Red Giant Stars in the Large Magellanic Cloud Clusters (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 2008
We present integrated JHK s 2MASS photometry and a compilation of integratedlight optical photoelectric measurements for 84 star clusters in the Magellanic Clouds. These clusters range in age from ≈ 200 Myr to > 10 Gyr, and have [Fe/H] values from −2.2 to −0.1 dex. We find a spread in the intrinsic colours of clusters with similar ages and metallicities, at least some of which is due to stochastic fluctuations in the number of bright stars residing in low-mass clusters. We use 54 clusters with the most reliable age and metallicity estimates as test particles to evaluate the performance of four widely used SSP models in the optical/NIR colour-colour space. All models reproduce the reddening-corrected colours of the old ( 10 Gyr) globular clusters quite well, but model performance varies at younger ages. In order to account for the effects of stochastic fluctuations in individual clusters, we provide composite B − V , B − J, V − J, V − K s and J − K s colours for Magellanic Cloud clusters in several different age intervals. The accumulated mass for most composite clusters are higher than that needed to keep luminosity variations due to stochastic fluctuations below the 10% level. The colours of the composite clusters are clearly distinct in optical-NIR colour-colour space for the following intervals of age: > 10 Gyr, 2 − 9 Gyr, 1 − 2 Gyr, and 200 Myr−1 Gyr. This suggests that a combination of optical plus NIR colours can be used to differentiate clusters of different age and metallicity.
The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud
The Astronomical Journal, 2000
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 three orders of magnitude in number density. First, we count the non-variable red and blue supergiants, 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 to 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 Lyraes 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 Lyraes and the AGB-bump. In this case, compared to the age-dependent luminosity predictions of stellar evolution theory, the red HB clump is too bright relative to the RR Lyraes and AGB-bump. Last, we show that the surface density profile of RR Lyraes is fit by an exponential, favoring a disk-like rather than spheroidal distribution. We conclude that the age of the LMC disk is probably similar to the age of the Galactic disk.
Astronomy & Astrophysics, 2014
Aims. To increase the number of studied star clusters (SCs) of the Large Magellanic Cloud (LMC), we present flux-calibrated integrated spectra in the optical range (λ = 3700-6800 Å) for six poorly studied LMC SCs of IVA type. This type corresponds to the age range between 200 and 400 Myr. We also aim at creating a new template spectrum representative of this age range at the metallicity level of the LMC. Methods. Foreground reddening E(B − V) values and ages are derived by applying the template matching method that consists of comparing the line strengths and continuum distribution of the cluster spectra with those of template cluster spectra with known properties. The equivalent width (EW) of the Balmer lines and the diagnostic diagrams involving the sum of EWs of selected spectral lines were also employed as age indicators. Results. For the first time, we provide estimates of the clusters' reddenings and ages. As expected, all the clusters appear to be of nearly the same age, their mean value being (400 ± 100) Myr, while the resulting mean E(B − V) values range between 0.00 and 0.10 mag. Conclusions. The present cluster sample complements previous ones in an effort to gather a spectral library with several clusters per age bin. By averaging the reddening-corrected integrated spectra, weighted by their signal-to-noise ratios (S/N), a new high S/N template spectrum of 400 Myr has been created.
Publications of the Astronomical Society of the Pacific, 2016
We present flux-calibrated integrated spectra in the optical range (3700-6800 A) obtained at Complejo Astronómico El Leoncito (CASLEO, Argentina) for a sample of 10 concentrated star clusters belonging to the Large Magellanic Cloud (LMC). No previous data exist for two of these objects (SL 142 and SL 624), while most of the remaining clusters have been only poorly studied. We derive simultaneously foreground E(B − V) reddening values and ages for the cluster sample by comparing their integrated spectra with template LMC cluster spectra and with two different sets of simple stellar population models. Cluster reddening values and ages are also derived from both available interstellar extinction maps and by using diagnostic diagrams involving the sum of equivalent widths of some selected spectral features and their calibrations with age, respectively. For the studied sample, we derive ages between 1 Myr and 240 Myr. In an effort to create a spectral library at the LMC metallicity level with several clusters per age range, the cluster sample here presented stands out as a useful complement to previous ones.
Astronomical Journal, 2007
We present results from a study of the distances and distribution of a sample of intermediate-age clusters in the Large Magellanic Cloud. Using deep nearinfrared photometry obtained with ISPI on the CTIO 4m, we have measured the apparent K-band magnitude of the core helium burning red clump stars in 17 LMC clusters. We combine cluster ages and metallicities with the work of Grocholski & Sarajedini to predict each cluster's absolute K-band red clump magnitude, and thereby calculate absolute cluster distances. An analysis of these data shows that the cluster distribution is in good agreement with the thick, inclined disk geometry of the LMC, as defined by its field stars. We also find that the old globular clusters follow the same distribution, suggesting that the
The Astronomical Journal, 1999
A comparative analysis of Washington color-magnitude diagrams (CMDs) for 14 star clusters and respective surrounding fields in the Large Magellanic Cloud (LMC) outer disk is presented. Each CCD frame including field and respective cluster covers an area of 1852 ′ . The stellar population sampled is of intermediate age and metallicity. CMD radial analysis involving star count ratios, morphology and integrated light properties are carried out. Luminosity functions (LFs) are also presented. Two main results are: (i) Within the range 4 < R(kpc) < 8, the distance from the LMC center is well correlated with the average age in the sense that inner fields are younger and; (ii) Beyond ≈8kpc the outer fields do not show evidence of a significant intermediate-age component in their stellar populations, as inferred from red giant clump star counts.
Spectral evolution of star clusters in the Large Magellanic Cloud
Astronomy & Astrophysics, 2006
Aims. Integrated spectroscopy of a sample of 17 blue concentrated Large Magellanic Cloud (LMC) clusters is presented and its spectral evolution studied. The spectra span the range ≈(3600−6800) Å with a resolution of ≈14 Å FWHM, being used to determine cluster ages and, in connection with their spatial distribution, to explore the LMC structure and cluster formation history. Methods. Cluster reddening values were estimated by interpolation, using the available extinction maps. We used two methods to derive cluster ages: (i) template matching, in which line strengths and continuum distribution of the cluster spectra were compared and matched to those of template clusters with known astrophysical properties, and (ii) equivalent width (EW) method, in which new age/metallicity calibrations were used together with diagnostic diagrams involving the sum of EWs of selected spectral lines (K Ca II, G band (CH), Mg I, Hδ, Hγ and Hβ). Results. The derived cluster ages range from 40 Myr (NGC 2130 and SL 237) to 300 Myr (NGC 1932 and SL 709), a good agreement between the results of the two methods being obtained. Combining the present sample with additional ones indicates that cluster deprojected distances from the LMC center are related to age in the sense that inner clusters tend to be younger. Conclusions. Spectral libraries of star clusters are useful datasets for spectral classifications and extraction of parameter information for target star clusters and galaxies. The present cluster sample complements previous ones, in an effort to gather a spectral library with several clusters per age bin.
Infrared Surface Brightness Fluctuations of Magellanic Star Clusters
The Astrophysical Journal, 2004
We present surface brightness fluctuations (SBFs) in the near-IR for 191 Magellanic star clusters available in the Second Incremental and All Sky Data releases of the Two Micron All Sky Survey (2MASS) and compare them with SBFs of Fornax Cluster galaxies and with predictions from stellar population models as well. We also construct color-magnitude diagrams (CMDs) for these clusters using the 2MASS Point Source Catalog (PSC). Our goals are twofold. The first is to provide an empirical calibration of near-IR SBFs, given that existing stellar population synthesis models are particularly discrepant in the near-IR. Second, whereas most previous SBF studies have focused on old, metal-rich populations, this is the first application to a system with such a wide range of ages ($10 6 to more than 10 10 yr, i.e., 4 orders of magnitude), at the same time that the clusters have a very narrow range of metallicities (Z $ 0:0006 0:01, i.e., 1 order of magnitude only). Since stellar population synthesis models predict a more complex sensitivity of SBFs to metallicity and age in the near-IR than in the optical, this analysis offers a unique way of disentangling the effects of age and metallicity. We find a satisfactory agreement between models and data. We also confirm that near-IR fluctuations and fluctuation colors are mostly driven by age in the Magellanic cluster populations and that in this respect they constitute a sequence in which the Fornax Cluster galaxies fit adequately. Fluctuations are powered by red supergiants with high-mass precursors in young populations and by intermediate-mass stars populating the asymptotic giant branch in intermediateage populations. For old populations, the trend with age of both fluctuation magnitudes and colors can be explained straightforwardly by evolution in the structure and morphology of the red giant branch. Moreover, fluctuation colors display a tendency to redden with age that can be fitted by a straight line. For the star clusters only, (H ÀK s) ¼ (0:21 AE 0:03) log (age) À (1:29 AE 0:22); once galaxies are included, (H ÀK s) ¼ (0:20 AE 0:02) log (age) À (1:25 AE 0:16). Finally, we use for the first time a Poissonian approach to establish the error bars of fluctuation measurements, instead of the customary Monte Carlo simulations.
2012
The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep, high-resolution images of the intermediate-age star cluster NGC 1846 in the Large Magellanic Cloud. We present new color-magnitude diagrams (CMDs) based on F435W, F555W, and F814W imaging. We test the previously observed broad main sequence turnoff region for ‘contamination ’ by field stars and (evolved) binary star systems. We find that while these impact the number of objects in this region, none can fully account for the large color spread. Our results therefore solidify the recent finding that stars in the main sequence turnoff region of this cluster have a large spread in color which is unrelated to measurement errors or contamination by field stars, and likely due to a ∼ 300 Myr range in the ages of cluster stars. An unbiased estimate of the stellar density distribution across the main sequence turnoff region shows that the spread is fairly continuous rather than strongly bimodal,...
Young star clusters immersed in intermediate-age fields in the Small Magellanic Cloud
Monthly Notices of the Royal Astronomical Society, 2007
We present CCD photometry in the Washington C and T 1 filters for six star clusters (B 34, NGC 256, NGC 265, NGC 294, IC 1611 and NGC 376) in the Small Magellanic Cloud (SMC) and their surrounding fields. The resultant colour-magnitude diagrams (CMDs) extend from T 1 ∼ 14 to as faint as T 1 ∼ 22 revealing the main-sequence turnoffs of the clusters. Adopting a metallicity of Z = 0.004, we compare our cluster photometry with theoretical isochrones in the Washington system in order to derive ages. To facilitate age determination of the surrounding fields, we use the magnitude difference between the helium-burning red clump stars and the main-sequence turnoff. Finally, we estimate mean metallicities for the field stars by comparing the location of the field red giant branch with standard giant branches for Galactic globular clusters of known abundance, corrected for age effects. Combining these results with our previous work, we find a clear trend of younger clusters being located closer to the centre of the SMC. In addition, there is a tendency for the mean metallicity and its dispersion to be greater inside 4 • of the SMC's centre as compared to outside this radius. As far as the properties of the field stars are concerned, we find little correlation between the ages of the clusters and those of the field stars against which they are projected. Clearly, more work needs to be done to clarify these trends.