95 research outputs found
Intermediate Mass Stars <--> Massive Stars. A workshop around causes and consequences of differing evolutionary paths
The post-main sequence evolution of stars of intermediate or large masses is
notoriously complex. In the recent past, a number of workshops and meetings
have focused on either the Asymptotic Giant Branch of intermediate mass stars,
or the evolution of massive stars. But how well defined is the boundary between
these categories of objects defined? How would an observer proceed to classify
stars into one or the other category? How do objects near the boundary evolve,
die, and contribute to the chemical evolution of their environment? During this
3-day international workshop, 26 high quality presentations were given by
specialists in the relevant fields of astrophysics, and stimulating discussions
followed. It is technically impossible to provide an exhaustive census of the
results and ideas that emerged. In this brief article, we choose to point to
key elements of the workshop, some of which are now the topic of new
collaborations and will lead to publications elsewhere. For the sake of
brevity, we deliberately cite only the contributors to the workshop and no
external references. Many bibliographic references can be found in the original
presentations, which can be retrieved through:
http://astro.u-strasbg.fr/observatoire/obs/stars2009/stars2009.html The
programme workshop, which includes the titles of the individual contributions,
is provided as an appendix.Comment: 8 pages, 4 figure
Analysing star cluster populations with stochastic models: the HST/WFC3 sample of clusters in M83
The majority of clusters in the Universe have masses well below 10^5 Msun.
Hence their integrated fluxes and colors can be affected by the random presence
of a few bright stars introduced by stochastic sampling of the stellar mass
function. Specific methods are being developed to extend the analysis of
cluster SEDs into the low-mass regime. In this paper, we apply such a method to
observations of star clusters, in the nearby spiral galaxy M83. We reassess
ages and masses of a sample of 1242 objects for which UBVIHalpha fluxes were
obtained with the HST/WFC3 images. Synthetic clusters with known properties are
used to characterize the limitations of the method. The ensemble of color
predictions of the discrete cluster models are in good agreement with the
distribution of observed colors. We emphasize the important role of the Halpha
data in the assessment of the fraction of young objects, particularly in
breaking the age-extinction degeneracy that hampers an analysis based on UBVI
only. We find the mass distribution of the cluster sample to follow a power-law
of index -2.1 +/-0.2, and the distribution of ages a power-law of index -1.0
+/-0.2 for M > 10^3.5 Msun and ages between 10^7 and 10^9 yr. An extension of
our main method, that makes full use of the probability distributions of age
and mass of the individual clusters, is explored. It produces similar power-law
slopes and will deserve further investigation. Although the properties derived
for individual clusters significantly differ from those obtained with
traditional, non-stochastic models in ~30% of the objects, the first order
aspect of the age and mass distributions are similar to those obtained
previously for this M83 sample in the range of overlap of the studies. We
extend the power-law description to lower masses with better mass and age
resolution and without most of the artifacts produced by the classical method.Comment: accepted for publication in ApJ, 29 pages, 20 figure
The Next Generation Virgo Cluster Survey. VIII. The Spatial Distribution of Globular Clusters in the Virgo Cluster
We report on a large-scale study of the distribution of globular clusters
(GCs) throughout the Virgo cluster, based on photometry from the Next
Generation Virgo Cluster Survey, a large imaging survey covering Virgo's
primary subclusters to their virial radii. Using the g', (g'-i')
color-magnitude diagram of unresolved and marginally-resolved sources, we
constructed 2-D maps of the GC distribution. We present the clearest evidence
to date showing the difference in concentration between red and blue GCs over
the extent of the cluster, where the red (metal-rich) GCs are largely located
around the massive early-type galaxies, whilst the blue (metal-poor) GCs have a
more extended spatial distribution, with significant populations present beyond
83' (215 kpc) along the major axes of M49 and M87. The GC distribution around
M87 and M49 shows remarkable agreement with the shape, ellipticity and boxiness
of the diffuse light surrounding both galaxies. We find evidence for spatial
enhancements of GCs surrounding M87 that may be indicative of recent
interactions or an ongoing merger history. We compare the GC map to the
locations of Virgo galaxies and the intracluster X-ray gas, and find good
agreement between these baryonic structures. The Virgo cluster contains a total
population of 6730014400 GCs, of which 35% are located in M87 and M49
alone. We compute a cluster-wide specific frequency S_N,CL=,
including Virgo's diffuse light. The GC-to-baryonic mass fraction is
e_b=and the GC-to-total cluster mass formation
efficiency is e_t=, values slightly lower than, but
consistent with, those derived for individual galactic halos. Our results show
that the production of the complex structures in the unrelaxed Virgo cluster
core (including the diffuse intracluster light) is an ongoing
process.(abridged)Comment: 23 pages, 17 figures. Accepted for publication in the Astrophysical
Journal. Figure 1 has reduced resolution. Revised version with updated
references, corrected typos -- no changes to result
The mass-to-light ratio of rich star clusters
We point out a strong time-evolution of the mass-to-light conversion factor
eta commonly used to estimate masses of unresolved star clusters from observed
cluster spectro-photometric measures. We present a series of gas-dynamical
models coupled with the Cambridge stellar evolution tracks to compute
line-of-sight velocity dispersions and half-light radii weighted by the
luminosity. We explore a range of initial conditions, varying in turn the
cluster mass and/or density, and the stellar population's IMF. We find that
eta, and hence the estimated cluster mass, may increase by factors as large as
3 over time-scales of 50 million years. We apply these results to an hypothetic
cluster mass distribution function (d.f.) and show that the d.f. shape may be
strongly affected at the low-mass end by this effect. Fitting truncated
isothermal (Michie-King) models to the projected light profile leads to
over-estimates of the concentration parameter c of delta c ~ 0.3 compared to
the same functional fit applied to the projected mass density.Comment: 6 pages, 2 figures, to appear in the proceedings of the "Young
massive star clusters", Granada, Spain, September 200
The Next Generation Virgo Cluster Survey. XX. RedGOLD Background Galaxy Cluster Detections
We build a background cluster candidate catalog from the Next Generation Virgo Cluster Survey (NGVS) using our detection algorithm RedGOLD. The NGVS covers 104 deg^2 of the Virgo cluster in the u*, g, r, i, z-bandpasses to a depth of g ~ 25.7 mag (5Ï). Part of the survey was not covered or has shallow observations in the r band. We build two cluster catalogs: one using all bandpasses, for the fields with deep r-band observations (~20 deg^2), and the other using four bandpasses (u*, g, i, z) for the entire NGVS area. Based on our previous CanadaâFranceâHawaii Telescope Legacy Survey W1 studies, we estimate that both of our catalogs are ~100% (~70%) complete and ~80% pure, at z †0.6 (z âŸ1), for galaxy clusters with masses of M âł 10^(14) Mâ. We show that when using four bandpasses, though the photometric redshift accuracy is lower, RedGOLD detects massive galaxy clusters up to z ~ 1 with completeness and purity similar to the five-band case. This is achieved when taking into account the bias in the richness estimation, which is ~40% lower at 0.5 †z 1.4 Ă 10^(14) Mâ and 0.08 < z < 0.5. Because of our different cluster richness limits and the NGVS depth, our catalogs reach lower masses than the published redMaPPer cluster catalog over the area, and we recover ~90%â100% of its detections
The Next Generation Virgo Cluster Survey - Infrared (NGVS-IR): I. A new Near-UV/Optical/Near-IR Globular Cluster selection tool
The NGVS-IR project (Next Generation Virgo Survey - Infrared) is a contiguous
near-infrared imaging survey of the Virgo cluster of galaxies. It complements
the optical wide-field survey of Virgo (NGVS). The current state of NGVS-IR
consists of Ks-band imaging of 4 deg^2 centered on M87, and J and Ks-band
imaging of 16 deg^2 covering the region between M49 and M87. In this paper, we
present the observations of the central 4 deg^2 centered on Virgo's core
region. The data were acquired with WIRCam on the Canada-France-Hawaii
Telescope and the total integration time was 41 hours distributed in 34
contiguous tiles. A survey-specific strategy was designed to account for
extended galaxies while still measuring accurate sky brightness within the
survey area. The average 5\sigma limiting magnitude is Ks=24.4 AB mag and the
50% completeness limit is Ks=23.75 AB mag for point source detections, when
using only images with better than 0.7" seeing (median seeing 0.54"). Star
clusters are marginally resolved in these image stacks, and Virgo galaxies with
\mu_Ks=24.4 AB mag arcsec^-2 are detected. Combining the Ks data with optical
and ultraviolet data, we build the uiK color-color diagram which allows a very
clean color-based selection of globular clusters in Virgo. This diagnostic plot
will provide reliable globular cluster candidates for spectroscopic follow-up
campaigns needed to continue the exploration of Virgo's photometric and
kinematic sub-structures, and will help the design of future searches for
globular clusters in extragalactic systems. Equipped with this powerful new
tool, future NGVS-IR investigations based on the uiK diagram will address the
mapping and analysis of extended structures and compact stellar systems in and
around Virgo galaxies.Comment: 23 pages, 18 figures. Accepted for publication in ApJ
XSL: The X-Shooter Spectral Library
We are building a new spectral library with the X-Shooter instrument on ESO's
VLT: XSL, the X-Shooter Spectral Library. We present our progress in building
XSL, which covers the wavelength range from the near-UV to the near-IR with a
resolution of R ~ 10000. At the time of writing we have collected spectra for
nearly 240 stars. An important feature of XSL is that we have already collected
spectra of more than 100 Asymptotic Giant Branch stars in the Galaxy and the
Magellanic Clouds.Comment: 8 pages, 4 figures, to be published in the proceedings of the
"GREAT-ESF Stellar Atmospheres in the Gaia Era Workshop", Brussels, Belgium,
23-24 June, 201
The properties of bright globular clusters, ultra-compact dwarfs and dwarf nuclei in the Virgo core: hints on origin of ultra-compact dwarf galaxies (UCDs)
Based on the data from the Next Generation Virgo cluster Survey (NGVS), we statistically study the photometric properties of globular clusters (GCs), ultra-compact dwarfs (UCDs) and dwarf nuclei in the Virgo core (M87) region. We found an obvious negative color (g - z) gradient in GC system associate with M87, i.e. GCs in the outer regions are bluer. However, such color gradient does not exist in UCD system, neither in dwarf nuclei system around M87. In addition, we found that many UCDs are surrounded by extended, low surface brightness envelopes. The dwarf nuclei and UCDs show different spatial distributions from GCs, with dwarf nuclei and UCDs (especially for the UCDs with visible envelopes) lying at larger distances to the Virgo center. These results support the view that UCDs (at least for a fraction of UCDs) are more tied to dwarf nuclei than to GCs
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