699 research outputs found
Ages and metallicities of star clusters: new calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for star clusters
from very young objects, through intermediate-age ones up to the oldest known
clusters. All the selected clusters have integrated spectra in the visible
range, as well as reliable determinations of their ages and metallicities. From
these spectra equivalent widths (EWs) of KCaII, Gband(CH) and MgI metallic, and
Hdelta, Hgamma and Hbeta Balmer lines have been measured homogeneously. The
analysis of these EWs shows that the EW sums of the metallic and Balmer H
lines, separately, are good indicators of cluster age for objects younger than
10 Gyr, and that the former is also sensitive to cluster metallicity for ages
greater than 10 Gyr. We propose an iterative procedure for estimating cluster
ages by employing two new diagnostic diagrams and age calibrations based on the
above EW sums. For clusters older than 10 Gyr, we also provide a calibration to
derive their overall metal contents.Comment: 9 pages, 4 figures, accepted by A&
On a possible origin for the lack of old star clusters in the Small Magellanic Cloud
We model the dynamical interaction between the Small and Large Magellanic
Clouds and their corresponding stellar cluster populations. Our goal is to
explore whether the lack of old clusters ( Gyr) in the Small
Magellanic Cloud (SMC) can be the result of the capture of clusters by the
Large Magellanic Cloud (LMC), as well as their ejection due to the tidal
interaction between the two galaxies. For this purpose we perform a suite of
numerical simulations probing a wide range of parameters for the orbit of the
SMC about the LMC. We find that, for orbital eccentricities ,
approximately 15 per cent of the SMC clusters are captured by the LMC. In
addition, another 20 to 50 per cent of its clusters are ejected into the
intergalactic medium. In general, the clusters lost by the SMC are the less
tightly bound cluster population. The final LMC cluster distribution shows a
spatial segregation between clusters that originally belonged to the LMC and
those that were captured from the SMC. Clusters that originally belonged to the
SMC are more likely to be found in the outskirts of the LMC. Within this
scenario it is possible to interpret the difference observed between the star
field and cluster SMC Age-Metallicity Relationships for ages Gyr.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
BS196: an old star cluster far from the SMC main body
We present B and V photometry of the outlying SMC star cluster BS196 with the
4.1-m SOAR telescope. The photometry is deep (to V~25) showing ~3 mag below the
cluster turnoff point (TO) at Mv=2.5 (1.03 Msun). The cluster is located at the
SMC distance. The CMD and isochrone fittings provide a cluster age of 5.0+-0.5
Gyr, indicating that this is one of the 12 oldest clusters so far detected in
the SMC. The estimated metallicity is [Fe/H]=-1.68+-0.10. The structural
analysis gives by means of King profile fittings a core radius Rc=8.7+-1.1
arcsec (2.66+-0.14 pc) and a tidal radius Rt=69.4+-1.7 arcsec (21.2+-1.2 pc).
BS196 is rather loose with a concentration parameter c=0.90. With
Mv=-1.89+-0.39, BS196 belongs to the class of intrinsically fainter SMC
clusters, as compared to the well-known populous ones, which starts to be
explored.Comment: 8 pages, 10 figures; accepted by MNRA
Mass distribution and structural parameters of Small Magellanic Cloud star clusters
In this work we estimate, for the first time, the total masses and mass
function slopes of a sample of 29 young and intermediate-age SMC clusters from
CCD Washington photometry. We also derive age, interstellar reddening and
structural parameters for most of the studied clusters by employing a
statistical method to remove the unavoidable field star contamination. Only
these 29 clusters out of 68 originally analysed cluster candidates present
stellar overdensities and coherent distribution in their colour-magnitude
diagrams compatible with the existence of a genuine star cluster. We employed
simple stellar population models to derive general equations for estimating the
cluster mass based only on its age and integrated light in the B, V, I, C and
T1 filter. These equations were tested against mass values computed from
luminosity functions, showing an excellent agreement. The sample contains
clusters with ages between 60 Myr and 3 Gyr and masses between 300 and 3000 Mo
distributed between ~0.5 deg. and ~2 deg. from the SMC optical centre. We
determined mass function slopes for 24 clusters, of which 19 have slopes
compatible with that of Kroupa IMF (2.3 +/- 0.7), considering the
uncertainties. The remaining clusters - H86-188, H86-190, K47, K63 and NGC242 -
showed flatter MFs. Additionally, only clusters with masses lower than ~1000 Mo
and flatter MF were found within ~0.6 deg. from the SMC rotational centre.Comment: 12 pages, 19 figures. Includes another 29 full-page figures of
supplementary material. Accepted for publication in the MNRA
Gemini/GMOS photometry of intermediate-age star clusters in the Large Magellanic Cloud
We present Gemini South GMOS g,i photometry of 14 intermediate-age Large
Magellanic Cloud (LMC) star clusters, namely: NGC 2155, 2161, 2162, 2173, 2203,
2209, 2213, 2231, 2249, Hodge 6, SL 244, 505, 674, and 769, as part of a
continuing project to investigate the extended Main Sequence Turnoff (EMSTO)
phenomenon. Extensive artificial star tests were made over the observed field
of view. These tests reveal the observed behaviour of photometric errors with
magnitude and crowding. The cluster stellar density radial profiles were traced
from star counts over the extent of the observed field. We adopt clus- ter
radii and build colour-magnitude diagrams (CMDs) with cluster features clearly
identified. We used the cluster (g,g-i) CMDs to estimate ages from the matching
of theoretical isochrones. The studied LMC clusters are confirmed to be
intermediate-age clusters, which range in age 9.10 < log(t) < 9.60. NGC 2162
and NGC 2249 look like new EMSTO candidates, in addition to NGC 2209, on the
basis of having dual red clumps.Comment: MNRAS, accepte
Spectral evolution of star clusters in the Large Magellanic Cloud: I. Blue concentrated clusters in the age range 40-300 Myr
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 ~3600-6800A with a resolution of ~14A FWHM, being used
to determine cluster ages and, in connection with their spatial distribution,
to explore the LMC structure and cluster formation history. 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 (KCaII, G band (CH), MgI, Hdelta, Hgamma and Hbeta).
The derived cluster ages range from 40Myr (NGC2130 and SL237) to 300Myr
(NGC1932 and SL709), 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. 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.Comment: 13 pages, 22 figures. Accepted for publication in A&
Analysis of the chemical evolution of the Galactic disk via dynamical simulations of the open cluster system
For several decades now, open clusters have been used to study the structure and chemical evolution of the disk of our Galaxy. Due to the fact that their ages and metallicities can be determined with relatively good precision, and since they can be observed even at great distances, they are excellent tracers of the variations in the abundance of heavy chemical elements with age and position in the Galactic disk. In the present work we analyze the star formation history and the chemical evolution of the disk of the Galaxy using numerical simulations of the dynamical evolution of the system of open clusters in the Milky Way. Starting from hypotheses on the history of cluster formation and the chemical enrichment of the disk, we model the present properties of the Galactic open cluster system. The comparison of these models with the observations allows us to examine the validity of the assumed hypotheses and to improve our knowledge about the initial conditions of the chemical evolution of the Galactic disk
Proximity Eliashberg theory of electrostatic field-effect-doping in superconducting films
We calculate the effect of a static electric field on the critical
temperature of a s-wave one band superconductor in the framework of proximity
effect Eliashberg theory. In the weak electrostatic field limit the theory has
no free parameters while, in general, the only free parameter is the thickness
of the surface layer where the electric field acts. We conclude that the best
situation for increasing the critical temperature is to have a very thin film
of a superconducting material with a strong increase of electron-phonon (boson)
constant upon charging.Comment: 9 pages, 5 figure
Age Determination of Fifteen Old to Intermediate-Age Small Magellanic Cloud Star Clusters
We present CMDs in the V and I bands for fifteen star clusters in the Small
Magellanic Cloud (SMC) based on data taken with the Very Large Telescope (VLT,
Chile). We selected these clusters from our previous work, wherein we derived
cluster radial velocities and metallicities from Calcium II infrared triplet
(CaT) spectra also taken with the VLT. We discovered that the ages of six of
our clusters have been appreciably underestimated by previous studies, which
used comparatively small telescopes, graphically illustrating the need for
large apertures to obtain reliable ages of old and intermediate-age SMC star
clusters. In particular, three of these clusters, L4, L6 and L110, turn out to
be amongst the oldest SMC clusters known, with ages of 7.9 +- 1.1, 8.7 +- 1.2
and 7.6 +- 1.0 Gyr, respectively, helping to fill a possible "SMC cluster age
gap" (Glatt et al. 2008). Using the present ages and metallicities from Parisi
et al. (2009), we analyze the age distribution, age gradient and age
metallicity relation (AMR) of a sample of SMC clusters measured homogeneously.
There is a suggestion of bimodality in the age distribution but it does not
show a constant slope for the first 4 Gyr (Piatti 2011), and we find no
evidence for an age gradient. Due to the improved ages of our cluster sample,
we find that our AMR is now better represented in the intermediate/old period
than that we derived in Parisi et al. (2009), where we simply took ages
available in the literature. Additionally, clusters younger than aprox. 4 Gyr
now show better agreement with the bursting model, but we confirm that this
model is not a good representation of the AMR during the intermediate-age/old
period. A more complicated model is needed to explain the SMC chemical
evolution in that period.Comment: 76 pages, 32 figures. Accepted for publication in A
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