cr II 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 Fe II 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 L alpha profile set constraints on the presence of a chromosphere, excluding a strong temperature rise in layers deeper than tau_R ~ 10^{-4}.