Search for associations containing young stars (SACY) - III. Ages and Li abundances

Abstract

Context. Our study is a follow-up of the SACY project, an extended high spectral resolution survey of more than two thousand optical counterparts of X-ray sources in the Southern Hemisphere targeted to search for young nearby associations. Nine associations have either been newly identified, or had their member list better defined. Groups belonging to the Sco-Cen-Oph complex are not considered in the present study. Aims. These nine associations, with ages between about 6Myr and 70Myr, form an excellent sample to study the Li depletion in the pre-main sequence(PMS) evolution. In the present paper we investigate the use of Li abundances as an independent clock to constrain the PMS evolution. Methods. Using our measurements of the equivalent widths of the Li resonance line and assuming fixed metallicities and microturbulence, we have calculated the LTE Li abundances for 376 members of different young associations. In addition we considered the effects of their projected stellar rotation. Results. We present the Li depletion as function of age in the first hundred million years for the first time for the most extended sample of Li abundances in young stellar associations. Conclusions. A clear Li depletion can be measured in the temperature range from 5000 K to 3500 K for the age span covered by the nine associations studied in this paper. The age sequence based on the Li-clock agrees well with the isochronal ages, ǫCha association being the only possible exception. The lithium depletion patterns for the associations presented here resemble those of the young open clusters with similar ages, strengthening the notion that the members proposed for these loose young associations have indeed a common physical origin. The observed scatter in the Li abundances hampers the use of Li to determine reliable ages for individual stars. For velocities above 20kms 1 rotation seems to play an important role inhibiting the Li depletion.