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Astron. Astrophys. 334, 633-645 (1998)


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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

1 Sterrenkundig Instituut, Utrecht Universiteit, Princetonplein 5, 3508 TA Utrecht, The Netherlands (noort@physics.usyd.edu.au, lanz@fys.ruu.nl)
2 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands (hennyl@sron.ruu.nl)
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA (kurucz@cfa.harvard.edu)
4 Institut d'Astrophysique de Paris, CNRS, 98 bis Boulevard Arago, F-75014 Paris, France (ferlet, hebrard, vidalmadjar@iap.fr)

Received 29 December 1997 / Accepted 9 March 1998

Abstract

We present high-quality HST/GHRS spectra in the Hydrogen L [FORMULA] 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, [FORMULA] 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 [FORMULA] profile set constraints on the presence of a chromosphere, excluding a strong temperature rise in layers deeper than [FORMULA].

Key words: stars: atmospheres – stars: chromospheres – stars: Sirius; Vega – line: formation – ultraviolet: stars

* Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

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

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