Lithium abundances, K line emission and ages of nearby solar type stars

Abstract

Li abundances and chromospheric emission fluxes measured in the core of the Ca n K line have been determined in over 100 field F5-G5 dwarfs and subgiants. Although both quantities are known statistically to decrease in older stars, the correlation between them is not good. In particular, there are a number of anomalous solar type stars which show high Li abundances and very little chromospheric flux; the converse is rare. This might be understood if the intensity of chromospheric emission undergoes a sudden decrease when stars reach an age of 1 to 2 x 10 9 years, before much Li depletion occurs. Some of the anomalous stars appear to be older than this, however. Such stars must have preserved their Li from main sequence destruction. Consistent time scales for Li depletion are fitted to Pleiades, Ursa Major cluster, Hyades, and solar data, and are applied to the field stars. The resulting age distribution indicates a slightly decreasing rate of star formation during the last 4-5 x 10 9 years. If some of the Li-strong stars are anomalous and not young, the age distribution would have to be modified accordingly, resulting in fewer young stars. There is no difference in mean metallicity between the youngest and oldest groups of field stars studied. Many of the earliest type stars observed (F5-F6) show significant Li depletion, which cannot be explained by the same process which depletes Li in the Sun. Different mechanisms are discussed which may explain Li depletion in the Sun and in earlier type stars. The highest Li abundances observed in any stars are about log n(Li) = 3.0, which is a factor of two below the abundance found in Type I carbonaceous chondrites. It is likely that the typical protostellar Li abundance is log n(Li) = 3.0, and that the meteorites are enriched in Li.