65 research outputs found
COCOA Code for Creating Mock Observations of Star Cluster Models
We introduce and present results from the COCOA (Cluster simulatiOn
Comparison with ObservAtions) code that has been developed to create idealized
mock photometric observations using results from numerical simulations of star
cluster evolution. COCOA is able to present the output of realistic numerical
simulations of star clusters carried out using Monte Carlo or \textit{N}-body
codes in a way that is useful for direct comparison with photometric
observations. In this paper, we describe the COCOA code and demonstrate its
different applications by utilizing globular cluster (GC) models simulated with
the MOCCA (MOnte Carlo Cluster simulAtor) code. COCOA is used to synthetically
observe these different GC models with optical telescopes, perform PSF
photometry and subsequently produce observed colour magnitude diagrams. We also
use COCOA to compare the results from synthetic observations of a cluster model
that has the same age and metallicity as the Galactic GC NGC 2808 with
observations of the same cluster carried out with a 2.2 meter optical
telescope. We find that COCOA can effectively simulate realistic observations
and recover photometric data. COCOA has numerous scientific applications that
maybe be helpful for both theoreticians and observers that work on star
clusters. Plans for further improving and developing the code are also
discussed in this paper.Comment: 18 pages, 12 figures, accepted for publication in MNRAS. Revised
manuscript has a new title, better quality figures and many other
improvements. COCOA can be downloaded from: https://github.com/abs2k12/COCOA
(comments are welcome
The Horizontal Branch in the UV Colour Magnitude Diagrams. II. The case of M3, M13 and M79
We present a detailed comparison between far-UV/optical colour Magnitude
Diagrams obtained with high-resolution Hubble Space Telescope data and suitable
theoretical models for three Galactic Globular Clusters: M3, M13 and M79. These
systems represents a classical example of clusters in the intermediate
metallicity regime that, even sharing similar metal content and age, show
remarkably different Horizontal Branch morphologies. As a consequence, the
observed differences in the colour distributions of Horizontal Branch stars
cannot be interpreted in terms of either first (metallicity) or a second
parameter such as age. We investigate here the possible role of variations of
initial Helium abundance (Y). Thanks to the use of a proper setup of far-UV
filters, we are able to put strong constraints on the maximum Y (Y_{max})
values compatible with the data. We find differences Delta Y_{max} ~ 0.02-0.04
between the clusters with M13 showing the largest value (Y_{max} ~ 0.30) and M3
the smallest (Y_{max} ~ 0.27). In general we observe that these values are
correlated with the colour extensions of their Horizontal Branches and with the
range of the observed Na-O anti-correlations.Comment: Accepted for publication by MNRAS. 15 pages, 15 figures, 1 tabl
Spatial mixing of binary stars in multiple-population globular clusters
We present the results of a study aimed at investigating the effects of
dynamical evolution on the spatial distribution and mixing of primordial binary
stars in multiple-population globular clusters.
Multiple stellar population formation models predict that second-generation
(SG) stars form segregated in the inner regions of a more extended
first-generation (FG) cluster. Our study, based on the results of a survey of
N-body simulations, shows that the spatial mixing process for binary stars is
more complex than that of single stars since additional processes such as
binary ionization, recoil and ejection following binary-single and
binary-binary interactions play a key role in determining the spatial
distribution of the population of surviving binaries. The efficiency and
relative importance of these additional effects depends on the binary binding
energy and determines the timescale of the spatial mixing of FG and SG
binaries. Our simulations illustrate the role of ionization, recoil and
ejection combined with the effects of mass segregation driven by two-body
relaxation and show that the complex interplay of all these processes results
in a significant extension of the time needed for the complete spatial mixing
of FG and SG binaries compared to that of single stars. Clusters in which FG
and SG single stars have already reached complete spatial mixing might be
characterized by a significant radial gradient in the ratio of the FG-to-SG
binary fraction. The implications of the delayed mixing of FG and SG binaries
for the differences between the kinematics of the two populations are
discussed.Comment: 8 pages, 7 figures, 1 table; accepted for publication in MNRA
The dynamical state of the globular clusters Rup 106 and IC 4499
The dynamical evolution of globular clusters is theoretically described by a
series of well known events typical of N-body systems. Still, the
identification of observational signatures able to empirically describe the
stage of dynamical evolution of a stellar system of the density typical of a
globular cluster, represents a challenge. In this paper we study the dynamical
age of the globular clusters Rup 106 and IC 4499. To this aim, we study the
radial distribution of the Blue Straggler Stars via the A+ parameter and of the
slope of the Main Sequence Mass Function. Both tracers show that Rup 106 and IC
4499 are dynamically young clusters where dynamical friction has just started
to segregate massive stars towards the clusters' centre. In fact, we observe
that the Blue Straggler stars are more centrally concentrated in both clusters
than the reference population. On the same line, we find that in both cases the
slope of the mass function significantly decreases as a function of the
cluster-centric distances. This result provides additional support for the use
of the the radial distribution of the blue stragglers as a powerful
observationally convenient indicator of the cluster dynamical age.Comment: Accepted for publication on A&
Deep multi-telescope photometry of NGC 5466. II. The radial behaviour of the mass function slope
We use a combination of data acquired with the Advanced Camera for Survey
(ACS) on board the Hubble Space Telescope and the Large Binocular Camera
(LBC-blue) mounted on the Large Binocular Telescope, to sample the main
sequence stars of the globular cluster NGC~5466 in the mass range
. We derive the cluster's Luminosity Function in several
radial regions, from the center of the cluster out to the tidal radius. After
corrections for incompleteness and field-contamination, this has been compared
to theoretical Luminosity Functions, obtained by multiplying a simple power law
Mass Function in the form dN/dm by the derivative of the
mass-luminosity relationship of the best-fit isochrone. We find that
varies from -0.6 in the core region to -1.9 in the outer region. This fact
allows us to observationally prove that the stars in NGC 5466 have experienced
the effects of mass segregation. We compare the radial variation of
from the center out to 5 core radii (r) in NGC 5466 and the globular
cluster M10, finding that the gradient of in the first 5r is more
than a factor of 2 shallower in NGC 5466 than in M10, in line with the
differences in the clusters' relaxation timescales. NGC 5466 is dynamically
younger than M10, with two-body relaxation processes only recently starting to
shape the distribution of main sequence stars. This result fully agrees with
the conclusion obtained in our previous works on the radial distribution of
Blue Straggler Stars, further confirming that this can be used as an efficient
clock to measure the dynamical age of stellar systems.Comment: Accepted for publications on Ap
Modelling the Observed Stellar Mass Function and its Radial Variation in Galactic Globular Clusters
We measure how the slope of the stellar mass function (MF) changes
as a function of clustercentric distance in five Galactic globular clusters
and compare to predictions from direct -body star cluster
simulations. Theoretical studies predict that (which traces the
degree of mass segregation in a cluster) should steepen with time as a cluster
undergoes two-body relaxation and that the amount by which the global MF can
evolve from its initial state due to stellar escape is directly linked to
. We find that the amount of mass segregation in M10, NGC 6218, and
NGC 6981 is consistent with their dynamical ages, but only the global MF of M10
is consistent with its degree of mass segregation as well. NGC 5466 and NGC
6101 on the other hand appear to be less segregated than their dynamical ages
would indicate. Furthermore, despite the fact that the escape rate of stars in
non-segregated clusters is independent of stellar mass, both NGC 5466 and NGC
6101 have near-flat MFs. We discuss various mechanisms which could produce
non-segregated clusters with near-flat MFs, including higher mass-loss rates
and black hole retention, but argue that for some clusters (NGC 5466 and NGC
6101) explaining the present-day properties might require either a
non-universal IMF or a much more complex dynamical history.Comment: 12 pages, 9 figures, Accepted for publication in MNRA
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