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.

The Advanced Camera for Surveys onboard the Hubble Space Telescope has been used to obtain deep, highresolution images of the intermediate-age star cluster NGC 1846 in the Large Magellanic Cloud. We present new color-magnitude diagrams (CMDs) based on F435W, F555W, and F814W imaging. We test the previously observed broad main-sequence turnoff (MSTO) region for "contamination" by field stars and (evolved) binary star systems. We find that while these impact the number of objects in this region, none can fully account for the large color spread. Our results therefore solidify the recent finding that stars in the MSTO region of this cluster have a large spread in color which is unrelated to measurement errors or contamination by field stars, and likely due to a ∼ 300 Myr range in the ages of cluster stars. An unbiased estimate of the stellar density distribution across the MSTO region shows that the spread is fairly continuous rather than strongly bimodal, as suggested previously. We fit the CMDs with several different sets of theoretical isochrones, and determine systematic uncertainties for population parameters when derived using any one set of isochrones. We note a degeneracy between age and [α/Fe], which can be lifted by matching the shape (curvature) of the full red giant branch (RGB) in the CMD. We find that stars in the upper part of the MSTO region are more centrally concentrated than those in any other region, including more massive RGB and asymptotic giant branch stars. We consider several possible formation scenarios which account for the unusual features observed in the CMD of NGC 1846.

We present high-precision UBV RI CCD photometry of the old open cluster NGC 188. Our colormagnitude diagram extends from near the red giant branch tip to as faint as D5 mag below the mainsequence turno †. From an analysis of these data along with published photometry for M67, we draw the following conclusions : (1) From the UBV two-color diagram, we Ðnd a reddening of E(B[V) \ 0.04^0.02 for M67 and E(B[V) \ 0.09^0.02 for NGC 188. (2) Based on main-sequence Ðtting to solar abundance isochrones, the distance moduli turn out to be for (m [ M) v \ 9.69^0.11 M67 and for NGC 188. (3) The comparison of the CMDs to theoretical iso-(m [ M) v \ 11.44^0.08 chrones indicates that an amount of core convective overshoot equivalent to 0.10 of a pressure scale height is appropriate for M67, while no overshoot is required to Ðt the CMD of NGC 188. These isochrones suggest that NGC 188 is 3.0^0.7 Gyr older than M67. (4) There is a clear indication of mass segregation in both M67 and NGC 188, with the most massive stars being more centrally (M/M _ [ 1.1) concentrated than those that are the least massive (0.8 º M/M _ [ 0.65).

We present techniques for obtaining precision astrometry using old photographic plates from assorted largeaperture reflectors in combination with recent CCD Mosaic Imager frames. At the core of this approach is a transformation of plate/CCD coordinates into a previously constructed astrometric reference frame around the open cluster NGC 188. This allows us to calibrate independently the optical field angle distortion for all telescopes and field correctors used in this study. Particular attention is paid to computing the differential color refraction, which has a marked effect in the case of NGC 188 as a result of the large zenith distances at which this cluster has been observed. Our primary result is a new catalog of proper motions and positions for 7812 objects down to V = 21 in the 0.75 deg 2 area around NGC 188. The precision for well-measured stars is 0.15 mas yr À1 for proper motions and 2 mas for positions on the system of the Tycho-2 catalog. In total, 1490 stars have proper-motion membership probabilities P l ! 10%. The sum of membership probabilities indicates that NGC 188 contains $1050 stars down to V = 21. Comprehensive lists of the candidate blue stragglers and red giant stars substantially enlarge the number of such stars known in NGC 188. We have also obtained a small correction to the proper motions from the mean '' motion '' of background galaxies. Thus, the absolute proper motion of NGC 188 is l abs x ¼ À2:56 AE 0:2 and l abs y ¼ þ0:18 AE 0:2 mas yr À1 .

It is now well established that globular clusters (GCs) exhibit star-to-star light-element abundance variations (known as multiple stellar populations, MPs). Such chemical anomalies have been found in (nearly) all the ancient GCs (more than 10 Gyr old) of our Galaxy and its close companions, but so far no model for the origin of MPs is able to reproduce all the relevant observations. To gain new insights into this phenomenon, we have undertaken a photometric Hubble Space Telescope survey to study clusters with masses comparable to that of old GCs, where MPs have been identified, but with significantly younger ages. Nine clusters in the Magellanic Clouds with ages between ∼ 1.5-11 Gyr have been targeted in this survey. We confirm the presence of multiple populations in all clusters older than 6 Gyr and we add NGC 1978 to the group of clusters for which MPs have been identified. With an age of ∼ 2 Gyr, NGC 1978 is the youngest cluster known to host chemical abundance spreads found to date. We do not detect evident star-to-star variations for slightly younger massive clusters (∼ 1.7 Gyr), thus pointing towards an unexpected age dependence for the onset of multiple populations. This discovery suggests that the formation of MPs is not restricted to the early Universe and that GCs and young massive clusters share common formation and evolutionary processes.