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

Our study is a follow-up of the SACY project, an extended survey 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. These associations, with ages between about 6 Myr and 70 Myr, form an excellent sample to study the Li depletion in the pre-main sequence (PMS) evolution. We investigate the use of Li abundances as an independent clock to constrain the PMS evolution. 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. 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. A clear Li depletion can be measured in the temperature range from 5000K to 3500K for the age span covered by these nine associations. The age sequence based on the Li-clock agrees well with the isochronal ages, $epsilon$Cha association being the only possible exception. The lithium depletion patterns for those associations resemble those of the young open clusters, 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. Rotation velocities above 20 km s$^{-1}$ seem to inhibit the Li depletion.

Transient jets in V617 Sagittarii

Some of the luminous Compact Binary Supersoft X-Ray sources (CBSS) have shown indications of jets, also called satellites due to their appearance in the spectra. In V Sagittae (V Sge) stars, the galactic counterparts of the CBSS, such features have been reported only for WX Cen. If V Sge stars are indeed the analogs of CBSS, one may expect transient jet emission in other objects of this class. Spectroscopic observations of the V Sge star V617 Sgr have been made, both at high photometric state and at decline. We show that V617 Sgr presents Halpha satellites at high photometric state with velocities of +/-780 km/s. This feature confirms, once more, the CBSS nature of the V Sge stars, however the details of the spectral characteristics also suggest that the two groups of stars display some intrinsic spectroscopic differences, which are likely to be due to a selection effect related to chemical abundance.