An ultra-high-resolution survey of the interstellar lithium-7 to lithium-6 isotope ratio in the solar neighborhood .

摘要

Previous studies on the interstellar 7Li/6Li ratio show variations in its value. In an effort to probe the extent of these variations, ultra-high-resolution (R ∼ 360,000), high signal-to-noise spectra of stars in the Perseus OB2 and Scorpius OB2 Associations were obtained. These observations confirm my earlier findings of an interstellar 7Li/6Li ratio of about 2 toward o Per, the value predicted from models of GCR spallation reactions. However, observations of the other nearby stars yield isotopic ratios closer to that seen in carbonaceous chondrite meteorites (7Li/6Li = 12.3). My results suggest that the 7Li/6Li ratio has not changed significantly during the last 4.5 billion years (Gyr) and that a ratio ∼12 represents most gas in the solar neighborhood.; Some directions reveal the presence of interesting processes taking place. If the Solar System value of about 12 originally represented the gas toward o Per, then an increase in the Li abundance by an order of magnitude is expected, but is not seen. The elemental K/Li ratio is not unusual, although Li and K are formed via different nucleosynthetic pathways. Fractionation cannot account for the low 7Li/6Li ratio; the abundance of LiH is too low. In attempt to explain the low ratio toward o Per, the role of stellar flares from young stars in IC 348 was investigated, but the amount of 6Li created in stellar flares is negligible. Another interesting result is evidence for enhanced depletion toward X Per, since both the Li and K abundances are lower by a factor of 4 when compared to other sight lines. Moreover, enhanced destruction of Li relative to H and K is suggested by my measurements of 20 Aql. A possible cause is the proximity of the line of sight toward 20 Aql to the supernova remnant, Radio Loop I, but a simple calculation for Li destruction indicates timescales much longer than the estimated age of the remnant.; From knowledge of the interstellar 7Li/6Li ratio, the amount of 7Li from stellar sources needed to reproduce the observed 7Li abundance was calculated. There appears to be a constant stellar contribution to 7Li in the solar neighborhood, indicating that one or two processes dominate 7Li production in the Galaxy. A continuation of the present study of interstellar Li is vital in understanding the evolution of this important element. Both interstellar and stellar observations of 7Li, s-process, and r-process elements are necessary to determine the exact amount of 7Li created by stars. Information on the 12C/13C ratio and oxygen isotopes is useful in constraining 7Li production in asymptotic and red giant branch stars. Crucial to our understanding of Li evolution are observations of the other light nuclei, 9Be, 10Be, and 11B. Only through such a comprehensive approach can the production pathways for the light elements be discerned. (Abstract shortened by UMI.)