Observing ROSAT sources in an area 20 • × 25 • centered at the high latitude (b = −59 • ) active star ER Eri, we found evidences for a nearby association, that we call the Horologium Association (HorA), formed by at least 10 very young stars, some of them being bona fide Post-T Tauri stars. We suggest other six stars as possible members of this proposed association. We examine several requirements that characterize a young stellar association. Although no one of them, isolated, gives an undisputed prove of the existence of the HorA, all together practically create a strong evidence for it. In fact, the Li line intensities are between those of the older classical T Tauri stars and the ones of the Local Association stars.The space velocity components, 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 of the Local Association, so that it could be one of its last episodes of star formation. In this region of the sky there are some hotter and non-X-ray active stars, with similar space velocities, that could be the massive members of the HorA, among them, the nearby Be star Achernar. The maximum of the mass distribution function of its probable members is around 0.7 -0.9 M ⊙ . We estimate its distance as ∼60 pc and its size as ∼50 pc. If spherical, this size would be larger than the surveyed area and many other members could have been missed.We also observed 3 control regions, two at northern and southern galactic latitudes and a third one in the known TW Hya Association (TWA), and the properties and distribution of their young stars strengthen the reality of the HorA. Contrary to the TWA, the only known binaries in the HorA are 2 very wide systems. The HorA is much more isolated from clouds and older (∼30 Myr) than the TWA and could give some clues about the lifetime of the disks around T Tauri stars. Actually, none of the proposed members is an IRAS source indicating an advanced stage of the evolution of their primitive accreting disks.ER Eri itself was found to be a RS CVn-like system.
PDS 365 is a newly detected rapidly rotating (v sin i = 20 km s −1 ) single low mass giant star which with HD 233517 and HD 219025 forms a remarkable ensemble of single K giants with the unique properties of rapid rotation, very strong Li lines, an asymmetrical Hα profile, and a large far infrared excess. Their v sin i values are between 18 and 23 km s −1 , and LTE Li abundances, log ε(Li), are between 2.9 and 3.9. Detailed analysis of PDS 365 reveals it to be a ∼ 1M ⊙ giant with a value of 12 C/ 13 C approximately equal to 12. A clear relation between high rotational velocities and very high Li abundances for K giant stars is found only when asymmetrical Hα profiles and large far-infrared excesses are present. If we consider single K giants, we find that among rapid (v sin i ≥ 8 km s −1 ) rotators, a very large proportion (∼ 50%) are Li-rich giants. This proportion is in contrast with a very low proportion (∼ 2%) of Li-rich stars among the much more common slowly rotating K giants. This striking difference is discussed in terms of proposed mechanisms for Li enrichment.
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