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Properties of a model star cluster with t = 100 Myr and "luminous" mass M lum = 3000 M⊙, consisting of N lum = 5300 stars. The cumulative mass distribution function Ξ(m) of the IMF (see Eq. 3) and the cumulative radial distribution function Ψ(r) of the King model (see Eq. 6), displayed in panels (a) and (b), respectively, are used to sample mass, m, and radial distance, r, by means of uniform random numbers, rnd. The stellar mass distribution, ξ(log (m)) = dN/d log(m), is displayed vs. mass, m, in panel (c), overplotted with slope α0 = −1.3 and α1 = −2.3 lines of IMF (see Eq. 2). The lower and upper mass limits are defined by the isochrone. The surface number density of stars, ψ(r) = dN/dS, is plotted vs. radial distance, r, in panel (d), obeying King (1962) model profile of rc = 0.75 pc and rt = 15 pc (see Eq. 5); half-light radius r h ∼ 1.7 pc is indicated by an arrow.
Source publication
We present a program tool, SimClust, designed for Monte-Carlo modeling of star clusters. It populates the available stellar isochrones with stars according to the initial mass function and distributes stars randomly following the analytical surface number density profile. The tool is aimed at simulating realistic images of extragalactic star cluste...
Contexts in source publication
Context 1
... function (Eq. 3) is continuous between 0 and 1 (see Figure 1a). Thus, a uniform random number, rnd, obtained from long periodicity gsl rng taus ran- domizer function of the GNU Scientific Library, is attributed to Ξ(m) = rnd. ...
Context 2
... example of the resulting stellar mass distribution, ξ (log (m)) = dN/d log (m), displayed vs. mass, m, of t = 100 Myr model star cluster of "luminous" mass M lum = 3000 M ⊙ , consisting of N lum = 5300 stars, is shown in Figure 1c, over- plotted with the IMF slope α 0 = −1.3 and α 1 = −2.3 lines (see Eq. 2). ...
Context 3
... Ψ (r) function (Eq. 6) is continuous between 0 and 1 (see Figure 1b). Thus, a uniform random number, rnd, is attributed to Ψ (r) = rnd to sample the distance r values from the King model via inverse function ...
Context 4
... iteratively by the bisection method until the solution of r converges. An example of the resulting surface number density of stars, ψ(r) = dN/dS, displayed vs. radial distance, r, of the model star cluster, consisting of N lum = 5300 stars, is shown in Figure 1d, overplotted with the King (1962) model profile of r c = 0.75 pc and r t = 15 pc (see Eq. 5); half-light 7 radius r h ∼ 1.7 pc is indicated by an arrow. ...
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Citations
Context. When trying to derive the star cluster physical parameters of the M33 galaxy using broad-band unresolved ground-based photometry, previous studies mainly made use of simple stellar population models, shown in the recent years to be oversimplified.
Aims. In this study, we aim to derive the star cluster physical parameters (age, mass, and extinction; metallicity is assumed to be LMC-like for clusters with age below 1 Gyr and left free for older clusters) of this galaxy using models that take stochastic dispersion of cluster integrated colors into account.
Methods. We use three recently published M33 catalogs of cluster optical broad-band photometry in standard UBVRI and in CFHT/MegaCam u*g'r'i'z' photometric systems. We also use near-infrared JHK photometry that we derive from deep 2MASS images. We derive the cluster parameters using a method that takes into account the stochasticity problem, presented in previous papers of this series.
Results. The derived differential age distribution of the M33 cluster population is composed of a two-slope profile indicating that the number of clusters decreases when age gets older. The first slope is interpreted as the evolutionary fading phase of the cluster magnitudes, and the second slope as the cluster disruption. The threshold between these two phases occurs at ~300 Myrs, comparable to what is observed in the M31 galaxy. We also model by use of artificial clusters the ability of the cluster physical parameter derivation method to correctly derive the two-slope profile for different photometric systems tested.
This paper aims to contribute to the debate taking place nowadays on the two extreme schemes of sampling the stellar masses within star clusters, known as Optimal Sampling and Random Sampling. We propose a new method for sampling of stellar masses in star clusters which allows a continuous transition between the Optimal Sampling and the Random Sampling. We use a sample of young star clusters from the literature to calibrate the amount of stochasticity generated by the proposed method.
Context. This study is the third of a series that investigates the degeneracy and stochasticity problems present in the determination of physical parameters such as age, mass, extinction, and metallicity of partially resolved or unresolved star cluster populations situated in external galaxies when using broad-band photometry. Aims. This work tests the derivation of parameters of artificial star clusters using models with fixed and free metallicity for the WFC3+ACS photometric system. Then the method is applied to derive parameters of a sample of 203 star clusters in the Andromeda galaxy observed with the HST. Methods. Following Papers I and II, the star cluster parameters are derived using a large grid of stochastic models that are compared to the observed cluster broad-band integrated WFC3+ACS magnitudes. Results. We derive the age, mass, and extinction of the sample of M 31 star clusters with one fixed metallicity in agreement with previous studies. Using artificial tests we demonstrate the ability of the WFC3+ACS photometric system to derive the metallicity of star clusters. We show that the metallicity derived using photometry of 36 massive M 31 star clusters is in a good agreement with the metallicity previously derived using spectroscopy taken from literature.
Context. An automatic tool to derive structural parameters of semi-resolved
star clusters located in crowded stellar fields in nearby galaxies is needed
for homogeneous processing of archival frames.
Aims. We have developed a program that automatically derives the structural
parameters of star clusters and estimates errors by accounting for individual
stars and variable sky background.
Methods. Models of observed frames consist of the cluster's surface
brightness distribution, convolved with a point spread function; the stars,
represented by the same point spread function; and a smoothly variable sky
background. The cluster's model is fitted within a large radius by using the
Levenberg-Marquardt and Markov chain Monte Carlo algorithms to derive
structural parameters, the flux of the cluster, and individual fluxes of all
well-resolved stars.
Results. FitClust, a program to derive structural parameters of semi-resolved
clusters in crowded stellar fields, was developed and is available for free
use. The program was tested on simulated cluster frames, and was used to
measure clusters of the M31 galaxy in Subaru Suprime-Cam frames.
Conclusions. Accounting for bright resolved stars and variable sky background
significantly improves the accuracy of derived structural parameters of star
clusters. However, their uncertainty remains dominated by the stochastic noise
of unresolved stars.
Stochasticity of bright stars introduces uncertainty and bias into derived
structural parameters of star clusters. We have simulated a grid of cluster
$V$-band images, observed with Subaru Suprime-Cam with age, mass, and size
representing a cluster population in the M31 galaxy and derived their
structural parameters by fitting King model to the surface brightness
distribution. We have found that clusters less massive than $10^4 M_\odot$ show
significant uncertainty in their core and tidal radii for all ages, while
clusters younger than 10 Myr have their sizes systematically underestimated for
all masses. This emphasizes the importance of stochastic simulations to asses
the true uncertainty of structural parameters in studies of semi-resolved and
unresolved clusters.
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 parameter space when stochasticity is taken into account in the
cluster models. We show that, without prior knowledge on the metallicity, the
optical bands ($UBVRI$) fail to allow a correct derivation of the age, mass,
and extinction because of the strong degeneracies between models of different
metallicities. Adding near-infrared information ($UBVRI$+$JHK$) slightly helps
in improving the parameter derivation, except for the metallicity. Adding
ultraviolet data (GALEX+$UBVRI$) helps significantly in deriving these
parameters and allows constraining the metallicity when the photometric errors
have a Gaussian distribution with standard deviations 0.05 mag for $UBVRI$ and
0.15 mag for the GALEX bands.
We report the discovery of 59 globular clusters (GCs) and two candidate GCs
in a search of the halo of M31, primarily via visual inspection of CHFT/MegaCam
imagery from the Pan-Andromeda Archaeological Survey (PAndAS). The superior
quality of these data also allow us to check the classification of remote
objects in the Revised Bologna Catalogue (RBC), plus a subset of GC candidates
drawn from SDSS imaging. We identify three additional new GCs from the RBC, and
confirm the GC nature of 11 SDSS objects (8 of which appear independently in
our remote halo catalogue); the remaining 188 candidates across both lists are
either foreground stars or background galaxies. Our new catalogue represents
the first uniform census of GCs across the M31 halo - we find clusters to the
limit of the PAndAS survey area at projected radii of up to R_proj ~ 150 kpc.
Tests using artificial clusters reveal that detection incompleteness cuts in at
luminosities below M_V = -6.0; our 50% completeness limit is M_V ~ -4.1. We
construct a uniform set of PAndAS photometric measurements for all known GCs
outside R_proj = 25$ kpc, and any new GCs within this radius. With these data
we update results from Huxor et al. (2011), investigating the luminosity
function (LF), colours and effective radii of M31 GCs with a particular focus
on the remote halo. We find that the GCLF is clearly bimodal in the outer halo
(R_proj > 30 kpc), with the secondary peak at M_V ~ -5.5. We argue that the GCs
in this peak have most likely been accreted along with their host dwarf
galaxies. Notwithstanding, we also find, as in previous surveys, a substantial
number of GCs with above-average luminosity in the outer M31 halo - a
population with no clear counterpart in the Milky Way.
Using data from the Pan-Andromeda Archaeological Survey (PAndAS), we have discovered four new globular clusters (GCs) associated
with the M31 dwarf elliptical (dE) satellites NGC 147 and NGC 185. Three of these are associated with NGC 147 and one with
NGC 185. All lie beyond the main optical boundaries of the galaxies and are the most remote clusters yet known in these systems.
Radial velocities derived from low-resolution spectra are used to argue that the GCs are bound to the dwarfs and are not part
of the M31 halo population. Combining PAndAS with United Kingdom Infrared Telescope (UKIRT)/WFCAM (Wide-Field Camera) data,
we present the first homogeneous optical and near-IR photometry for the entire GC systems of these dEs. Colour–colour plots
and published colour–metallicity relations are employed to constrain GC ages and metallicities. It is demonstrated that the
clusters are in general metal poor ([Fe/H] < −1.25 dex), while the ages are more difficult to constrain. The mean (V − I)0 colours of the two GC systems are very similar to those of the GC systems of dEs in the Virgo and Fornax clusters, as well
as the extended halo GC population in M31. The new clusters bring the GC-specific frequency (SN) to ∼9 in NGC 147 and ∼5 in NGC 185, consistent with values found for dEs of similar luminosity residing in a range of environments.
The evolution of AGB stars is notoriously complex. The confrontation of AGB population models with observed stellar populations is a useful alternative to the detailed study of individual stars in efforts to converge towards a reliable evolution theory. I review here the impact of studies of star clusters on AGB models and AGB population synthesis, deliberately leaving out any more complex stellar populations. Over the last 10 years, despite much effort, the absolute uncertainties in the predictions of the light emitted by intermediate-age populations have not been reduced to a satisfactory level. Observational sample definitions, as well as the combination of the natural variance in AGB properties with small number statistics, are largely responsible for this situation. There is hope that the constraints may soon become strong enough, thanks to large unbiased surveys of star clusters, resolved colour-magnitude diagrams, and new analysis methods that can account for the stochastic nature of AGB populations in clusters.
We obtained CCD observations of the open cluster NGC 752 with the 1.8m Vatican Advanced Technology Telescope (Mt. Graham, Arizona) with a 4K CCD camera and eight intermediate-band filters of the Stromvil (Strömgren + Vilnius) system. Four 12' × 12' fields were observed, covering the central part of the cluster. The good-quality multicolor data made it possible to obtain precise estimates of distance moduli, metallicity and foreground reddening for individual stars down to the limiting magnitude, V = 17.5, enabling photometric identification of faint cluster members. The new observations provide an extension of the lower main sequence to three magnitudes beyond the previous (photographic) limit. A relatively small number of photometric members identified at fainter magnitudes seems to be indicative of actual dissolution of the cluster from the low-mass end.
