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.

We derive the star formation history (SFH) for several regions of the Large Magellanic Cloud (LMC), using deep near-infrared data from the VISTA near-infrared Y JK s survey of the Magellanic system (VMC). The regions include three almost-complete 1.4 deg 2 tiles located ∼3.5• away from the LMC centre in distinct directions. They are split into 21.0 × 21.5 (0.12 deg 2 ) subregions, and each of these is analysed independently. To this dataset, we add two 11.3 × 11.3 (0.036 deg 2 ) subregions selected based on their small and uniform extinction inside the 30 Doradus tile. The SFH is derived from the simultaneous reconstruction of two different colour-magnitude diagrams (CMDs), using the minimization code StarFISH together with a database of "partial models" representing the CMDs of LMC populations of various ages and metallicities, plus a partial model for the CMD of the Milky Way foreground. The distance modulus (m− M) 0 and extinction A V is varied within intervals ∼0.2 and ∼0.5 mag wide, respectively, within which we identify the best-fitting star formation rate SFR(t) as a function of lookback time t, age-metallicity relation (AMR), (m− M) 0 and A V . Our results demonstrate that VMC data, due to the combination of depth and little sensitivity to differential reddening, allow the derivation of the space-resolved SFH of the LMC with unprecedented quality compared to previous wide-area surveys. In particular, the data clearly reveal the presence of peaks in the SFR(t) at ages log(t/yr) 9.3 and 9.7, which appear in most of the subregions. The most recent SFR(t) is found to vary greatly from subregion to subregion, with the general trend of being more intense in the innermost LMC, except for the tile next to the N11 complex. In the bar region, the SFR(t) seems remarkably constant over the time interval from log(t/yr) 8.4 to 9.7. The AMRs, instead, turn out to be remarkably similar across the LMC. Thanks to the accuracy in determining the distance modulus for every subregion -with typical errors of just ∼0.03 mag -we make a first attempt to derive a spatial model of the LMC disk. The fields studied so far are fit extremely well by a single disk of inclination i = 26.2 ± 2.0• , position angle of the line of nodes θ 0 = 129.1 ± 13.0• , and distance modulus of (m− M) 0 = 18.470 ± 0.006 mag (random errors only) up to the LMC centre. We show that once the (m− M) 0 values or each subregion are assumed to be identical to those derived from this best-fitting plane, systematic errors in the SFR(t) and AMR are reduced by a factor of about two.

We use the open clusters (OCs) with known parameters available in the WEBDA database and in recently published papers to derive properties related to the disk structure such as the thin-disk scale height, displacement of the Sun above the Galactic plane, scale length and the OC age-distribution function. The sample totals 654 OCs, consisting basically of Trumpler types I to III clusters whose spatial distribution traces out the local geometry of the Galaxy. We find that the population of OCs with ages younger than 200 Myr distributes in the disk following an exponential-decay profile with a scale height of z h = 48 ± 3 pc. For the clusters with ages in the range 200 Myr to 1 Gyr we derive z h = 150 ± 27 pc. Clusters older than 1 Gyr distribute nearly uniformly in height from the plane so that no scale height can be derived from exponential fits. Considering clusters of all ages we obtain an average scale height of z h = 57 ± 3 pc. We confirm previous results that z h increases with Galactocentric distance. The scale height implied by the OCs younger than 1 Gyr outside the Solar circle is a factor ∼1.4−2 larger than z h of those interior to the Solar circle. We derive the displacement of the Sun above the Galactic plane as z = 14.8 ± 2.4 pc, which agrees with previous determinations using stars. As a consequence of the completeness effects, the observed radial distribution of OCs with respect to Galactocentric distance does not follow the expected exponential profile, instead it falls off both for regions external to the Solar circle and more sharply towards the Galactic center. We simulate the effects of completeness assuming that the observed distribution of the number of OCs with a given number of stars above the background, measured in a restricted zone outside the Solar circle, is representative of the intrinsic distribution of OCs throughout the Galaxy. Two simulation models are considered in which the intrinsic number of observable stars are distributed: (i) assuming the actual positions of the OCs in the sample, and (ii) random selection of OC positions. As a result we derive completeness-corrected radial distributions which agree with exponential disks throughout the observed Galactocentric distance range 5-14 kpc, with scale lengths in the range R D = 1.5−1.9 kpc, smaller than those inferred by means of stars. In particular we retrieve the expected exponential-disk radial profile for the highly depleted regions internal to the Solar circle. The smaller values of R D may reflect intrinsic differences in the spatial distributions of OCs and stars. We derive a number-density of Solar-neighbourhood (with distances from the Sun d ≤ 1.3 kpc) OCs of ρ = 795 ± 70 kpc −3 , which implies a total number of (Trumpler types I to III) OCs of ∼730 of which ∼47% would already have been observed. Extrapolation of the completeness-corrected radial distributions down to the Galactic center indicates a total number of OCs in the range (1.8−3.7) × 10 5 . These estimates are upper-limits because they do not take into...

We analyse deep images from the VISTA survey of the Magellanic Clouds in the Y JK s filters, covering 14 deg 2 (10 tiles), split into 120 subregions, and comprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply a colour-magnitude diagram reconstruction method that returns their best-fitting star formation rate SFR(t), age-metallicity relation (AMR), distance and mean reddening, together with 68% confidence intervals. The distance data can be approximated by a plane tilted in the East-West direction with a mean inclination of 39 • , although deviations of up to ±3 kpc suggest a distorted and warped disk. After assigning to every observed star a probability of belonging to a given age-metallicity interval, we build high-resolution population maps. These dramatically reveal the flocculent nature of the young star-forming regions and the nearly smooth features traced by older stellar generations. They document the formation of the SMC Wing at ages < 0.2 Gyr and the peak of star formation in the SMC Bar at ∼ 40 Myr. We clearly detect periods of enhanced star formation at 1.5 Gyr and 5 Gyr. The former is possibly related to a new feature found in the AMR, which suggests ingestion of metal-poor gas at ages slightly larger than 1 Gyr. The latter constitutes a major period of stellar mass formation. We confirm that the SFR(t) was moderately low at even older ages.

Colour–magnitude diagrams (CMDs) of the Small Magellanic Cloud (SMC) star cluster NGC 419, derived from Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) data, reveal a well‐delineated secondary clump located below the classical compact red clump typical of intermediate‐age populations. We demonstrate that this feature belongs to the cluster itself, rather than to the underlying SMC field. Then, we use synthetic CMDs to show that it corresponds very well to the secondary clump predicted to appear as a result of He‐ignition in stars just massive enough to avoid e−‐degeneracy settling in their H‐exhausted cores. The main red clump instead is made of the slightly less massive stars which passed through e− degeneracy and ignited He at the tip of the red giant branch. In other words, NGC 419 is the rare snapshot of a cluster while undergoing the fast transition from classical to degenerate H‐exhausted cores. At this particular moment of a cluster's life, the colour distance between the main‐sequence turn‐off and the red clump(s) depends sensitively on the amount of convective core overshooting, Λc. By coupling measurements of this colour separation with fits to the red clump morphology, we are able to estimate simultaneously the cluster mean age (1.35+0.11−0.04 Gyr) and overshooting efficiency (Λc= 0.47+0.14−0.04). Therefore, clusters like NGC 419 may constitute important marks in the age scale of intermediate‐age populations. After eye inspection of other CMDs derived from HST/ACS data, we suggest that the same secondary clump may also be present in the Large Magellanic Cloud clusters NGC 1751, 1783, 1806, 1846, 1852 and 1917.