Chengyuan Li

The Search for Multiple Populations in Magellanic Cloud Clusters II: The Detection of Multiple Populations in Three Intermediate-Age SMC Clusters

This is the second paper in our series about the search for multiple populations in Magellanic Cloud star clusters using the Hubble Space Telescope. Here we report the detection of multiple stellar populations in the colour-magnitude diagrams of the intermediate-age clusters Lindsay 1, NGC 416 and NGC 339. With ages between 6.0 and 7.5 Gyr, these clusters are the youngest ones in which chemical abundance spreads have been detected so far. This confirms that the appearance of multiple populations is not restricted to only ancient globular clusters, but may also be a common feature in clusters as young as 6 Gyr. Our results are in agreement with a recent spectroscopic study of Lindsay 1. We found that the fraction of enriched stars in NGC 416 is ~45% whereas it is ~25% in NGC 339 and ~36% in Lindsay 1. Similar to NGC 121, these fractions are lower than the average value for globular clusters in the Milky Way.

A young cluster with an extended main-sequence turnoff: confirmation of a prediction of the stellar rotation scenario

We present Hubble Space Telescope photometry of NGC 1850, a ~100 Myr, ~105 M⊙ cluster in the Large Magellanic Cloud. The colour-magnitude diagram clearly shows the presence of an extended main-sequence turnoff (eMSTO). The use of non-rotating stellar isochrones leads to an age spread of ~40 Myr. This is in good agreement with the age range expected when the effects of rotation in the main-sequence turnoff (MSTO) stars are wrongly interpreted in terms of age spread. We also do not find evidence for multiple, isolated episodes of star formation bursts within the cluster, in contradiction to scenarios that invoke actual age spreads to explain the eMSTO phenomenon. NGC 1850 therefore continues the trend of eMSTO clusters, where the inferred age spread is proportional to the age of the cluster. While our results confirm a key prediction of the scenario where stellar rotation causes the eMSTO feature, direct measurements of the rotational rate of MSTO stars is required to definitively confirm or refute whether stellar rotation is the origin of the eMSTO phenomenon or if it is due to an as yet undiscovered effect.

Consequences of Dynamical Disruption and Mass Segregation for the Binary Frequencies of Star Clusters

The massive (13,000–26,000 M⊙), young (15–30 Myr) Large Magellanic Cloud star cluster NGC 1818 reveals an unexpected increasing binary frequency with radius for F-type stars (1.3–2.2 M⊙). This is in contrast to many older star clusters that show a decreasing binary frequency with radius. We study this phenomenon with sophisticated N-body modeling, exploring a range of initial conditions, from smooth virialized density distributions to highly substructured and collapsing configurations. We find that many of these models can reproduce the cluster’s observed properties, although with a modest preference for substructured initial conditions. Our models produce the observed radial trend in binary frequency through disruption of soft binaries (with semi-major axes, a & 3000 AU), on approximately a crossing time (� 5.4 Myr), preferentially in the cluster core. Mass segregation subsequently causes the binaries to sink towards the core. After roughly one initial half-mass relaxation time (trh(0) � 340 Myr) the radial binary frequency distribution becomes bimodal, the innermost binaries having already segregated towards the core, leaving a minimum in the radial binary frequency distribution that marches outwards with time. After 4–6 trh(0), the rising distribution in the halo disappears, leaving a radial distribution that rises only towards the core. Thus, both a radial binary frequency distribution that falls towards the core (as observed for NGC 1818) and one that rises towards the core (as for older star clusters) can arise naturally from the same evolutionary sequence owing to binary disruption and mass segregation in rich star clusters.

THE VMC SURVEY. XI. RADIAL STELLAR POPULATION GRADIENTS IN THE GALACTIC GLOBULAR CLUSTER 47 TUCANAE

We present a deep near-infrared color-magnitude diagram of the Galactic globular cluster 47 Tucanae, obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA) as part of the VISTA near-infrared Y, J, K s survey of the Magellanic System (VMC). The cluster stars comprising both the subgiant and red giant branches exhibit apparent, continuous variations in color-magnitude space as a function of radius. Subgiant branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core; similarly, red-giant-branch stars in the clusters periphery are bluer than their more centrally located cousins. The observations can very well be described by adopting an age spread of ~0.5 Gyr as well as radial gradients in both the clusters helium abundance (Y) and metallicity (Z), which change gradually from (Y = 0.28, Z = 0.005) in the cluster core to (Y = 0.25, Z = 0.003) in its periphery. We conclude that the clusters inner regions host a significant fraction of second-generation stars, which decreases with increasing radius; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population.

Age as a major factor in the onset of multiple populations in stellar clusters

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

Formation of new stellar populations from gas accreted by massive young star clusters.

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The search for multiple populations in Magellanic Cloud Clusters – III. No evidence for multiple populations in the SMC cluster NGC 419

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

DIFFERENT DYNAMICAL AGES FOR THE TWO YOUNG AND COEVAL LMC STAR CLUSTERS NGC 1805 AND NGC 1818 IMPRINTED ON THEIR BINARY POPULATIONS

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THE VMC SURVEY. XVIII. RADIAL DEPENDENCE OF THE LOW-MASS, 0.55–0.82 M⊙ STELLAR MASS FUNCTION IN THE GALACTIC GLOBULAR CLUSTER 47 TUCANAE

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 (

THE RADIAL DISTRIBUTIONS OF THE TWO MAIN-SEQUENCE COMPONENTS IN THE YOUNG MASSIVE STAR CLUSTER NGC 1856

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