A New Association of Post–T Tauri Stars near the Sun

Observing ROSAT sources in 20 x 25 deg centered at the high latitude activestar ER Eri, we found evidences for a new young nearby association (~30Myrat~60pc), the Horologium Association (HorA), formed by at least 10 probable and6 possible members, some being Post-T Tauri stars. We examine severalrequirements that characterize a young association and they, together, create astrong evidence for the reality of the HorA. In fact, the Li line intensitiesare between those of the oldest classical T Tauri stars and the ones of theLocal Association stars. The space velocities of the HorA relative to the Sun,U= -9.5+/-1.0, V = -20.9 +/- 1.1, W = -2.1 +/- 1.9, are not far from those ofthe Local Association. We suggest that some hotter and non-X-ray active stars,with similar space velocities, could be massive members of the HorA, amongthem, the nearby Be star Achernar. The maximum of the mass distributionfunction of the HorA is around 0.8 solar masses. At its distance, the projectedsize of the HorA, ~50 pc, would be larger than our surveyed area and many othermembers could have been missed. We also observed 3 control regions, two atnorthern and southern galactic latitudes and a third one in the known TW HyaAssociation (TWA), and the properties and distribution of their young starsstrengthen the reality of the HorA. Contrary to the TWA, the only knownbinaries in the HorA are 2 very wide systems. The HorA is much more isolatedfrom clouds and older than the TWA and could give some clues about the lifetimeof the disks around T Tauri stars. Actually, none of the proposed members is anIRAS source indicating an advanced stage of the evolution of their accretingdisks. ER Eri itself was found to be a RS CVn-like system.Comment: 25 pages, 5 eps figures, to appear in Astron.