Can rotation explain the multiple main sequence turn-offs of Magellanic Cloud star clusters?

Many intermediate age star clusters in the Magellanic Clouds present multiple main sequence turn-offs (MMSTO), which challenge the classical idea that star formation in such objects took place over short timescales. It has been recently suggested that the presence of fast rotators among main sequence stars could be the cause of such features (Bastian & de Mink 2009), hence relaxing the need for extended periods of star formation. In this letter, we compute evolutionary tracks and isochrones of models with and without rotation. We find that, for the same age and input physics, both kinds of models present turn-offs with an almost identical position in the colour-magnitude diagrams. As a consequence, a dispersion of rotational velocities in coeval ensembles of stars could not explain the presence of MMSTOs. We construct several synthetic colour-magnitude diagrams for the different kinds of tracks and combinations of them. The models that best reproduce the morphology of observed MMSTOs are clearly those assuming a significant spread in the stellar ages - as long as ~400 Myr - added to a moderate amount of convective core overshooting. Only these models produce the detailed "golf club" shape of observed MMSTOs. A spread in rotational velocities alone cannot do anything similar. We also discuss models involving a mixture of stars with and without overshooting, as an additional scenario to producing MMSTOs with coeval populations. We find that they produce turn-offs with a varying extension in the CMD direction perpendicular to the lower main sequence, which are clearly not present in observed MMSTOs.Comment: To appear in MNRAS Letters. Figs. 2 and 3 are in colou

The Contributions of Interactive Binary Stars to Double Main Sequence Turn-Offs and Dual Red Clump of Intermediate-Age Star Clusters

Double or extended main-sequence turn-offs (DMSTOs) and dual red clump (RC) were observed in intermediate-age clusters, such as in NGC 1846 and 419. the DMSTOs are interpreted as that the cluster has two distinct stellar popula- tions with differences in age of about 200-300 Myr but with the same metallicity. The dual RC is interpreted as a result of a prolonged star formation. Using a stellar population-synthesis method, we calculated the evolutions of binary-star stellar population (BSP). We found that binary interactions and merging can reproduce the dual RC in the color-magnitude diagrams of an intermediate-age cluster, whereas in actuality only a single population exists. Moreover, the binary interactions can lead to an extended MSTO rather than DMSTOs. However, the rest of main sequence, subgiant branch and first giant branch are hardly spread by the binary interactions. Part of the observed dual RC and extended MSTO may be the results of binary interactions and merger.Comment: 10 pages, 2 figures; accepted by ApJ

The VMC Survey. XXII. Hierarchical Star Formation in the 30 Doradus-N158-N159-N160 Star-forming Complex

We study the hierarchical stellar structures in a ∼1.5 deg2 area covering the 30 Doradus-N158-N159-N160 star-forming complex with the VISTA Survey of Magellanic Clouds. Based on the young upper main-sequence stars, we find that the surface densities cover a wide range of values, from log(pc2) ≲ -2.0 to log(pc2) ≳ 0.0. Their distributions are highly non-uniform, showing groups that frequently have subgroups inside. The sizes of the stellar groups do not exhibit characteristic values, and range continuously from several parsecs to more than 100 pc; the cumulative size distribution can be well described by a single power law, with the power-law index indicating a projected fractal dimension D2 = 1.6 ± 0.3. We suggest that the phenomena revealed here support a scenario of hierarchical star formation. Comparisons with other star-forming regions and galaxies are also discussed.Fil: Sun, Ning-Chen. Peking University; ChinaFil: Grijs, Richard De. Peking University; ChinaFil: Subramanian, Smitha. Peking University; ChinaFil: Cioni, Maria-Rosa L.. Universita Zu Berlin. Universita Postdam; AlemaniaFil: Rubele, Stefano. Università di Padova; ItaliaFil: Bekki, Kenji. University of Western Australia; AustraliaFil: Ivanov, Valentin D.. European Southern Observatory; ChileFil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Ripepi, Vincenzo. Osservatorio Astronomico di Capodimonte; Itali

The VMC survey - XV : The Small Magellanic Cloud-Bridge connection history as traced by their star cluster populations

Date of Acceptance: 19/03/2015We present results based on YJKs photometry of star clusters located in the outermost, eastern region of the Small Magellanic Cloud (SMC). We analysed a total of 51 catalogued clusters whose colour-magnitude diagrams (CMDs), having been cleaned from field-star contamination, were used to assess the clusters' reality and estimate ages of the genuine systems. Based on CMD analysis, 15 catalogued clusters were found to be possible non-genuine aggregates. We investigated the properties of 80 per cent of the catalogued clusters in this part of the SMC by enlarging our sample with previously obtained cluster ages, adopting a homogeneous scale for all. Their spatial distribution suggests that the oldest clusters, log(t/yr) ≥ 9.6, are in general located at greater distances to the galaxy's centre than their younger counterparts - 9.0 ≤ log(t/yr) ≤ 9.4 - while two excesses of clusters are seen at log(t/yr) ~9.2 and log(t yr-1) ˜ 9.7. We found a trail of younger clusters which follow the wing/bridge components. This long spatial sequence does not only harbour very young clusters, log(t yr-1) ~7.3, but it also hosts some of intermediate ages, log(t/yr) ~9.1. The derived cluster and field-star formation frequencies as a function of age are different. The most surprising feature is an observed excess of clusters with ages of log(t/yr) < 9.0, which could have been induced by interactions with the LMC.Peer reviewedFinal Accepted Versio

The star formation history of the Large Magellanic Cloud star cluster NGC1751

The HST/ACS colour-magnitude diagrams (CMD) of the populous LMC star cluster NGC1751 present both a broad main sequence turn-off and a dual clump of red giants. We show that the latter feature is real and associate it to the first appearance of electron-degeneracy in the H-exhausted cores of the cluster stars. We then apply to the NGC1751 data the classical method of star formation history (SFH) recovery via CMD reconstruction, for different radii corresponding to the cluster centre, the cluster outskirts, and the underlying LMC field. The mean SFH derived from the LMC field is taken into account during the stage of SFH-recovery in the cluster regions, in a novel approach which is shown to significantly improve the quality of the SFH results. For the cluster centre, we find a best-fitting solution corresponding to prolonged star formation for a for a timespan of 460 Myr, instead of the two peaks separated by 200 Myr favoured by a previous work based on isochrone fitting. Remarkably, our global best-fitting solution provides an excellent fit to the data - with chi^2 and residuals close to the theoretical minimum - reproducing all the CMD features including the dual red clump. The results for a larger ring region around the centre indicate even longer star formation, but in this case the results are of lower quality, probably because of the differential extinction detected in the area. Therefore, the presence of age gradients in NGC1751 could not be probed. Together with our previous findings for the SMC cluster NGC419, the present results for the NGC1751 centre argue in favour of multiple star formation episodes (or continued star formation) being at the origin of the multiple main sequence turn-offs in Magellanic Cloud clusters with ages around 1.5 Gyr.Comment: To appear soon in MNRAS. 12 pages, better printed in colou

Discovery of two distinct red clumps in NGC419: a rare snapshot of a cluster at the onset of degeneracy

Colour-magnitude diagrams (CMD) of the SMC star cluster NGC419, derived from HST/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 electron-degeneracy settling in their H-exhausted cores. The main red clump instead is made of the slightly less massive stars which passed through electron-degeneracy and ignited He at the tip of the RGB. In other words, NGC419 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, Lambda_c. 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(-0.04,+0.11) Gyr) and overshooting efficiency (Lambda_c=0.47(-0.04,+0.14)). Therefore, clusters like NGC419 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 LMC clusters NGC1751, 1783, 1806, 1846, 1852, and 1917.Comment: To appear in MNRAS Letters (www.blackwell-synergy.com). Better printed in colou

Star clusters with dual red clumps

A few star clusters in the Magellanic Clouds exhibit composite structures in the red-clump region of their colour-magnitude diagrams. The most striking case is NGC419 in the SMC, where the red clump is composed of a main blob as well as a distinct secondary feature. This structure is demonstrated to be real and corresponds to the simultaneous presence of stars which passed through electron degeneracy after central-hydrogen exhaustion and those that did not. This rare occurrence in a single cluster allows us to set stringent constraints on its age and on the efficiency of convective-core overshooting during main-sequence evolution. We present a more detailed analysis of NGC419, together with a first look at other populous LMC clusters which are apparently in the same phase: NGC1751, NGC1783, NGC1806, NGC1846, NGC1852 and NGC1917. We also compare these Magellanic Cloud cases with their Galactic counterparts, NGC752 and NGC7789. We emphasise the extraordinary potential of these clusters as absolute calibration marks on the age scale of stellar populations.Comment: contributed talk at IAUS 266 'Star clusters: basic galactic building blocks', eds R. de Grijs and J. Lepine. A high resolution version of Fig. 1 is available in http://stev.oapd.inaf.it/~lgirardi/NGC419_hr.pd