Young Large Magellanic Cloud Cluster Research Articles

Decoding the Bifurcated Red Giant Branch as a Tracer of Multiple Stellar Populations in the Young Large Magellanic Cloud Cluster NGC 2173

Abstract Multiple stellar populations (MPs) representing star-to-star light-element abundance variations are common in nearly all ancient Galactic globular clusters (GCs). Here we provide the strongest evidence yet that the populous, ∼1.7 Gyr old Large Magellanic Cloud cluster NGC 2173 also exhibits light-element abundance variations. Thus, our results suggest that NGC 2173 is the youngest cluster for which MPs have been confirmed to date. Our conclusion is based on the distinct bifurcation at the tip of its red giant branch in high-quality color–magnitude diagrams generated from Hubble Space Telescope imaging observations. Our results are further supported by a detailed analysis of “pseudo-UBI” maps, which reveal clear evidence of a bimodality in the cluster’s red giant branch color distribution. Young clusters in the Magellanic Clouds can provide critical insights into galaxy evolution histories. Our discovery of MPs in NGC 2173 suggests that ancient Galactic GCs and young massive clusters might share a common formation process.

Feedback in Forming Star Clusters: The Mass–Radius Relation and Mass Function of Molecular Clumps in the Large Magellanic Cloud

Abstract We derive the mass–radius relation and mass function of molecular clumps in the Large Magellanic Cloud (LMC) and interpret them in terms of the simple feedback model proposed by Fall et al. (FKM). Our work utilizes the dendrogram-based catalog of clumps compiled by Wong et al. from 12CO and 13CO maps of six giant molecular clouds in the LMC observed with the Atacama Large Millimeter Array. The Magellanic Clouds are the only external galaxies for which this type of analysis is possible at the necessary spatial resolution (∼1 pc). We find that the mass–radius relation and mass function of LMC clumps have power-law forms, R ∝ M α and , with indices α = 0.36 ± 0.03 and β = −1.8 ± 0.1 over the mass ranges 102 M ⊙ ≲ M ≲ 105 M ⊙ and 102 M ⊙ ≲M ≲ 104 M ⊙, respectively. With these values of α and β for the clumps (i.e., protoclusters), the predicted index for the mass function of young LMC clusters from the FKM model is β ≈ 1.7, in good agreement with the observed index. The situation portrayed here for clumps and clusters in the LMC replicates that in the Milky Way.

Multiple stellar populations in NGC 1866

We performed a comprehensive study of the stellar populations in the young Large Magellanic Cloud cluster NGC 1866, combining the analysis of its best-studied Cepheids with that of a very accurate colour–magnitude diagram (CMD) obtained from the most recentHubbleSpace Telescope photometry. We used a Bayesian method based on new PARSEC stellar evolutionary tracks with overshooting and rotation to obtain ages and initial rotation velocities of five well-studied Cepheids of the cluster. We find that four of the five Cepheids belong to an initially slowly rotating young population (of 176 ± 5 Myr), while the fifth is significantly older, either 288 ± 20 Myr for models with high initial rotational velocity (ωi ∼ 0.9), or 202 ± 5 Myr for slowly rotating models. The complementary analysis of the CMD rules out the latter solution while strongly supporting the presence of two distinct populations of ∼176 Myr and ∼288 Myr, respectively. Moreover, the observed multiple main sequences and the turn-offs indicate that the younger population is mainly made of slowly rotating stars, as is the case of the four younger Cepheids, while the older population is made mainly of initially fast rotating stars, as is the case of the fifth Cepheid. Our study reinforces the notion that some young clusters like NGC 1866 harbour multiple populations. This work also hints that the first population, i.e. the older, may inherit the angular momentum from the parent cloud while stars of the second population, i.e. the younger, do not.

An Unexpected Detection of Bifurcated Blue Straggler Sequences in the Young Globular Cluster NGC 2173∗

Abstract The bifurcated patterns in the color–magnitude diagrams of blue straggler stars (BSSs) have attracted significant attention. This type of special (but rare) pattern of two distinct blue straggler sequences is commonly interpreted as evidence that cluster core-collapse-driven stellar collisions are an efficient formation mechanism. Here, we report the detection of a bifurcated blue straggler distribution in a young Large Magellanic Cloud cluster, NGC 2173. Because of the cluster’s low central stellar number density and its young age, dynamical analysis shows that stellar collisions alone cannot explain the observed BSSs. Therefore, binary evolution is instead the most viable explanation of the origin of these BSSs. However, the reason why binary evolution would render the color–magnitude distribution of BSSs bifurcated remains unclear.

Observational hints of a real age spread in the young LMC star cluster NGC 1971

Abstract We report that the serendipitous young Large Magellanic Cloud cluster NGC 1971 exhibits an extended main-sequence turn-off (eMSTO) possibly originated mostly by a real age spread. We used CT1 Washington photometry to produce a colour–magnitude diagram (CMD) with the fiducial cluster features. From its eMSTO, we estimated an age spread of ∼170 Myr (observed age range 100–280 Myr), once observational errors, stellar binarity, overall metallicity variations and stellar rotation effects were subtracted in quadrature from the observed age width.

An analysis of the population of extended main-sequence turn-off clusters in the Large Magellanic Cloud

We combine a number of recent studies of the extended main sequence turn-off (eMSTO) phenomenon in intermediate age stellar ($1-2$ Gyr) clusters in the Large Magellanic Cloud (LMC) in order to investigate its origin. By employing the largest sample of eMSTO LMC clusters so far used, we show that cluster core radii, masses, and dynamical state are not related to the genesis of eMSTOs. Indeed, clusters in our sample have core radii, masses and age-relaxation time ratios in the range $\approx$ 2--6 pc, 3.35- 5.50 (log($M_{cls}$/$M_\odot$) and 0.2-8.0, respectively. These results imply that the eMSTO phenomenon is not caused by actual age spreads within the clusters. Furthermore, we confirm from a larger cluster sample recent results including young eMSTO LMC clusters, that the FWHM at the MSTOs correlates most strongly with cluster age, suggesting that a stellar evolutionary effect is the underlying cause.

No evidence for significant age spreads in young massive LMC clusters

Recent discoveries have put the picture of stellar clusters being simple stellar populations into question. In particular, the color-magnitude diagrams of intermediate age (1-2 Gyr) massive clusters in the Large Magellanic Cloud (LMC) show features that could be interpreted as age spreads of 100-500 Myr. If multiple generations of stars are present in these clusters then, as a consequence, young (<1 Gyr) clusters with similar properties should have age spreads of the same order. In this paper we use archival Hubble Space Telescope (HST) data of eight young massive LMC clusters (NGC 1831, NGC 1847, NGC 1850, NGC 2004, NGC 2100, NGC 2136, NGC 2157 and NGC 2249) to test this hypothesis. We analyzed the color-magnitude diagrams of these clusters and fitted their star formation history to derive upper limits of potential age spreads. We find that none of the clusters analyzed in this work shows evidence for an extended star formation history that would be consistent with the age spreads proposed for intermediate age LMC clusters. Tests with artificial single age clusters show that the fitted age dispersion of the youngest clusters is consistent with spreads that are purely induced by photometric errors. As an additional result we determined a new age of NGC 1850 of ~100 Myr, significantly higher than the commonly used value of about 30 Myr, although consistent with early HST estimates.

Data Characterization Using Artificial-Star Tests: Performance Evaluation

ABSTRACTTraditional artificial-star tests are widely applied to photometry in crowded stellar fields. However, to obtain reliable binary fractions (and their uncertainties) of remote, dense, and rich star clusters, one needs to recover huge numbers of artificial stars. Hence, this will consume much computation time for data reduction of the images to which the artificial stars must be added. In this article, we present a new method applicable to data sets characterized by stable, well-defined, point-spread functions, in which we add artificial stars to the retrieved-data catalog instead of to the raw images. Taking the young Large Magellanic Cloud cluster NGC 1818 as an example, we compare results from both methods and show that they are equivalent, while our new method saves significant computational time.

Integrated parameters of star clusters: a comparison of theory and observations

This paper presents integrated magnitudes and colours for synthetic clusters. The integrated parameters have been obtained for the whole cluster population as well as for the main-sequence (MS) population of star clusters. We have also estimated observed integrated magnitudes and colours of the MS population of galactic open clusters, Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) star clusters. It is found that the colour evolution of the MS population of star clusters is not affected by the stochastic fluctuations, however, these fluctuations significantly affect the colour evolution of the whole cluster population. The fluctuations are maximum in (V−I) colour in the age range 6.7 < log(age) < 7.5. Evolution of integrated colours of the MS population of clusters in the Milky Way, LMC and SMC, obtained in the present study is well explained by the present synthetic cluster model. The observed integrated (B−V) colours of the MS population of LMC star clusters having age ≥500 Myr seem to be distributed around the Z= 0.004 model, whereas (V−I) colours are found to be bluer than those predicted by the Z= 0.004 model. The (V−I) versus (B−V) two-colour diagram for the MS population of the Milky Way star clusters shows a fair agreement between the observations and present model, however, the diagrams for LMC and SMC clusters indicate that observed (V−I) colours are relatively bluer. Possible reasons for this anomaly have been discussed. Comparison of the synthetic (U−B) versus (B−V) relation with the observed integrated parameters of the whole cluster population of the Milky Way, LMC and SMC star clusters indicates that the majority of the bluest clusters [(B−V)0 < 0.0] follow the MS population relation. The colour evolution of young Milky Way, LMC and SMC clusters [6.5 ≤ log(age) ≤8.0] also indicates that a large number of young clusters follow the MS population relation. Therefore, in the absence of a careful modelling of stochastic effects, age determination of young star clusters by comparing their integrated colours with whole cluster synthetic colours may yield erroneous results.

ASSESSMENT OF STELLAR STRATIFICATION IN THREE YOUNG STAR CLUSTERS IN THE LARGE MAGELLANIC CLOUD

(abridged) We present a comprehensive study of stellar stratification in young star clusters in the Large Magellanic Cloud (LMC). We apply our recently developed effective radius method for the assessment of stellar stratification on imaging data obtained with the Advanced Camera for Surveys of three young LMC clusters to characterize the phenomenon and develop a comparative scheme for its assessment in such clusters. The clusters of our sample, NGC 1983, NGC 2002 and NGC 2010, are selected on the basis of their youthfulness, and their variety in appearance, structure, stellar content, and surrounding stellar ambient. Our photometry is complete for magnitudes down to m_814 ~ 23 mag, allowing the calculation of the structural parameters of the clusters, the estimation of their ages and the determination of their stellar content. Our study shows that each cluster in our sample demonstrates stellar stratification in a quite different manner and at different degree from the others. Specifically, NGC 1983 shows to be partially segregated only for the faintest stars of the cluster, NGC 2002 shows evidence of strong stellar stratification for both bright and faint stars, and NGC 2010 is found not to be segregated. For the parametrization of the phenomenon of stellar stratification and its quantitative comparison among these clusters, we propose the slope derived from the change in the effective radius over the corresponding magnitude range as indicative parameter of the degree of stratification in the clusters. A positive value of this slope indicates mass segregation in the cluster, while a negative or zero value signifies the lack of the phenomenon.

Pristine CNO Abundances from Magellanic Cloud B Stars. II. Fast Rotators in the Large Magellanic Cloud Cluster NGC 2004

We present spectroscopic abundance analyses of three main-sequence B stars in the young Large Magellanic Cloud cluster NGC 2004. All three targets have projected rotational velocities around 130 km s-1. Techniques are presented that allow the derivation of stellar parameters and chemical abundances in spite of these high v sin i values. Together with previous analyses of stars in this cluster, we find no evidence among the main-sequence stars for effects due to rotational mixing up to v sin i ≈ 130 km s-1. Unless the equatorial rotational velocities are significantly larger than the v sin i values, this finding is probably in line with theoretical expectations. NGC 2004/B30, a star of uncertain evolutionary status located in the blue Hertzsprung gap, clearly shows signs of mixing in its atmosphere. To verify the effects due to rotational mixing will therefore require homogeneous analysis of statistically significant samples of low-metallicity main-sequence B stars over a wide range of rotational velocities.

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.

Spatial Distributions of Young Large Magellanic Cloud Clusters as Tracers of a Bar Perturbation

Spatial Distributions of Young Large Magellanic Cloud Clusters as Tracers of a Bar Perturbation

The LMC Cluster NGC 1866. I. A Revised Photometric Sequence and a Reconnaissance of the Surrounding Field Population

view Abstract Citations (13) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The LMC Cluster NGC 1866. I. A Revised Photometric Sequence and a Reconnaissance of the Surrounding Field Population Walker, Alistair R. Abstract A photometric sequence consisting of 53 stars measured in the V, B, and I bands has been set up in the vicinity of the rich, young Large Magellanic Cloud cluster, NGC 1866. Many of these stars are relatively isolated and are thus suitable for use as local standard stars for photometric studies of the cluster. Color magnitude diagrams to a limit of V = 20.5 for various parts of the 13 x 13 arcmin field, centered on the cluster, show that NGC 1866 is embedded in a mix of populations which includes stars younger than the cluster itself, the dominant 0.5-3 Gyr population, plus some very old stars represented by four RR Lyrae variables. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117550 Bibcode: 1995AJ....110..638W Keywords: STARS: EVOLUTION; ISM: INDIVIDUAL: NGC 1866 full text sources ADS | data products SIMBAD (49) NED (1) CDS (1)

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 - 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.

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 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