Monte Carlo simulations of metal-poor star clusters (original) (raw)
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Predicted colours for simple stellar populations
Astronomy and Astrophysics Supplement Series, 2000
This paper presents theoretical integrated colours of old stellar populations as computed adopting an homogeneous set of stellar models, covering all the major evolutionary phases of globular cluster stars. We show that adopting Reimers parameterization of mass loss rates, the choice η = 0.4 gives synthetic CM diagrams of simple stellar populations in agreement with the typical dependence of globular cluster CM diagrams on metallicity. We present theoretical U − B, B − V , V − R and V − I integrated colours for cluster metallicity in the range Z = 0.0001 to 0.02 and for age between 8 and 15 Gyr. The stochastic occurrence of luminous post-AGB stars is briefly discussed. We find that for an age t ∼ 15 Gyr the predicted integrated colours appear in good agreement with available data for both galactic and LMC old globulars. We discuss the uncertainties of integrated colours due to statistical fluctuations in the number of luminous stars, giving for each colour the expected uncertainty as a function of the cluster integrated V-magnitude. Comparison with Kurth et al. (1999) discloses that the still existing differences in the evolutionary results appear of minor relevance as far as integrated cluster colours is concerned. Finally, we show that reasonable uncertainties either in the cluster age or in the efficiency of mass loss have marginal effects on the predicted colours.
An assessment of broad-band optical colours as age indicators for star clusters
Monthly Notices of the Royal Astronomical Society, 2008
We present an empirical assessment of the use of broad-band optical colours as age indicators for unresolved extragalactic clusters and investigate stochastic sampling effects on integrated colours. We use the integrated properties of Galactic open clusters (OCs) as models for unresolved extragalactic clusters. The population synthesis code STARBURST99 (SB99) and four optical colours were used to estimate how well we can recover the ages of 62 well-studied Galactic OCs with published ages. We provide a method for estimating the ages of unresolved clusters and for reliably determining the uncertainties in the age estimates. Our results support earlier conclusions based on comparisons to synthetic clusters, namely the (U − B) colour is critical to the estimation of the ages of star-forming regions. We compare the observed optical colours with those obtained from SB99 using the published ages and get good agreement. The scatter in the (B − V) observed − (B − V) model is larger for lower luminosity clusters, perhaps due to stochastic effects.
Astronomy and Astrophysics, 2007
Aims. We analyzed HST/WFPC2 colour-magnitude diagrams (CMDs) of 15 populous Large Magellanic Cloud (LMC) stellar clusters with ages between ∼ 0.3 Gyr and ∼ 3 Gyr. These (V, B-V) CMDs are photometrically homogeneous and typically reach V∼22. Accurate and self-consistent physical parameters (age, metallicity, distance modulus and reddening) were extracted for each cluster by comparing the observed CMDs with synthetic ones. Methods. These determinations involved simultaneous statistical comparisons of the main-sequence fiducial line and the red clump position, offering objective and robust criteria to determine the best models. The models explored a regular grid in the parameter space covered by previous results found in the literature. Control experiments were used to test our approach and to quantify formal uncertainties. Results. In general, the best models show a satisfactory fit to the data, constraining well the physical parameters of each cluster. The age-metallicity relation derived by us presents a lower spread than similar results found in the literature for the same clusters. Our results are in accordance with the published ages for the oldest clusters, but reveal a possible underestimation of ages by previous authors for the youngest clusters. Our metallicity results in general agree with the ones based on spectroscopy of giant stars and with recent works involving CMD analyses. The derived distance moduli implied by the most reliable solutions, correlate with the reddening values, as expected from the non-negligible three-dimensional distribution of the clusters within the LMC. Conclusions. The inferred spatial distribution for these clusters is roughly aligned with the LMC disk, being also more scattered than recent numerical predictions, indicating that they were not formed in the LMC disk. The set of ages and metallicities homogeneously derived here can be used to calibrate integrated light studies applied to distant galaxies.
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.
Properties of young star cluster systems: the age signature from near-infrared integrated colours
Astronomy & Astrophysics, 2013
A recent JHK s study of several grand-design spiral galaxies, including NGC 2997, shows a bimodal distribution of their system of star clusters and star forming complexes in colour-magnitude and colour-colour diagrams. In a comparison with stellar population models including gas, the (J − H) vs (H − K s ) diagram reveals that embedded clusters, still immersed in their parental clouds of gas and dust, generally have a redder (H − K s ) colour than older clusters, whose gas and dust have already been ejected. This bimodal behaviour is also evident in the colour-magnitude diagram M K vs (J − K s ), where the brightest clusters split into two sequences separating younger from older clusters. In addition, the reddening-free index Q d =(H − K s ) − 0.884 (J − H) has been shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. Aims. We aim to study the behaviour of these photometric indices for star cluster systems in the Local Group. In particular, we investigate the effectiveness of the Q d index in sorting out clusters of different ages at their early evolutionary stages. In addition, the whole set of homogeneous measurements will serve as a template for analyses of the populations belonging to distant galaxies that are unresolved clusters or complexes. Methods. Surface photometry was carried out for 2MASS images of populous clusters younger than ∼ 100 Myr whose ages were available. The integrated magnitude and colours were measured to a limiting radius and combined to generate the photometric diagrams. Some clusters, particularly the embedded ones, were studied for the first time using this method. Results. The integrated magnitudes and colours extracted from the surface photometry of the most populous clusters/complexes in the Local Group shows the expected bimodal distribution in the colour-colour and colour-magnitude diagrams. In particular, we confirm the index Q d as a powerful tool for distinguishing clusters younger than about 7 Myr from older clusters.
The Astrophysical Journal, 1978
New infrared observations of globular clusters have been obtained which show that both infrared and optical colors are strongly correlated with metallicity and which provide an empirical calibration of abundance effects in composite stellar systems. Models have been constructed, based on the isochrones of Ciardullo and Demarque, with Z-values between 0.0001 and 0.04, and slope of the initial mass function s between 0 and 4. Metal-poor models with s < 2.35 (the Salpeter function) give good agreement with the empirical calibration. Metal-rich models are compared with observations of the central regions of early-type galaxies, and imply that galaxies which have-19 > M v >-23 correspond to a range in metallicity of 0.0 < [M/H] < +0.3. Models with s = 2.35 adequately fit the observations; proper accounting of metallicity effects on narrow-band infrared features does not require s < 2, as previously published models have suggested. An upper limit on s of 3.2 is determined. Subject headings: cluster: globular-galaxies: stellar content-stars: abundances
2014
This paper is the second of a series that investigates the stochasticity and degeneracy problems that hinder the derivation of the age, mass, extinction, and metallicity of unresolved star clusters in external galaxies when broad-band photometry is used. While Paper I concentrated on deriving age, mass, and extinction of star clusters for one fixed metallicity, we here derive these parameters in case when metallicity is let free to vary. The results were obtained using several different filter systems (UBVRI, UBVRIJHK, GALEX+UBVRI), which allowed to optimally reduce the different degeneracies between the cluster physical parameters. The age, mass, and extinction of a sample of artificial star clusters were derived by comparing their broad-band integrated magnitudes with the magnitudes of a large grid of cluster models with various metallicities. A large collection of artificial clusters was studied to model the different degeneracies in the age, mass, extinction, and metallicity par...
Predicted HST FOC and broad band colours for young and intermediate simple stellar populations
Astronomy and Astrophysics Supplement Series, 1999
This paper presents theoretical HST and broad band colours from population synthesis models based on an homogeneous set of stellar evolutionary tracks as computed under canonical (no overshooting) assumptions, covering the range of cluster ages from t = 8 Myr to t = 5 Gyr for three different metallicities (Z = 0.02, 0.006, and 0.001). Statistical fluctuations in the cluster population have been investigated, assessing the predicted fluctuations of the various colours as a function of the cluster integrated absolute magnitude. We show that the red leak in HST UV filters deeply affects the predicted fluxes and colours. However, we find that for F152M−F307M≤ 0.5 and for F170M−F278M≤ 0.5 (which means ages lower than 1 Gyr) the HST UV colours can still be used to infer reliable indications on the age of distant clusters. Moreover, one finds that the age calibration of these colours is scarcely affected by the amount of original helium or by the assumed IMF. On this basis, we present a calibration of the HST UV two-colours (F152M−F307M vs F170M−F278M) in terms of cluster ages for the three above quoted metallicities. We suggest the combined use of HST UV colours and IR colours (V−K in particular) to disentangle the metallicity-age effect in integrated colours of young stellar populations (t ≤ 1Gyr).
Chemical effects on the development of the colour-magnitude relation of cluster galaxies
2011
We investigate the development of the colour-magnitude re- lation (CMR) of cluster galaxies. This study is carried out using a semi- analytic model of galaxy formation and evolution coupled to a sample of simulated galaxy clusters of different masses, reinforcing the conclusions reached by Jim\'enez et al. (2009). We compare both simulated and obeserved CMRs in different colour-magnitude planes, finding a very good agreement in all cases. This indicates that model parameters are correctly tuned, giving accurate values of the main properties of galaxies for further use in our study. In the present work, we perform a statistical analysis of the relative contribution to the stellar mass and metallicity of galaxies along the CMR by the different processes involved in their formation and evolution (i.e. quiescent star formation, disc instability events and galaxy mergers). Our results show that a mix of minor and major dry mergers at low redshifts is relevant in the evolution of the most luminous galaxies in the CMR. These processes contribute with low metallicity stars to the remnant galaxies, thus increasing the galaxy masses without significantly altering their colours. These results are found for all simulated clusters, supporting the idea of the universality of the CMR in agreement with observational results.
Monthly Notices of the Royal Astronomical Society, 2011
We present SOAR/OSIRIS cross-dispersed near-infrared (NIR) integrated spectra of 12 Galactic globular clusters that are employed to test Maraston (2005, hereafter M05) NIR Evolutionary Population Synthesis (EPS) models, and to provide spectral observational constraints to calibrate future models. We measured Equivalent Widths (W λ ) of the most prominent NIR absorption features: λ 1.49µm, Mg I λ 1.58µm, Fe I/Mg I, λ 1.59µm, Si I, λ 1.71µm, Mg I, λ 2.21µm, Na I and λ 2.26µm, Ca I as well as the λ 1.62µm, λ 2.29µm, CO and λ 2.05µm, CN molecular bands. Optical W λ of G-band (4300Å), Hβ, Mg 2 , Fe I (4531Å, 527Å and 5335Å), and Na I (5897Å) were also measured. The globular clusters W λ were compared with model predictions with ages within 4 -15 Gyr, and metallicities between 1 200 Z⊙ and 2 Z⊙. Observed integrated colours (B − V , V − I and V − K s ) were also compared with models. The NIR integrated spectra among our sample appear qualitatively similar in most the absorption features. The M05 models can properly predict the optical W λ observed in globular clusters. Regarding the NIR, they do underestimate the strength of Mg I 1.49µm, but they can reproduce the observed W λ of Fe I 1.58µm, Si I 1.59µm, and CO 2.29µm, in about half of our sample. The remaining objects require the inclusion of intermediate-age populations. Thus, we suggest that the presence of C-and O-rich stars in models is important to reproduce the observed strengths of metallic lines. Another possibility is the lack of α-enhancement in the models. In the case of the optical and NIR Fe I lines, standard models and those that include blue horizontal branch stars, produce similar results. A similar trend is observed for Na I 5895Å, while in the case of the G-band, the models with blue horizontal branch do describe better the observations. For most of the sample the optical to NIR colours are well described by the M05 models.In general, M05 models can provide reliable information on the NIR stellar population of galaxies, but only when W λ and colours are taken together, in other words, W λ and continuum fluxes should be simultaneously fitted. However, the results should be taken with caution, since the models tend to predict results biased towards young ages.