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

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Abstract
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. ER Eri itself was found to be not a member, but a background RS CVn–like system. We also observed three 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 two 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.

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