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

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 <sup>4

1</sup> 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 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

* 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

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Contents

• 1. Introduction
• 2. Observations and data reduction
  • 2.1. HST observations
  • 2.2. Data reduction
• 3. Qualitative description of the spectra
  • 3.1. The L region
  • 3.2. The 1300 Å region
  • 3.3. Summary of observation results
• 4. Model atmospheres
  • 4.1. General approach
  • 4.2. LTE line-blanketed photospheres
  • 4.3. Non-LTE extended model atmospheres
  • 4.4. CRD and PRD approximations
  • 4.5. Treating Fe ii outside LTE
• 5. Chromospheric emissions in early-A stars?
• 6. Non-LTE Fe ii line formation
  • 6.1. Schuster mechanism
  • 6.2. Radiative interlocking
  • 6.3. The ODF approach
  • 6.4. Case A: H -
  • 6.5. Case B: D -
  • 6.6. Case C: All observed terms
• 7. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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