We report the discovery of five T Tauri star systems, two of which are resolved binaries, in the vicinity of the nearest known region of recent star formation, the TW Hydrae Association. The newly discovered systems display the same signatures of youth (namely high X-ray flux, large Li abundance and strong chromospheric activity) and the same proper motion as the original five members. These similarities firmly establish the group as a bona fide T Tauri association, unique in its proximity to Earth and its complete isolation from any known molecular clouds.At an age of ∼10 Myr and a distance of ∼50 pc, the association members are excellent candidates for future studies of circumstellar disk dissipation and the formation of brown dwarfs and planets. Indeed, as an example, our speckle imaging revealed a faint, very likely companion 2 ′′ north of CoD-33 • 7795 (TWA 5). Its color and brightness suggest a spectral type ∼M8.5 which, at an age of ∼10 7 years, implies a mass ∼ 20 M Jupiter .

We present a revised catalog of 2106 Galactic stars, selected without kinematic bias, and with available radial velocities, distance estimates, and metal abundances in the range 0.0 ≤ [Fe/H] ≤ −4.0. This update of the Beers & Sommer-Larsen (1995) catalog includes newly-derived homogeneous photometric distance estimates, revised

The ages of the most common stars-low-mass (cool) stars like the Sun, and smaller-are difficult to derive 1,2 because traditional dating methods use stellar properties that either change little as the stars age 3,4 or are hard to measure 5-8 . The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology 2,9-11 . To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years 12-16 , and gyrochronology ages for older stars have been inferred from model predictions 2,7,11,17 . Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a welldefined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.Prior observations in star clusters with ages ≲300million years (Myr) have shown that cool stars begin their main-sequence phase with a dispersion in their rotation periods, P, spanning two orders of magnitude [12][13][14]18 (0.1-10 d). However, this dispersion diminishes rapidly with cluster age, t, as they lose angular momentum through magnetically channelled winds 19 , causing their periods to increase and converge to a well-defined relationship with stellar mass, M, by the age (600 Myr) of the Hyades cluster 15,20 . These observations suggest that cool main-sequence stars older than the Hyades probably occupy a single surface, P = P(t, M), in P-t-M space, which can be defined by measurements of their periods and photometric colours (a proxy for stellar mass) in a series of age-ranked clusters (Fig. 1). Measuring stellar rotation periods in clusters older than the Hyades will confirm or deny the existence of such a surface, and, if it exists, define its shape and thickness.Models of cool-star rotational evolution also describe a convergence of rotation with age 2,11,17,21 . However, they differ in their predictions of the location and shape of the P-t-M surface and, beyond the common assumption that the rotation period of the Sun is typical for stars of its mass and age, were until 2011 16 unconstrained at ages greater than 600 Myr.

We present color-magnitude diagram analysis of deep Hubble Space Telescope imaging of a mass-limited sample of 18 intermediate-age (1 -2 Gyr old) star clusters in the Magellanic Clouds, including 8 clusters for which new data was obtained. We find that all star clusters in our sample feature extended main sequence turnoff (eMSTO) regions that are wider than can be accounted for by a simple stellar population (including unresolved binary stars). FWHM widths of the MSTOs indicate age spreads of 200 -550 Myr. We evaluate dynamical evolution of clusters with and without initial mass segregation. Our main results are: (1) the fraction of red clump (RC) stars in secondary RCs in eMSTO clusters scales with the fraction of MSTO stars having pseudo-ages 1.35 Gyr; (2) the width of the pseudo-age distributions of eMSTO clusters is correlated with their central escape velocity v esc , both currently and at an age of 10 Myr. We find that these two results are unlikely to be reproduced by the effects of interactive binary stars or a range of stellar rotation velocities. We therefore argue that the eMSTO phenomenon is mainly caused by extended star formation within the clusters;(3) we find that v esc ≥ 15 km s −1 out to ages of at least 100 Myr for all clusters featuring eMSTOs, while v esc ≤ 12 km s −1 at all ages for two lower-mass clusters in the same age range that do not show eMSTOs. We argue that eMSTOs only occur for clusters whose early escape velocities are higher than the wind velocities of stars that provide material from which second-generation stars can form. The threshold of 12 -15 km s −1 is consistent with wind velocities of intermediate-mass AGB stars and massive binary stars in the literature.

Context. The solar-age open cluster M 67 (C0847+120, NGC 2682) is a touchstone in studies of the old Galactic disk. Despite its outstanding role, the census of cluster membership for M 67 at fainter magnitudes and their properties are not well-established. Aims. Using proprietary and archival ESO data, we have obtained astrometric, photometric, and radial velocities of stars in a 34 × 33 arcmin 2 field centered on the old open cluster M 67. Methods. The two-epoch archival observations separated by 4 years and acquired with the Wide-Field Imager at the 2.2 m MPG/ESO telescope were reduced with our new astrometric techniques, as described in the first paper in this series. The same observations served to derive calibrated BV I photometry in M 67. Radial velocities were measured using the archival and new spectroscopic data obtained at the VLT. Results. We have determined relative proper motions and membership probabilities for ∼2400 stars. The precision of proper motions for optimally exposed stars is 1.9 mas yr −1 , gradually degrading down to ∼5 mas yr −1 at V = 20. Our relatively precise proper motions at V > 16 were first obtained in this magnitude range for M 67. Radial velocities were measured for 211 stars in the same field. We also present a detailed comparison with recent theoretical isochrones from several independent groups. Conclusions. For the M 67 area, we provide positions, calibrated BV I photometry, relative proper motions, membership probabilities, and radial velocities. We demonstrate that ground-based CCD mosaic observations just a few years apart are producing proper motions, allowing reliable membership determination. We have produced a catalog that is electronically available to the astronomical community. This is paper 33 of the WIYN Open Cluster Study (WOCS).