Large Magellanic Cloud Clusters Research Articles

Near-Infrared photometry of the LMC cluster Reticulum

AbstractWe present U, B, V, I (SUSI@NTT) and J,Ks (SOFI@NTT) photometry of the Large Magellanic Cloud (LMC) cluster Reticulum. The observing strategy and data reduction (DAOPHOTII/ALLFRAME) allowed us to reach an accuracy of the order of 0.01 – 0.03 mag in all the bands at limiting magnitudes typical of RR Lyrae stars. Reticulum hosts a sizable sample of RR Lyrae stars (32), and we supply an accurate distance estimate using the RR Lyrae K-band Period-Luminosity-Metallicity (PLZK) relation. This method presents several advantages when compared with the MV vs [Fe/H] relation, since it is only marginally affected by off-ZAHB evolutionary effects and reddening corrections.

Fundamental parameters of the LMC clusters NGC 1836, NGC 1860, NGC 1865, SL 444, LW 224 and SL 548

Complementing our recent Washington photometric studies on intermediate age and young Large Magellanic Cloud (LMC) clusters, we now turn our attention to six previously unstudied star clusters in the transition range 200-700 Myr. We study NGC 1836, 1860 and 1865, which are projected on the LMC bar; SL 444, also located in the central disc but outside the bar; and LW 224 and SL 548, both located in the outer disc. We derive ages and metallicities from extracted T 1 versus C-T 1 colour-magnitude diagrams (CMDs), using theoretical isochrones recently computed for the Washington photometric system. For the metallicity determinations, these CMDs are particularly sensitive. We also estimate ages and metallicities of the surrounding fields of NGC 1860 and 1865 by employing the δT 1 index defined in Geisler et al. (1997, AJ, 114, 1920) and theoretical isochrones. By adding the present cluster sample to those of our previous studies, we now gather 37 LMC clusters with homogeneous parameter determinations, which are employed to probe the chemical enrichment of the LMC and its spatial distribution. On average, inner disc clusters turned out to be not only younger than the outer ones, but also more metal-rich; some have solar metal content. Furthermore, inner clusters located to the west of the LMC centre are younger and more metal-rich than their eastern counterparts. We propose that a bursting formation mechanism, with an important formation event centred at ∼2.0 Gyr, provides a better description of the cluster age-metallicity relation than a closed-box chemical evolution model. In the outer disc, the field star formation seems to have lasted until 2 Gyr ago while it continued in the inner disc for almost 1 Gyr longer.

Ages and metallicities of eight star clusters and their surrounding fields in the inner disc of the Large Magellanic Cloud

We present Washington system colour–magnitude diagrams for 8 star clusters and their surrounding fields which, with one exception, lie within the inner parts of the Large Magellanic Cloud (LMC) disc. Careful attention is paid to separating out the cluster and field star distributions. Ages and metallicities are then determined in a consistent manner for both populations in two different ways. We first compare the colour–magnitude diagrams (CMDs) with new theoretical isochrones in the Washington system. We also derive ages using the magnitude difference between the red clump and the turnoff, and derive metallicities by comparing the giant branches to standard calibrating clusters. For this latter metallicity derivation, we present age-dependent metallicity corrections for intermediate age clusters (IACs) based on the new isochrones. The two methods for both age and metallicity determination are in good agreement with each other. All clusters are found to be IACs (1–3 Gyr), with [Fe/H] from −0.4 to −0.9. We find that the stellar population of each star cluster is generally quite similar to that of the field where it is embedded, sharing its mean age and metallicity. Combining the present sample with a revision of that of Bica et al. studied similarly, we find that our metallicities for IACs are intermediate in metallicity to those for clusters of similar age studied by Olszewski et al. and by Beasley, Hoyle & Sharples. A combined age–metallicity relation is presented which shows that LMC clusters formed between 1–3 Gyr ago with a mean metallicity (–0.5 dex) and metallicity spread (0.23 dex) independent of age. Good agreement is found with the bursting model of Pagel & Tautvaivienė. No evidence for a metallicity gradient is found.

The Distance to the Large Magellanic Cloud Cluster NGC 1866 and the Surrounding Field

We use the main-sequence stars in the LMC cluster NGC 1866 and red clump stars in the local field to obtain two independent estimates of the LMC distance. We apply an empirical main-sequence-fitting technique based on a large sample of subdwarfs with accurate Hipparcos parallaxes in order to estimate the cluster distance modulus, and the multicolor red clump method to derive distance and reddening of the LMC field. We find that the main-sequence fitting and the red clump distance moduli are in significant disagreement; the NGC 1866 distance modulus is equal to (m - M)0,NGC 1866 = 18.33 ± 0.08 (consistent with a previous estimate using the same data and theoretical main-sequence isochrones), while the field stars provide (m - M)0,field = 18.53 ± 0.07. This difference reflects the more general dichotomy in the LMC distance estimates found in the literature. Various possible causes for this disagreement are explored, with particular attention paid to the still uncertain metallicity of the cluster and the star formation history of the field stars.

Core radius evolution of star clusters

We use N-body simulations of star clusters to investigate the possible dynamical origins of the observed spread in core radius among intermediate-age and old star clusters in the Large Magellanic Cloud (LMC). Two effects are considered, a time-varying external tidal field and variations in primordial hard binary fraction. Simulations of clusters orbiting a point-mass galaxy show similar core radius evolution for clusters on both circular and elliptical orbits and we therefore conclude that the tidal field of the LMC has not yet significantly influenced the evolution of the intermediate-age clusters. The presence of large numbers of hard primordial binaries in a cluster leads to core radius expansion; however, the magnitude of the effect is insufficient to explain the observations. Further, the range of binary fractions required to produce significant core radius growth is inconsistent with the observational evidence that all the LMC clusters have similar stellar luminosity functions.

Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. II. Analysis with the Yale Models

Using Yale stellar evolution models, we present an analysis of the color-magnitude diagrams (CMDs) of three intermediate-age LMC clusters, namely, NGC 2173, SL 556, and NGC 2155, obtained with the Very Large Telescope. The main goal of our project is to investigate the amount of convective core overshoot necessary to reproduce the CMDs of relatively metal-poor, intermediate-age stellar populations, to check whether the extrapolation that is usually made from solar metallicity is valid. In the process, we obtained values for the binary fraction of each cluster, together with refined age estimates. Our method involved the comparison of the observed CMDs with synthetic CMDs computed using various values of the overshoot parameter and binary fraction. We conclude that a moderate amount of overshoot and some fraction of binary stars are essential for reproducing the observed shapes around the turnoff in the CMDs of all three clusters: unresolved binary stars fill in the expected core contraction gap and make a unique sequence near the gap, which cannot be reproduced by single stars alone, even with a larger amount of overshoot. We utilize ratios of the number of stars in different areas around the core contraction gap to constrain the binary fraction, which is around 10%–20% (for primary-to-secondary mass ratio ≥0.7) in all three clusters. Even if binary stars contaminate the core contraction gap, it is shown that the overshoot parameter can be inferred from the color dispersion of the stars around the contraction gap, regardless of the assumed binary fraction. From our overall analysis such as shape of isochrones, star counts, color distribution, and synthetic CMD comparisons, we conclude that overshoot ∼20% of the local pressure scale height best reproduces the CMD properties of all three clusters. The best age estimates are 1.5, 2.1, and 2.9 Gyr for NGC 2173, SL 556, and NGC 2155, respectively.

Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud

We have compiled a pseudo-snapshot data set of two-colour observations from the Hubble Space Telescope archive for a sample of 53 rich LMC clusters with ages of 106–1010 yr. We present surface brightness profiles for the entire sample, and derive structural parameters for each cluster, including core radii, and luminosity and mass estimates. Because we expect the results presented here to form the basis for several further projects, we describe in detail the data reduction and surface brightness profile construction processes, and compare our results with those of previous ground-based studies. The surface brightness profiles show a large amount of detail, including irregularities in the profiles of young clusters (such as bumps, dips and sharp shoulders), and evidence for both double clusters and post-core-collapse (PCC) clusters. In particular, we find power-law profiles in the inner regions of several candidate PCC clusters, with slopes of approximately −0.7, but showing considerable variation. We estimate that 20 ± 7 per cent of the old cluster population of the Large Magellanic Cloud (LMC) has entered PCC evolution, a similar fraction to that for the Galactic globular cluster system. In addition, we examine the profile of R136 in detail and show that it is probably not a PCC cluster. We also observe a trend in core radius with age that has been discovered and discussed in several previous publications by different authors. Our diagram has better resolution, however, and appears to show a bifurcation at several hundred Myr. We argue that this observed relationship reflects true physical evolution in LMC clusters, with some experiencing small-scale core expansion owing to mass loss, and others large-scale expansion owing to some unidentified characteristic or physical process.

Surface brightness profiles and structural parameters for globular clusters in the Fornax and Sagittarius dwarf spheroidal galaxies

We present radial surface brightness profiles for all five globular clusters in the Fornax dwarf spheroidal galaxy, and for the four present members of the Sagittarius dwarf spheroidal galaxy. These profiles are derived from archival Hubble Space Telescope observations, and have been calculated using the same techniques with which we measured profiles in our previous studies of Large and Small Magellanic Cloud (LMC and SMC) clusters, apart from some small modifications. From the surface brightness profiles, we have determined structural parameters for each cluster, including core radii and luminosity and mass estimates. We also provide a brief summary of literature measurements of other parameters for these clusters, including their ages, metallicities and distances. Our core radius measurements are mostly in good agreement with those from previous lower resolution studies, although for several clusters our new values are significantly different. The profile for Fornax cluster 5 does not appear to be well fitted by a King-type model and we suggest that it is a post-core-collapse candidate. We examine the distribution of cluster core radii in each of the two dwarf galaxy systems and compare these with the distribution of core radii for old LMC clusters. The three distributions match within the limits of measurement errors and the small-sample sizes. We discuss the implications of this in the context of the radius—age trend we have previously highlighted for the Magellanic Cloud clusters.

ITAL]K[/ITAL]-Band Red Clump Distances to the Large Magellanic Cloud Clusters Hodge 4 and NGC 1651

We present near-infrared color-magnitude diagrams (CMDs) for the Large Magellanic Cloud (LMC) star clusters Hodge 4 and NGC 1651, revealing the helium-burning red clump (RC) stars for the first time in the JK passbands. From these diagrams, which extend to K ~ 19, and existing optical CMDs that reveal the clusters' main-sequence turnoffs, we determine the following cluster parameters. For Hodge 4, we estimate a metallicity of [Fe/H] = -0.17 ± 0.04, an age of 1.7 ± 0.3 Gyr, and a RC absolute magnitude of MK(RC) = -1.64 ± 0.17. Along with an adopted reddening of E(J-K) = 0.03 ± 0.01 based on the Burstein & Heiles and Schlegel et al. reddening maps, we find a distance of (m - M)0 = 18.52 ± 0.17 for Hodge 4. In the case of NGC 1651, we derive [Fe/H] = -0.07 ± 0.10, an age of 1.8 ± 0.3 Gyr, MK(RC) = -1.56 ± 0.12, E(J-K) = 0.06 ± 0.01, and (m - M)0 = 18.55 ± 0.12, all measured in the same manner as for Hodge 4. Based on these two clusters, we calculate a mean LMC distance of (m - M)0 = 18.54 ± 0.10.

Testing stellar population models with star clusters in the Large Magellanic Cloud

We present high signal-to-noise integrated spectra of 24 star clusters in the Large Magellanic Cloud (LMC), obtained using the FLAIR spectrograph at the UK Schmidt telescope. The spectra have been placed onto the Lick/IDS system in order to test the calibration of Simple Stellar Population (SSP) models (Maraston & Thomas 2000; Kurth, Fritz-von Alvensleben & Fricke 1999). We have compared the SSP-predicted metallicities of the clusters with those from the literature, predominantly taken from the Ca-Triplet spectroscopy of Olszewski et al.(1991). We find that there is good agreement between the metallicities in the range - 2.10 ≤ (Fe/H) ≤ 0. However, the Mg2 index (and to a lesser degree Mg b) systematically predict higher metallicities (up to +0.5 dex higher) than h Fei . Among the possible explanations for this are that the LMC clusters possess (�/Fe) > 0. Metallicities are presented for eleven LMC clusters which have no previous measurements. We compare SSP ages for the clusters, derived from the H�, H and HLick/IDS indices, with the available literature data, and find good agreement for the vast ma- jority. This includes six old globular clusters in our sample, which have ages consistent with their HST CMD ages and/or integrated colours. However, two globular clusters, NGC 1754 and NGC 2005, identified as old (∼ 15 Gyr) on the basis of HST CMDs, have Hline-strengths which lead ages which are too young (∼ 8 and ∼ 6 Gyr re- spectively). These findings are inconsistent with their CMD-derived values at the 3� level. Comparison between the horizontal branch morphology and the Balmer line- strengths of these clusters suggests that the presence of blue horizontal branch stars has increased their Balmer indices by up to ∼ 1.0 u We conclude that the Lick/IDS indices, used in conjunction with contemporary SSP models, are able to reproduce the ages and metallicities of the LMC clusters reassuringly well. The required extrapolations of the fitting-functions and stellar li- braries in the models to younger ages and low metallicities do not lead to serious systematic errors. However, due to the significant contribution of horizontal branch stars to Balmer indices, SSP model ages derived for metal-poor globular clusters are ambiguous without a priori knowledge of horizontal branch morphology.

On the nature of a secondary main-sequence turn-off in the rich LMC cluster NGC 1868

Evidence for a second main-sequence turn-off in a deep colour–magnitude diagram (CMD) of NGC 1868 is presented. The data were obtained with Hubble Space Telescope (HST) WFPC2 and reach down to m555≃ 25 mag. Besides the usual τ≃ 0.8 Gyr turn-off found in previous analyses, another possible turn-off is seen at m555≃ 21 mag (MV≃ 2.5), which is consistent with an age of τ≃ 3 Gyr. This CMD feature stands out clearly especially when contaminating field Large Magellanic Cloud (LMC) stars are statistically removed. The background subtracted CMD also visibly displays a red subgiant branch extending about 1.5 mag below the younger turn-off and the clump of red giants. The significance of the secondary turn-off in NGC 1868 was confirmed with Monte Carlo simulations and bootstrapping techniques. Star counts in selected regions in the cluster CMD indicate a mass ratio of old population/young population in the range 5 ≲Mold/Myoung≲ 12 per cent, depending on the mass-function slope. The existence of such a subpopulation in NGC 1868 is significant even in the presence of uncertainties in background subtraction. The possibility that the secondary turn-off is associated with the field star population was examined by searching for similar features in CMDs of field stars. Statistically significant excesses of stars redwards of the main sequence were found in all such fields in the range 20 ≲m555≲ 22 mag. These however are much broader features that do not resemble the main-sequence termination of a single population. We also discuss other alternative explanations for the feature at m555≃ 21 mag, such as unresolved binarism, peculiar stars or CMD discontinuities associated with the Böhm–Vitense gap.

Constraining the LMC cluster age gap: Washington photometry of NGC 2155 and SL 896 (LW 480)

We carried out Washington system photometry of the intermediate-age Large Magellanic Cloud (LMC) star clusters NGC 2155 and SL 896 (LW 480). We derive ages and metallicities from the T 1 versus C - T 1 colour-magnitude diagrams (CMDs). For the first time an age has been obtained for SL 896, 2.3 ′ 0.5 Gyr. For NGC 2155 we derive 3.6 ′ 0.7 Gyr. The two clusters basically define the lower age limit of the LMC age gap. In particular, NGC 2155 is confirmed as the oldest intermediate-age LMC cluster so far studied. The derived metallicities are [Fe/H] = -0.9 ′ 0.2 and -0.6 ′ 0.2 for NGC2155 and SL 896, respectively. We also studied the CMDs of the surrounding fields, which have a dominant turn-off comparable to that of the clusters themselves, and similar metallicity, showing that one is dealing with an intermediate-age disc where clusters and field stars have the same origin. We inserted the present clusters in the LMC and Small Magellanic Cloud (SMC) age-metallicity relations, using a set of homogeneous determinations with the same method as in our previous studies, now totalling 15 LMC clusters and four SMC clusters, together with some additional values from the literature. The LMC and SMC age-metallicity relations appear to be remarkably complementary, since the SMC was actively star-forming during the LMC quiescent age gap epoch.

Variable Stars in Clusters and the Distance Scale - Some Recent Results Concerning the LMC Cluster NGC 1866

After summarizing the importance of clusters and variable stars in the context of the distance scale, we focus on two new, and independent, programs to determine the distance to the Large Magellanic Cloud cluster NGC 1866.

The Distance of the Large Magellanic Cloud Cluster NGC 1866

Hubble Space Telescope V,I photometry of stars in the Large Magellanic Cloud Cluster NGC 1866 shows a well defined cluster main sequence down to V=25 mag, with little contamination from field or foreground stars. We use the main sequence fitting procedure to link the distance of NGC 1866 to the Hipparcos determination of the distance for the Hyades MS stars, making use of evolutionary prescriptions to allow for differences in the chemical composition. On this basis we find a true distance modulus for NGC 1866 of 18.35 +/- 0.05 mag. If the cluster is assumed to lie in the LMC plane then the LMC modulus is 0.02 mag less.

A Method for Determining the Star Formation History of a Mixed Stellar Population

We present a method to determine the star formation history of a mixed stellar population from its photometry. We perform a chi-squared minimization between the observed photometric distribution and a model photometric distribution, based on theoretical isochrones. The initial mass function, distance modulus, interstellar reddening, binary fraction and photometric errors are incorporated into the model, making it directly comparable to the data. The model is a linear combination of individual synthetic color-magnitude diagrams (CMDs), each of which represents the predicted photometric distribution of a stellar population of a given age and metallicity. We present extensive tests of the algorithm, using both simulated and actual photometric data. From a preliminary application of the algorithm to a subregion of the Large Magellanic Cloud (LMC), we find that the that the lull in star formation observed among the LMC's cluster population between 3 and 8 Gyr ago is also present in the field population. The method was designed with flexibility and generality in mind, and we make the code available for use by the astronomical community.

Spectroscopy of Star Cluster Candidates and H [CSC]ii[/CSC] Regions in NGC 6822

We present optical spectroscopy of four star clusters and six H II regions in the nearby, dwarf irregular galaxy NGC 6822. From the data, we estimate ages of 2.5 × 107, 2.0 × 108, 1.4 × 109, and 10 ± 2 × 109 yr for clusters H VI, C21, H VIII, and H VII, respectively. For clusters H VI and H VII we estimate [Fe/H] ≈ -1.46 ± 0.26 and ~-2.0, lower than Large and Small Magellanic Cloud clusters at similar ages. Mass estimates for H VI, H VII, and H VIII demonstrate that clusters with typical globular cluster masses (>104 M⊙) have formed over the lifetime of NGC 6822. For six H II regions, ionic abundances are derived for element species of N, O, S, and Ne. These show that there is a correlation of abundance with position, the highest oxygen abundance H II regions being located off the main body of the galaxy.

The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud

We present a 9 million star color-magnitude diagram (9M CMD) of the Large Magellanic Cloud (LMC) bar. The 9M CMD reveals a complex superposition of different-age and -metallicity stellar populations, with important stellar evolutionary phases occurring over 3 orders of magnitude in number density. First, we count the nonvariable red and blue supergiants and the associated Cepheid variables and measure the stellar effective temperatures defining the Cepheid instability strip. Lifetime predictions of stellar evolution theory are tested, with implications for the origin of low-luminosity Cepheids. The highly evolved asymptotic giant branch (AGB) stars in the 9M CMD have a bimodal distribution in brightness, which we interpret as discrete old populations (≳1 Gyr). The faint AGB sequence may be metal-poor and very old. Comparing the mean properties of giant branch and horizontal-branch (HB) stars in the 9M CMD with those of clusters, we identify NGC 411 and M3 as templates for the admixture of old stellar populations in the bar. However, there are several indications that the old and metal-poor field population has a red HB morphology: the RR Lyrae variables lie preferentially on the red edge of the instability strip, the AGB bump is very red, and the ratio of AGB bump stars to RR Lyrae variables is quite large. If the HB second parameter is age, the old and metal-poor field population in the bar likely formed after the oldest LMC clusters. Lifetime predictions of stellar evolution theory lead us to associate a significant fraction of the ∼1 million red HB clump giants in the 9M CMD with the same old and metal-poor population producing the RR Lyrae stars and the AGB bump. In this case, compared with the age-dependent luminosity predictions of stellar evolution theory, the red HB clump is too bright relative to the RR Lyrae stars and AGB bump. Last, we show that the surface density profile of RR Lyrae variables is fitted by an exponential, favoring a disklike rather than a spheroidal distribution. We conclude that the age of the LMC disk is probably similar to the age of the Galactic disk.

Hubble Space TelescopeObservations of the Oldest Star Clusters in the Large Magellanic Cloud

We present V, V-I color-magnitude diagrams (CMDs) for three old star clusters in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257, and Hodge 11. Our data extend ~3 mag below the main-sequence turnoff, allowing us to determine accurate relative ages and the blue straggler frequencies. Based on a differential comparison of the CMDs, any age difference between the three LMC clusters is less than 1.5 Gyr. Comparing their CMDs to those of M92 and M3, the LMC clusters, unless their published metallicities are significantly in error, are the same age as the old Galactic globulars. The similar ages to Galactic globulars are shown to be consistent with hierarchial clustering models of galaxy formation. The blue straggler frequencies are also similar to those of Galactic globular clusters. We derive a true distance modulus to the LMC of (m - M)0 = 18.46 ± 0.09 [assuming (m - M)0 = 14.61 for M92] using these three LMC clusters.

The Low End of the Initial Mass Function in Young Large Magellanic Cloud Clusters. I. The Case of R136

We report the result of a study in which we have used very deep broadband V and I Wide Field Planetary Camera 2 images of the R136 cluster in the Large Magellanic Cloud from the Hubble Space Telescope archive to sample the luminosity function below the detection limit of 2.8 M☉ previously reached. In these new deeper images, we detect stars down to a limiting magnitude of mF555W = 24.7 (1 mag deeper than previous works) and identify a population of red stars evenly distributed in the surrounding of the R136 cluster. A comparison of our color-magnitude diagram with recently computed evolutionary tracks indicates that these red objects are pre-main-sequence stars in the mass range 0.6-3 M☉. We construct the initial mass function (IMF) in the 1.35-6.5 M☉ range and find that, after correcting for incompleteness, the IMF shows a definite flattening below 2 M☉. We discuss the implications of this result for the R136 cluster and for our understanding of starburst galaxy formation and evolution in general.

Optical spectroscopy of the candidate luminous white dwarf in the young Large Magellanic Cloud cluster NGC 1818

An optical spectrum of the Elson et al. (1998) candidate luminous white dwarf in the young LMC cluster NGC1818 shows conclusively that it is not a degenerate star. A model atmosphere fit gives Teff�31,500K and log g�4.4, typical of a garden-variety main sequence B star. However, if it is a true LMC member then the star is underluminous by almost three magnitudes. Its position in the cluster colour-magnitude diagram also rules out the possibility that this is an ordinary B star. The luminosity is, however, consistent with a �0.5M⊙ post-AGB or post-EHB object, although if it has evolved via single star evolution from a high mass (7.6 9.0M⊙) progenitor then we might expect it to have a much higher mass, �0.9M⊙. Alternatively, it has evolved in a close binary. In this case the object offers no implications for the maximum mass for white dwarf progenitors, or the initial-final mass relation. Finally, we suggest that it could in fact be an evolved member of the LMC disk, and merely projected by chance onto NGC1818. Spectroscopically, though, we cannot distinguish between these evolutionary states without higher resolution (echelle) data.