Intermediate-age Magellanic Cloud Clusters Research Articles

Multiple stellar populations in Magellanic Cloud clusters – IV. The double main sequence of the young cluster NGC 1755

Nearly all the star clusters with ages of ~1-2 Gyr in both Magellanic Clouds exhibit an extended main-sequence turn off (eMSTO) whose origin is under debate. The main scenarios suggest that the eMSTO could be either due to multiple generations of stars with different ages or to coeval stellar populations with different rotation rates. In this paper we use Hubble-Space-Telescope images to investigate the ~80-Myr old cluster NGC1755 in the LMC. We find that the MS is split with the blue and the red MS hosting about the 25% and the 75% of the total number of MS stars, respectively. Moreover, the MSTO of NGC1755 is broadened in close analogy with what is observed in the ~300-Myr-old NGC1856 and in most intermediate-age Magellanic-Cloud clusters. We demonstrate that both the split MS and the eMSTO are not due to photometric errors, field-stars contamination, differential reddening, or non-interacting binaries. These findings make NGC1755 the youngest cluster with an eMSTO. We compare the observed CMD with isochrones and conclude that observations are not consistent with stellar populations with difference in age, helium, or metallicity only. On the contrary, the split MS is well reproduced by two stellar populations with different rotation, although the fit between the observed eMSTO and models with different rotation is not fully satisfactory. We speculate whether all stars in NGC1755 were born rapidly rotating, and a fraction has slowed down on a rapid timescale, or the dichotomy in rotation rate was present already at star formation. We discuss the implication of these findings on the interpretation of eMSTO in young and intermediate-age clusters.

The HST survey of Magellanic-Cloud clusters and of their stellar populations

AbstractA large number of intermediate-age (~1-2-Gyr old) globular clusters (GCs) in the Large and the Small Magellanic Cloud (MC) exhibit either bimodal or extended main-sequence (MS) turn off and dual red clump (RC). Moreover, recent papers have shown that the MS of the young clusters NGC 1844 and NGC 1856 is either broadened or split. These features of the color-magnitude diagram (CMD) are not consistent with a single isochrone and suggest that star clusters in MCs have experienced a prolonged star formation, in close analogy with Milky-Way GCs with multiple stellar populations. As an alternative, stellar rotation or interacting binaries can be responsible of the CMD morphology. In the following I will summarize the observational scenario and provide constraints on the nature of the complex CMD of young and intermediate-age MC clusters from our ongoing photometric survey with the Hubble Space Telescope (HST).

THE INSIDIOUS BOOSTING OF THERMALLY PULSING ASYMPTOTIC GIANT BRANCH STARS IN INTERMEDIATE-AGE MAGELLANIC CLOUD CLUSTERS

(Abridged) In the recent controversy about the role of TP-AGB stars in evolutionary population synthesis (EPS) models of galaxies, one particular aspect is puzzling: TP-AGB models aimed at reproducing the lifetimes and integrated fluxes of the TP-AGB phase in Magellanic Cloud (MC) clusters, when incorporated into EPS models, are found to overestimate the TP-AGB contribution in resolved star counts and integrated spectra of galaxies. In this paper, we call attention to a particular evolutionary aspect that in all probability is the main cause of this conundrum. As soon as stellar populations intercept the ages at which RGB stars first appear, a sudden change in the lifetime of the core He-burning phase causes a temporary boost in the production rate of subsequent evolutionary phases, including the TP-AGB. For a timespan of about 0.1 Gyr, triple TP-AGB branches develop at slightly different initial masses, causing their frequency and contribution to the integrated luminosity of the stellar population to increase by a factor of 2. The boost occurs just in the proximity of the expected peak in the TP-AGB lifetimes, and for ages of 1.6 Gyr. Coincidently, this relatively narrow age interval happens to contain the few very massive MC clusters that host most of the TP-AGB stars used to constrain stellar evolution and EPS models. This concomitance makes the AGB-boosting particularly insidious in the context of present EPS models. The effect brings about three main consequences. (1) Present estimates of the TP-AGB contribution to the integrated light of galaxies derived from MC clusters, are biased towards too large values. (2) The relative TP-AGB contribution of single-burst populations falling in this critical age range cannot be accurately derived by the fuel consumption theorem. (3) A careful revision of AGB star populations in intermediate-age MC clusters is urgently demanded.

Keck Spectroscopy of Candidate Proto–Globular Clusters in NGC 1275

?????Keck spectroscopy of five proto?globular cluster candidates in NGC 1275 has been combined with Hubble Space Telescope Wide Field Planetary Camera 2 photometry to explore the nature and origin of these objects and discriminate between merger and cooling-flow scenarios for globular cluster formation. The objects we have studied are not H II regions, but rather star clusters, yet their integrated spectral properties do not resemble young or intermediate-age Magellanic Cloud clusters or Milky Way open clusters. The clusters' Balmer absorption appears to be too strong to be consistent with any of the standard Bruzual & Charlot evolutionary models at any metallicity. If the Bruzual & Charlot models are adopted, an initial mass function (IMF) that is skewed to high masses provides a better fit to the data of the proto?globular cluster candidates. A truncated IMF with a mass range of 2?3 M? reproduces the observed Balmer equivalent widths and colors at ~450 Myr. Formation in a continuous cooling flow appears to be ruled out since the age of the clusters is much larger than the cooling time, the spatial scale of the clusters is much smaller than the cooling-flow radius, and the deduced star formation rate in the cooling flow favors a steep rather than a flat IMF. A merger would have to produce clusters only in the central few kiloparsecs, presumably from gas in the merging galaxies that was channeled rapidly to the center. Widespread shocks in merging galaxies cannot have produced these clusters. If these objects are confirmed to have a relatively flat, or truncated, IMF, it is unclear whether they will evolve into objects that we would regard as bona fide globular clusters.

Carbon stars in clusters in the Galaxy and the Magellanic Clouds

Spectra and infrared (JHK) magnitudes have been obtained for 23 of the brightest giants in the SMC clusters NGC 419 (18), NGC 121 (2) and Kron 3 (3) and for 10 giants in the LMC cluster NGC 1846. In addition, infrared observations have been made of the giant branches of the galactic clusters NGC 2477 and 2660 both of which have been reported to contain carbon stars. We estimate that carbon stars first appear on the asymptotic giant branch of the intermediate age Magellanic Cloud clusters at Mbol = −4.3 to −4.6, although there appears to be a separate class of fainter 13C-rich carbon stars (J stars) in both the clusters and the fields of the Magellanic Clouds. Stars exhibiting bands of ZrO as well as TiO (spectral type MS) were commonly found at the tip of the oxygen-rich giant branch; these stars have obviously begun dredging up carbon and s-process elements (e.g., Zr) and their existence at this phase of evolution confirms the association of the S stars with helium shell-flashing stars in the transition phase between pure M and C spectral types. By examining data on the frequency of occurrence of carbon stars in the fields of the Clouds and the Galaxy, we deduce that dredge-up at helium shell flashes cannot produce carbon stars from stars with masses (at the base of the first giant branch) of ≲ 0.9 M⊙ if [Fe/H] ≈ −1, or ≲ 1.3 M⊙ if [Fe/H] ≈ 0.