Spatial Study with the Very Large Telescope of a New Resolved Edge-on Circumstellar Dust Disk Discovered at the Periphery of the (rho) Ophiuchi Dark Cloud

In an effort to probe the extent of variations in the interstellar 7Li/6Li ratio seen previously, ultra-high-resolution (R ~360,000), high signal-to-noise spectra of stars in the Perseus OB2 and Scorpius OB2 associations were obtained. These measurements confirm our earlier findings of an interstellar 7Li/6Li ratio of about 2 toward (omicron) Per, the value predicted from models of Galactic cosmic-ray spallation reactions. Observations of other nearby stars yield limits consistent with the isotopic ratio of ~ 12 seen in carbonaceous chondrite meteorites. If this ratio originally represented the gas toward (omicron) Per, then to decrease the original isotope ratio to its current value an order of magnitude increase in the Li abundance is expected, but it is not seen. The elemental K/Li ratio is not unusual, although Li and K are formed via different nucleosynthetic pathways. Several proposals to account for the low 7Li/6Li ratio were considered, but none seems satisfactory. Analysis of the Li and K abundances from our survey highlighted two sight lines where depletion effects are prevalent. There is evidence for enhanced depletion toward X Per, since both abundances are lower by a factor of 4 when compared to other sight lines. Moreover, a smaller Li/H abundance is observed toward 20 Aql, but the K/H abundance is normal, suggesting enhanced Li depletion (relative to K) in this direction. Our results suggest that the 7Li/6Li ratio has not changed significantly during the last 4.5 billion years and that a ratio of ~12 represents most gas in the solar neighborhood. In addition, there appears to be a constant stellar contribution of 7Li, indicating that one or two processes dominate its production in the Galaxy.