Large Magellanic Cloud Cluster NGC Research Articles

Erratum: “Infrared Surface Brightness Fluctuations of Magellanic Star Clusters” (ApJ, 611, 270 [2004

Because their coordinates were mistyped, the Two Micron All-Sky Survey (2MASS) point source list for the Large Magellanic Cloud (LMC) cluster NGC 2002 (SWB class I), and the images and point source list for the Small Magellanic Cloud (SMC) cluster IC 1655 (SWB class II) were retrieved incorrectly. After rectifying the problem, the surface brightness fluctuation (SBF) magnitudes of the supercluster class SWB I are now 0.15 mag brighter, and those of the SWB II are 0.05 mag fainter. The colors of the two superclusters have also changed, albeit very slightly, and hence the fits of fluctuation color versus age for the supercluster set have also changed slightly. We list the new values of the fluctuation magnitudes, fluctuation colors, and fitting parameters in the tables here, which correspond to Tables 4 and 6 in the original paper.

Time dependent mixing in He-burning cores: The case of NGC 1866

We examine the helium burning phase of the convective core of intermediate mass stars, and investigate the role of the coupling of nuclear burning and mixing on the extension and duration of the blue loop phase. We compare the theoretical scenario with the distribution of stars in the colour–magnitude (CM) diagram of the Large Magellanic Cloud (LMC) cluster NGC 1866, whose densely populated clump of He-burning stars is equally populated on the blue and red side. We compare the distributions expected by adopting either a diffusive scheme within the instability regions, in which nuclear burning and mixing are self-consistently coupled, or the traditional instantaneous mixing approximation. We analyze with particular care the sensitivity of the results to: a) the e-folding distance with which the velocity of convective eddies decays outside the formal border of the convective regions fixed by the Schwarzschild criterion; b) the convective model adopted to evaluate the temperature gradient; c) the rate of reaction O. Models not including convective overshoot are also commented.

Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - I. The old Large Magellanic Cloud clusters NGC 1928, 1939 and Reticulum

We present the results of photometric measurements from images of the Large Magellanic Cloud (LMC) globular clusters NGC 1928, 1939 and Reticulum taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Exposures through the F555W and F814W filters result in high-accuracy colour-magnitude diagrams (CMDs) for these three clusters. This is the first time that CMDs for NGC 1928 and 1939 have been published. All three clusters possess CMDs with features indicating them to be > 10 Gyr old, including main-sequence turn-offs at V ∼ 23 and well-populated horizontal branches (HBs). We use the CMDs to obtain metallicity and reddening estimates for each cluster. NGC 1939 is a metal-poor cluster, with [Fe/H] = -2.10 ± 0.19, while NGC 1928 is significantly more metal rich, with [Fe/H] = -1.27 ± 0.14. The abundance of Reticulum is intermediate between the two, with [Fe/H] = -1.66 ± 0.12 - a measurement which matches well with previous estimates. All three clusters are moderately reddened, with values ranging from E(V - I) = 0.07 ± 0.02 for Reticulum and E(V - I) = 0.08 ± 0.02 for NGC 1928, to E(V - I) = 0.16 ± 0.03 for NGC 1939. After correcting the CMDs for extinction we estimate the HB morphology of each cluster. NGC 1928 and 1939 possess HBs consisting almost exclusively of stars to the blue of the instability strip, with NGC 1928 in addition showing evidence for an extended blue HB. In contrast, Reticulum has an intermediate HB morphology, with stars across the instability strip. Using a variety of dating techniques we show that these three clusters are coeval with each other and the oldest Galactic and LMC globular clusters, to within ∼2 Gyr. The census of known old LMC globular clusters therefore now numbers 15 plus the unique, younger cluster ESO121-SC03. The NGC 1939 field contains another cluster in the line of sight, NGC 1938. A CMD for this object shows it to be less than ∼400 Myr old, and it is therefore unlikely to be physically associated with NGC 1939.

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.

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.

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.

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.

The Intermediate‐Mass Stellar Population of the Large Magellanic Cloud Cluster NGC 1818 and the Universality of the Stellar Initial Mass Function

We present stellar photometry from Hubble Space Telescope images of NGC 1818, a young populous star cluster in the Large Magellanic Cloud. The cluster stars in both the core and the outlying regions are well resolved, and the photometry extends to a V magnitude of 26, corresponding approximately to a K4 V star. With the use of isochrones, we compute a stellar initial mass function (IMF) for stars from 0.85 to 9 M☉. The slope of the mass function is -1.23 ± 0.08, which is close to the Salpeter (1955) slope of -1.35. The NGC 1818 star cluster represents a star forming event intermediate between that of open clusters and of globular clusters in terms of the mass of stars formed and their spatial concentration. The products of the star forming event itself, as a diagnostic of the physical processes, indicate that star formation in NGC 1818 proceeded in a manner similar to that in events that are both less and more concentrated or rich in stars. We compare IMF slopes that have been measured from star counts in clusters and associations in Local Group galaxies, and we conclude that for young stellar clusters and associations the IMF is independent of the spatial concentration of the stars formed, the richness of stars formed, galactic characteristics including metallicity, and, at least down to 0.85 M☉, the stellar mass range.

The Large Magellanic Cloud cluster NGC 2214

Johnson {\it BV} CCD observations have been made of the young Large Magellanic Cloud cluster NGC~2214 and a nearby field using the Anglo-Australian Telescope. It has been suggested in the literature that this elliptical cluster is actually two clusters in the process of merging. No evidence is found from profile fitting or the colour--magnitude diagrams to support this contention. Completeness factors are estimated for the CCD frames. These values are used in conjunction with luminosity functions to estimate the initial mass function (IMF) for NGC~2214. A power law is assumed for the IMF, with a good fit being found for the exponent (1 + $x$) = 2.01~$\pm$ 0.09. There is some indication that the low-mass end (less than or equal to 3 Solar Masses) has a lower gradient than the high-mass end of the derived IMF. This value is in reasonable agreement with literature values for other Magellanic IMFs, and not substantially different from those of the poorly determined Galactic IMFs, suggesting the possibility of a `universal' IMF over the Magellanic Clouds and our Galaxy in the mass range $\sim 1$ to $\sim 10$ Solar Masses.

The distance to the Large Magellanic Cloud cluster NGC 1866 from its Cepheid members

The distance to the Large Magellanic Cloud cluster NGC 1866 from its Cepheid members

The Age of the Large Magellanic Cloud Cluster NGC 1953

view Abstract Citations (2) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Age of the Large Magellanic Cloud Cluster NGC 1953 Mould, J. R. ; Xystus, D. A. ; Da Costa, G. S. ; Schommer, R. A. Abstract NGC 1953 is a globular cluster in the Large Magellanic Cloud. A color-magnitude diagram (C-M) is fitted with appropriate isochrones which indicate that the age of the cluster is 250±50 million years. The C-M diagram also suggests that this cluster may contain a number of Cepheid variables. Spectroscopic observations reveal that the most luminous asymptotic giant branch star in the cluster is an S star, thus observationally confirming that the third dredge-up mechanism does occur in stars with initial masses at least as large as 3.4 Msun. The spectrum of this AGB star also shows Balmer lines in emission, indicative of mass loss, and thus it is likely that the evolution of this star on the AGB will terminate before the transition from M star to C star is completed. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173260 Bibcode: 1993ApJ...416..582M Keywords: globular clusters: individual (NGC 1953) -- Magellanic Clouds -- stars: AGB and post-AGB stars: evolution full text sources ADS | data products SIMBAD (5) NED (1)

The distance to the Large Magellanic Cloud - Constraints from Cepheids in Large Magellanic Cloud star clusters

We have used recent observational data for the Cepheids in the rich, young Large Magellanic Cloud clusters NGC 1866 and NGC 2031 to constrain the cluster distances with the mass-equivalency (ME) method. The basis of this approach is to fix the cluster distances by requiring the Cepheid evolutionary and pulsational masses to be equal. Using evolutionary models incorporating a mild amount of core and envelope over-shooting along with recent pulsational models, we derive distance moduli of 18.51 +/- 0.21 and 18.32 +/- 0.20 for NGC 1866 and NGC 2031, respectively. The quoted errors are dominated by the uncertainties in the heavy element abundances of the clusters (assumed to be 0.3 dex for both clusters), with a smaller contribution due to the apparently intrinsic spread in the masses of the Cepheids in each cluster. For the ME method, we find that {DELTA}(m - M)-0_/δZ_1_ = 0.69, where Z_1_ = log (Z/0.016). This result implies that the cluster distances can be determined to better than +/- 5% if the cluster abundances can be measured to better than about +/- 0.15 dex. The distance moduli we derive for NGC 1866 and NGC 2031 are consistent with other recent values derived for the LMC, meaning that the models used in this analysis avoid the classical evolutionary/pulsational Cepheid mass discrepancy. The results of our analysis are based on models using the Los Alamos opacities; had we used models incorporating the new Livermore opacities instead, we estimate that the cluster distance moduli would be larger by at most 0.1 mag.

The age of the Large Magellanic Cloud cluster NGC 2031

view Abstract Citations (10) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Age of the Large Magellanic Cloud Cluster NGC 2031 Mould, J. R. ; Xystus, D. A. ; Da Costa, G. S. Abstract NGC 2031 is a globular cluster in the Large Magellanic Cloud. A color- magnitude diagram is fitted with appropriate isochrones which indicate that the age of the cluster is 140 +/- 20 Myr. The cluster is similar to the well-known young cluster NGC 1866. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/172573 Bibcode: 1993ApJ...408..108M Keywords: Chronology; Globular Clusters; Magellanic Clouds; Stellar Evolution; Color-Magnitude Diagram; Stellar Spectrophotometry; Astrophysics full text sources ADS | data products SIMBAD (25) NED (1)

CCD photometry of two young Large Magellanic Cloud clusters NGC 2004 and 2100

In this paper we present results of CCD UBV photometry of stars in two young LMC clusters: NGC 2004 and 2100. A comparison of recent CCD investigations of these clusters shows large zero-point discrepancies

The young Large Magellanic Cloud clusters NGC 2004 and 2100 and their short-period variables

In this paper we present results of intensive CCD monitoring in the V-band of stars in two young LMC clusters: NGC 2004 and 2100. Each star in these clusters was observed about 10- 1 5 times per night for about 12 nights, with the aim of detecting β Cep stars in these metal-poor clusters and thereby testing the hypothesis that pulsation in these stars is driven by ionization of metals. Our results strongly support this idea, since we have failed to find any β Cep variables. The probability that the difference between the relative numbers of β Cep stars in the Galaxy and in these two clusters is due to chance is less than 0.1 per cent

The Large Magellanic Cloud clusters NGC 1835 - Photometry of the RR Lyrae stars

view Abstract Citations (26) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Large Megallanic Cloud Cluster NGC 1835: Photometry of the RR Lyrae Stars Walker, Alistair R. Abstract Photometry is presented for the RR Lyrae variables in the Large Magellanic Cloud (LMC) cluster NGC 1835. Sixteen new variables have been discovered. Due to the extreme crowding of LMC field stars, the scatter of the light curves and mean magnitudes is greater than that found for other LMC clusters studied under similar observational conditions. The derived parameters of the light curves yield E( B - V) = 0.13 and [Fe/H] = - 1.8, consistent with previous measurements. The complete sample of RR Lyraes has mean magnitudes <V> = 19.38, <B> = 19.70. If a distance modulus of 18.5 mag for the LMC is assumed, then in the mean the NGC 1835 RR Lyraes have <M_V_> = 0.48, almost identical to the average found by the same procedure for six other LMC clusters. Publication: The Astronomical Journal Pub Date: February 1993 DOI: 10.1086/116450 Bibcode: 1993AJ....105..527W Keywords: Astronomical Photometry; Light Curve; Lyra Constellation; Magellanic Clouds; Star Clusters; Variable Stars; Charge Coupled Devices; Globular Clusters; Photographic Measurement; Photographic Plates; Stellar Magnitude; Astrophysics; MAGELLANICS CLOUDS; STARS: VARIABLES: RR LYRAE full text sources ADS | data products SIMBAD (59) NED (2) CDS (1)

The young Large Magellanic Cloud cluster NGC 2214 with a single supergiant branch

The young LMC cluster NGC 2214 has been suspected to be in the stage of merging two clusters, because it shows a significantly elliptical core structure embedded in an almost spherical halo and two central subcomponents. Recently Sagar, Richtler, & de Boer [AA, 249, L5, 1991] have claimed that NGC 2214 has two distinct supergiant branches with ages of ∼60 Myr and ∼170 Myr, respectively. Based on deep UBVI CCD photometry, we suggest that NGC 2214 shows only one supergiant branch of age ∼60 Myr

The variable star population of NGC 1866

We report the results of an extensive photometric and radial velocity study of the variable stars in the Large Magellanic Cloud (LMC) cluster NGC 1866. We find:

CCD photometry of the Large Magellanic Cloud cluster NGC 1866 - Observational results

view Abstract Citations (60) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CCD Photometry of the Large Magellanic Cloud Cluster NGC 1866: Observational Results Brocato, E. ; Buonanno, R. ; Castellani, V. ; Walker, A. R. Abstract The first part of this paper is devoted to a new CCD investigation of stars in the field of the LMC cluster NGC 1866. Two main frames have been analyzed, one covering the cluster's central region and the other covering an area about 10' from the cluster's center, and data has been collected for 2207 objects. On this basis, a C-M diagram has been derived, which shows a well-developed blue sequence reaching a limiting magnitude of about V = 21 mag together with the already known sequence of the cluster's He-burning giants. A new feature in our data is represented by the evidence for a second group of less luminous giants clumping around V = 19 mag, which we interpret as field He-burning giants. The second part of the paper is devoted to a comparison of these data with theoretical calculations. On the basis of new evolutionary tracks we find that the composition Y = 0.28, Z = 0.02 already used by Becker and Mathews offers an adequate fitting of the observed range in color spanned by cluster He giants. Theoretical isochrones for this composition have been produced and are compared with the observed distribution of cluster stars. A good agreement with the observed C-M diagram is achieved for a cluster age of the order of τ = 100 Myr, whereas field stars appear to be significantly older, with ages of about 500 Myr or more. The distribution of cluster stars is studied, and both the luminosity function of the blue sequence (BS) and number of He giants relative to BS stars are exanined. We find that both these cluster characteristics can be reproduced with a Salpeter slope for the initial mass function, overcoming previous difficulties raised by suggestions of a serious disagreement between theory and observations in this cluster. The Appendix reports data for the evolutionary models and for the corresponding isochrones, covering both the H- and He-burning phases. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1989 DOI: 10.1086/191363 Bibcode: 1989ApJS...71...25B Keywords: Astronomical Photometry; Globular Clusters; Magellanic Clouds; Stellar Evolution; Charge Coupled Devices; Color-Magnitude Diagram; Giant Stars; Star Distribution; Tables (Data); Astrophysics; CLUSTERS: GLOBULAR; GALAXIES: MAGELLANIC CLOUDS; PHOTOMETRY; STARS: EVOLUTION full text sources ADS | data products SIMBAD (17) CDS (2) Related Materials (1) Catalog: 1996yCat..20710025B