Isotopic lithium abundances in five metal-poor disk stars

Abstract

High resolution (R =~ 110 000), very high S/N spectra centered on the 6707.8 Ä Li I line have been obtained with the ESO Coudé Echelle Spectrometer for five, metal-poor (-0.8 < [Fe/H] < -0.6) 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 Li I line as a function of the lithium isotope ratio. This has led to a detection of \element[][6]{Li} in HD 68284 and HD 130551 with element [][6]{Li}/element [][7]{Li} =~ 0.05, whereas the other stars, HR 2883, HR 3578 and HR 8181, have \element[][6]{Li}/\element[][7]{Li} close to zero. By comparing T_eff -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 \element[][6]{Li} present have a significantly higher mass, {cal M}/{cal M}_⊙ =~ 1.05, than the other three stars for which values between 0.85 and 1.0 {cal M}_⊙ 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 \element[][6]{Li} abundances are in agreement with standard cosmic ray production of \element[][6]{Li} in the galactic disk and a moderate depletion (0.5 dex) in the stars. Recent models for the evolution of \element[][6]{Li} including alpha + alpha fusion reactions and predicting a high lithium isotopic ratio, element [][6]{Li}/element [][7]{Li} =~ 0.3 at [Fe/H] = -0.6, require a high degree of \element[][6]{Li} depletion ( =~ 1.0 dex) to fit the observations. Furthermore, these models imply a \element[][7]{Li} abundance about 0.2 dex higher than observed for metal-poor disk stars. Based on observations carried out at the European Southern Observatory, La Silla, Chile