Chemical evolution of the orion association. III. The lithium abundance of F and G stars

We derive lithium abundances for a sample of 25 late F to early G dwarfs in the direction of the Orion association from spectra obtained with the McDonald Observatory's 2.1 m telescope plus a Cassegrain cross-dispersed echelle spectrometer at a spectral resolution of 60,000. Iron abundances are also derived for the slowly rotating stars. A kinematical discussion, combined with information on rotation and X-ray emission for the stars, led us to conclude that 10 stars in our sample are members of the Ic and Id subgroups of Orion, while two are probable members of the Ib subgroup, with the remaining stars being field stars in the direction of Orion. The Li abundances obtained for the seven members of the Ic subgroup with v sin i > 20 km s^-1 show a small scatter, comparable to our uncertainties in the abundance determinations themselves. The mean non-LTE Li abundance for the rapidly rotating Ic members is log ∈(Li) = 3.2 ± 0.1 - very close to the solar system meteoritic value of 3.3. If the Orion interstellar gas is representative of local Galactic gas, there is apparently little evidence of an increase in the Li abundance over the last 5 Gyr.