Astron. Astrophys. 348, 211-221 (1999)

Isotopic lithium abundances in five metal-poor disk stars ^*

P.E. Nissen ¹, D.L. Lambert ², F. Primas ³ and V.V. Smith <sup>4

1</sup> Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark
² Department of Astronomy, University of Texas, Austin, TX 78712-1083, USA
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
⁴ Department of Physics, University of Texas at El Paso, El Paso, TX 79968-0515, USA

Received 14 April 1999 / Accepted 28 May 1999

Abstract

High resolution (), very high S/N spectra centered on the 6707.8 Å LiI line have been obtained with the ESO Coudé Echelle Spectrometer for five, metal-poor () disk stars in the turnoff region of the HR-diagram. The instrumental and stellar atmospheric line broadening have been determined from two unblended iron lines and used in a model atmosphere synthesis of the profile of the LiI line as a function of the lithium isotope ratio. This has led to a detection of ⁶Li in HD 68284 and HD 130551 with ⁶Li/⁷Li 0.05, whereas the other stars, HR 2883, HR 3578 and HR 8181, have ⁶Li/⁷Li close to zero.

By comparing -values and absolute magnitudes based on Hipparcos parallaxes with recent stellar evolutionary tracks, the masses of the stars have been derived. It is shown that the two stars with ⁶Li present have a significantly higher mass, , than the other three stars for which values between 0.85 and 1.0 are obtained.

The results are discussed in terms of models for the galactic evolution of the light elements and depletion of the lithium isotopes in stellar envelopes. It is shown that the measured ⁶Li abundances are in agreement with standard cosmic ray production of ⁶Li in the galactic disk and a moderate depletion (0.5 dex) in the stars. Recent models for the evolution of ⁶Li including fusion reactions and predicting a high lithium isotopic ratio, ⁶Li/⁷Li 0.3 at , require a high degree of ⁶Li depletion ( dex) to fit the observations. Furthermore, these models imply a ⁷Li abundance about 0.2 dex higher than observed for metal-poor disk stars.

Key words: stars: abundances stars: atmospheres stars: fundamental parameters stars: interiors ISM: cosmic rays

* Based on observations carried out at the European Southern Observatory, La Silla, Chile

Send offprint requests to: P.E. Nissen

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Contents

• 1. Introduction
• 2. Selection of stars
• 3. Observations and data reduction
• 4. The isotope ratio ⁶Li/⁷Li
  • 4.1. The center-of-gravity wavelength of the LiI line
  • 4.2. Synthetic spectra
  • 4.3. analysis
  • 4.4. Systematic errors of ⁶Li/⁷Li
• 5. Discussion
  • 5.1. Stellar masses
  • 5.2. Galactic evolution and stellar depletion of ⁶Li
• 6. Concluding remarks
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999
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