Young LMC Cluster Research Articles

A black hole detected in the young massive LMC cluster NGC 1850

ABSTRACT We report on the detection of a black hole (NGC 1850 BH1) in the ∼100-Myr-old massive cluster NGC 1850 in the Large Magellanic Cloud. It is in a binary system with a main-sequence turn-off star (4.9 ± 0.4 M⊙), which is starting to fill its Roche lobe and is becoming distorted. Using 17 epochs of Very Large Telescope/Multi-Unit Spectroscopic Explorer observations, we detected radial velocity variations exceeding 300 km s−1 associated with the target star, linked to the ellipsoidal variations measured by the fourth phase of the Optical Gravitational Lensing Experiment in the optical bands. Under the assumption of a semidetached system, the simultaneous modelling of radial velocity and light curves constrains the orbital inclination of the binary to 38° ± 2°, resulting in a true mass of the unseen companion of $11.1_{-2.4}^{+2.1}\,{\rm M}_{\odot }$. This represents the first direct dynamical detection of a black hole in a young massive cluster, opening up the possibility of studying the initial mass function and the early dynamical evolution of such compact objects in high-density environments.

Chromosome maps of young LMC clusters: an additional case of coeval multiple populations

ABSTRACT Recent studies have revealed that the multiple populations (MPs) phenomenon does not occur only in ancient and massive Galactic globular clusters (GCs), but it is also observed in external galaxies, where GCs sample a wide age range with respect to the Milky Way. However, for a long time, it was unclear whether we were looking at the same phenomenon in different environments or not. The first evidence that the MPs phenomenon is the same regardless of cluster age and host galaxy came out recently, when an intermediate-age cluster from the Small Magellanic Cloud, Lindsay 1, and a Galactic GC have been directly compared. By complementing those data with new images from the Hubble Space Telescope (HST), we extend the comparison to two clusters of different ages: NGC 2121 (∼2.5 Gyr) and NGC 1783 (∼1.5 Gyr), from the Large Magellanic Cloud. We find a clear correlation between the RGB (red giant branch) width in the pseudo-colour CF275W, F343N, F438W and the age of the cluster itself, with the older cluster having larger σ(CF275W, F343N, F438W)RGB and vice versa. Unfortunately, the σ values cannot be directly linked to the N-abundance variations within the clusters before properly taking account the effect of the first dredge-up. Such HST data also allow us to explore whether multiple star formation episodes occurred within NGC 2121. The two populations are indistinguishable, with an age difference of only 6 ± 12 Myr and an initial helium spread of 0.02 or lower. This confirms our previous results, putting serious constraints on any model proposed to explain the origin of the chemical anomalies in GCs.

NGC 1866: First Spectroscopic Detection of Fast-rotating Stars in a Young LMC Cluster

Abstract High-resolution spectroscopic observations were taken of 29 extended main-sequence turnoff (eMSTO) stars in the young (∼200 Myr) Large Magellanic Cloud (LMC) cluster, NGC 1866, using the Michigan/Magellan Fiber System and MSpec spectrograph on the Magellan-Clay 6.5 m telescope. These spectra reveal the first direct detection of rapidly rotating stars whose presence has only been inferred from photometric studies. The eMSTO stars exhibit Hα emission (indicative of Be-star decretion disks), others have shallow broad Hα absorption (consistent with rotation ≳150 km s−1), or deep Hα core absorption signaling lower rotation velocities (≲150 km s−1). The spectra appear consistent with two populations of stars—one rapidly rotating, and the other, younger and slowly rotating.

Young LMC clusters: the role of red supergiants and multiple stellar populations in their integrated light and CMDs

The optical integrated spectra of three LMC young stellar clusters (NGC 1984, NGC 1994 and NGC 2011) exhibit concave continua and prominent molecular bands which deviate significantly from the predictions of single stellar population (SSP) models. In order to understand the appearance of these spectra, we create a set of young stellar population (MILES) models, which we make available to the community. We use archival International Ultraviolet Explorer integrated UV spectra to independently constrain the cluster masses and extinction, and rule out strong stochastic effects in the optical spectra. In addition, we also analyze deep colour-magnitude diagrams of the clusters to provide independent age determinations based on isochrone fitting. We explore hypotheses including age-spreads in the clusters, a top-heavy initial mass function, different SSP models and the role of red supergiant stars (RSG). We find that the strong molecular features in the optical spectra can only be reproduced by modeling an increased fraction of about 20 per cent by luminosity of RSG above what is predicted by canonical stellar evolution models. Given the uncertainties in stellar evolution at Myr ages, we cannot presently rule-out the presence of Myr age-spreads in these clusters. Our work combines different wavelengths as well as different approaches (resolved data as well as integrated spectra for the same sample) in order to reveal the complete picture. We show that each approach provides important information but in combination can we better understand the cluster stellar populations.

Ages and luminosities of young SMC/LMC star clusters and the recent star formation history of the Clouds

In this paper we discuss the age and spatial distribution of young (age$<$1Gyr) SMC and LMC clusters using data from the Magellanic Cloud Photometric Surveys. Luminosities are calculated for all age-dated clusters. Ages of 324 and 1193 populous star clusters in the Small and the Large Magellanic Cloud have been determined fitting Padova and Geneva isochrone models to their resolved color-magnitude diagrams. The clusters cover an age range between 10Myr and 1Gyr in each galaxy. For the SMC a constant distance modulus of $(m-M)_0$ = 18.90 and a metallicity of Z = 0.004 were adopted. For the LMC, we used a constant distance modulus of $(m-M)_0$ = 18.50 and a metallicity of Z = 0.008. For both galaxies, we used a variable color excess to derive the cluster ages. We find two periods of enhanced cluster formation in both galaxies at 160Myr and 630Myr (SMC) and at 125Myr and 800Myr (LMC). We present the spatially resolved recent star formation history of both Clouds based on young star clusters. The first peak may have been triggered by a close encounter between the SMC and the LMC. In both galaxies the youngest clusters reside in the supergiant shells, giant shells, the inter-shell regions, and toward regions with a high H$\alpha$ content, suggesting that their formation is related to expansion and shell-shell interaction. Most of the clusters are older than the dynamical age of the supergiant shells. No evidence for cluster dissolution was found. Computed V band luminosities show a trend for fainter magnitudes with increasing age as well as a trend for brighter magnitudes with increasing apparent cluster radii.

The distance to the LMC cluster NGC 1866; clues from the cluster Cepheid population

Recent investigations aimed at estimating the distance to t he young LMC cluster NGC 1866 have made use of Red Clump stars in the surrounding LMC field, together with em pirical and theoretical Main-Sequence fitting methods, and have found significantly different distances for the field an d the cluster, the latter being closer by in distance modulus by �(DM)�0.20 mag. In this paper we (re-)consider the Cepheid star population of NGC 1866, to try to shed some light on this discrepancy. By combining various extensive photometric datasets in B, V,I and single-epoch 2MASS JHK photometry, PL relationships for the cluster Cepheids are obtained. A comparison between the field LMC and cluster PL relationships for the reddening free Wesenheit index gives a firm determina tion of the distance between the cluster and the LMC main body (0.04 mag in distance modulus, the cluster being more distant) which, coupled to a model for the geometry of the LMC disk, provides �(DM) ranging between 0.0 and 0.11 mag. The simultaneous comparison of the PL relationships in B, V and I for the cluster and LMC field gives an estimate of the cluster r eddening, which results to be E(B V ) = 0.12 ± 0.02. This determination is higher than the canonical value of 0.06 mag used in all previous studies, but we show that it is not in contradiction with a re-analysis of independent esti mates. The adoption of the LMC extinction law recently presented by Gordon et al. (2003) does not change these results. The cluster Main Sequence fitting distance obtained with this new redd ening is DM = 18.58 ± 0.08, fully compatible with the Red Clump value of DM = 18.53 ±0.07(random) +0.02 −0.05 (systematic) and the Cepheid constraint on �(DM). Finally, we determined the distance to the cluster by using a Cepheid Wesenheit PL relationship with slope coming from LMC observations, and absolute magnitude zero point calibrated on Hipparcos parallaxes of Galactic Cepheids, in the assumption that the relationship is independent of metallicity; the resulting DM = 18.65 ± 0.10 is not an accurate estimate of the LMC distance because of possible metallicity effects but, when compared to the revised Main Sequence fitting value, it points out to a possibly weak depen dence of the Wesenheit PL relationship on the Cepheid chemical composition, at least in the period range between 2.5 and 3.5 days.

NGC 1866: A workbench for stellar evolution

The young LMC cluster NGC 1866 represents the ideal laboratory for testing stellar evolutionary models: in particular, it can be used to discriminate among classical stellar models, in which the extension of the convective regions is fixed by the classical Schwarzschild criterion, from models with overshooting, in which an ``extra-mixing'' is considered to take place beyond the classical limit of the convective zone. Addressing this subject in a recent work, Testa et al. (1999) reached the conclusion that the classical scheme for the treatment of convection represents a good and sufficient approximation for convective interiors. Using their own data, we repeat here the analysis. First we revise the procedure followed by Testa et al. (1999) to correct the data for completeness, second we calculate new stellar models with updated physical input for both evolutionary schemes, finally we present many simulations of the colour-magnitude diagrams and luminosity functions of the cluster using the ratio of the integrated luminosity function of main sequence stars to the number of giants as the normalization factor of the simulations. We also take into account several possible physical agents that could alter the color-magnitude diagram and the luminosity function: they are unresolved binary stars, dispersion in the age, stochastic effects in the initial mass function. Their effect is analyzed separately, with the conclusion that binary stars have the largest impact. The main result of this study is that the convective overshoot hypothesis (together with a suitable percentage of unresolved binaries) is really needed to fully match the whole pattern of data. The main drawback of the classical models is that they cannot reproduce the correct ratio of main sequence to post-main sequence stars.

Dynamical Evolution of Rich Star Clusters in the LMC

We combine the results of a large HST study of rich LMC clusters with state of the art N-body simulations. We summarise recent highlights from our project: the estimation of age spreads in young LMC clusters, the measurement of the binary fraction of NGC1818, and the determination of deep cluster luminosity functions (LF). We also present preliminary results from an investigation of the core radius versus age relation evident among LMC clusters.

HST and Keck Observations of a Young Globular Cluster in the Spiral Galaxy NGC 6946

Using new (Cycle 9) HST / WFPC2 data, we have studied a peculiar star forming region in the nearby Sc-type spiral NGC 6946. The region has a bubble-like shape and a diameter of about 600 pc. Near the centre is an extremely luminous young globular cluster with MV = −13.2 mag. The cluster has a compact core with a core radius of about 1.3 pc but is surrounded by an extended halo with a power-law luminosity profile similar to that observed for young LMC clusters. From Keck / HIRES high-dispersion spectroscopy we measure a velocity dispersion of 10 km s−1, leading to a dynamical mass estimate of about 1.7 × 106 solar masses. The dynamical mass estimate is comparable to the expectation from population synthesis modelling under the assumption of a Salpeter IMF extending down to 0.1 M⊙.

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.

Coalescence and Star Formation in Collapsing Flattened Systems

We report on the results of N-body simulations of star formation resulting from mergers during the collapse of dynamically cold, flattened systems of cloudlets. Such evolution is expected to occur in several models of cluster star formation. As was found previously in the case of spherical systems, the resulting star clusters have half-mass radii that are significantly smaller than the initial values for the systems. Stars that form early in the collapse have large final orbital radii and retain a strong memory of the initial ellipticity of the system. Stars that form later have smaller final radii and retain less memory. If we examine only those stars within the final half-mass radii of the models, we find that they follow a much less ellipsoidal distribution than do stars at larger radii. Their ellipticities, however, generally exceed those of young, dynamically unevolved massive star clusters in the LMC. The best comparisons with the observed ellipticities are found in models for which the initial system minor-to-major axis ratios b/a≳0.2, system kinetic-to-potential energy ratios Q≳0.1, covering factors fτ≲0.2, or initial cloud masses Mi≪MG. In the above, fτ is the fraction of the projected area of the system covered by the cloudlets, and MG is the critical mass for gravitational instability of the cloudlets, which are assumed to evolve isothermally. The ratio Mi/MG is taken to be one of the model parameters. All of these ranges of parameters tend to delay the onset of star formation until late in the collapse, when the system is less elliptical. Systems with Mi≪MG tend to produce very flat stellar initial mass functions, much flatter than observed. Unless, therefore, fτ≲0.2 results from the fragmentation of the parent cloud, systems such as the young LMC clusters are unlikely to have formed from extremely flattened or dynamically cold initial conditions, even in the presence of dissipation during star formation.

On the Nature of the Peculiar Hot Star in the Young Large Magellanic CloudCluster NGC 1818

The blue star reported in the field of the young LMC cluster NGC1818 by Elson et al. (1998) has the wrong luminosity and radius to be a "luminous white dwarf" member of the cluster. In addition, unless the effective temperature quoted by the authors is a drastic underestimate, the luminosity is much too low for it to be a cluster member in the post-AGB phase. Other possibilities, including that of binary evolution, are briefly discussed. However, the implication that the massive main sequence turnoff stars in this cluster can produce white dwarfs (instead of neutron stars) from single-star evolution needs to be reconsidered.

Mass Segregation in Young Large Magellanic Cloud Clusters. I. NGC 2157

We have carried out Wide Field Planetary Camera 2 V- and I-band imaging of the young LMC cluster NGC 2157. Construction of a color-magnitude diagram and isochrone fitting yield an age of τ = 108 yr, a reddening E(B - V) = 0.1, and a distance modulus of 18.4 mag. Our data cover the mass range 0.75 M⊙ ≤ m ≤ 5.1 M⊙. We find that the cluster mass function changes significantly from the inner regions to the outer regions, becoming steeper (larger number of low-mass stars relative to high-mass stars) at larger radii. The age of NGC 2157 is comparable to its two-body relaxation timescale only in the cluster core. The observed steepening of the mass function at larger radii is therefore most likely an initial condition of the cluster stars. Such initial conditions are predicted in models of cluster star formation in which dissipative processes act more strongly upon more massive stars.

The Age of the Large Magellanic Cloud Cluster NGC 1651

The age of NGC 1651 is 1.6 ± 0.4 billion yr. This letter shows that accurate age determinations of young LMC clusters are achievable with aperture photometry of Wide Field Planetary Camera 2 (WFPC2) images taken in a single orbit of the HST.

Moment analysis applied to LMC star clusters

Statistical moment-based ellipse fitting was performed on observations of Large Magellanic Cloud clusters, confirming that trends are evident in their position angles and ellipticities, as had been reported in the literature. Artificial cluster images with known parameters were generated, and subjected to the same analysis techniques, revealing apparent trends caused by stochastic processes. Caution should therefore be exercised in the interpretation of observational trends in young LMC clusters.

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

Dynamics of the young binary LMC cluster NGC 1850

BV CCD images and echelle spectra of 52 supergiants are used to examine the age and internal dynamics of the young binary LMC cluster NGC 1850. A color-magnitude diagram was constructed and found to contain three distinct populations of stars. Using star-subtracted, median filtered BV images, attempts were made to determine ellipticity parameters, BV luminosity profiles were constructed to project radii greater than 40. A power law model was applied to test for the presence of a tidal cutoff in the luminosity profile. Of the 52 stars with echelle spectra, a subset of 36 were used to study the cluster dynamics. A rotational signal in the radial velocities has been detected at the 93-percent confidence level, implying a rotation axis at a position angle of 100 +/- 40 deg. Values for the slope of the mass function were determined using mass luminosity relationships and different forms for the IMF. The current cluster age is similar to its central relaxation time.

Dynamics of the young LMC cluster NGC 1866

The 1.0-m telescope at Las Campanas was used to obtain V-band CCD images of the young LMC cluster NGC 1866 out to a projected radius of 100 pc. Radial velocities with a mean precision of 1.0 km/s were measured for 69 member supergiants using the echelle spectrograph on the 2.5-m telescope. The total cluster luminosity is 6.5 +/-0.3 x 10 exp 5 solar luminosities. A star-subtracted, median-filtered U frame was employed to determine photometric shape parameters. The stellar radial velocities indicate that NGC 1866 has a systemic velocity of 301.2 +/-1.0 km/s. A comparison of the derived values for NGC 1866 and the theoretical nu-0/sigma-0 - epsilon relationship indicates that the observed flattening is consistent with rotation. The quality of the fits for models with rotation is clearly superior to those without, and isotropic orbits agree best with the data in both cases.