Fe II emission lines in the UV spectrum of Sirius-A and Vega *
M. van Noort 1,
T. Lanz 1,
H.J.G.L.M. Lamers 1, 2,
R.L. Kurucz 3,
R. Ferlet 4,
G. Hébrard 4 and
A. Vidal-Madjar 4
Received 29 December 1997 / Accepted 9 March 1998
We present high-quality HST/GHRS spectra in the Hydrogen L 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 Å. 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 profile set constraints on the presence of a chromosphere, excluding a strong temperature rise in layers deeper than .
Key words: stars: atmospheres stars: chromospheres stars: Sirius; Vega line: formation ultraviolet: stars
Present address: School of Physics, University of Sydney, NSW 2006, Australia
Send offprint requests to: T. Lanz
© European Southern Observatory (ESO) 1998
Online publication: May 15, 1998