497 research outputs found
Structural Parameters of Seven SMC Intermediate-Age and Old Star Clusters
We present structural parameters for the seven intermediate-age and old star
clusters NGC121, Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in
the Small Magellanic Cloud. We fit King profiles and Elson, Fall, and Freeman
profiles to both surface-brightness and star count data taken with the Advanced
Camera for Surveys aboard the Hubble Space Telescope. Clusters older than 1 Gyr
show a spread in cluster core radii that increases with age, while the youngest
clusters have relatively compact cores. No evidence for post core collapse
clusters was found. We find no correlation between core radius and distance
from the SMC center, although consistent with other studies of dwarf galaxies,
some relatively old and massive clusters have low densities. The oldest SMC
star cluster, the only globular NGC121, is the most elliptical object of the
studied clusters. No correlation is seen between ellipticity and distance from
the SMC center. The structures of these massive intermediate-age (1-8 Gyr) SMC
star clusters thus appear to primarily result from internal evolutionary
processes.Comment: 16 pages, 13 figure
Assessment of Stellar Stratification in Three Young Star Clusters in the Large Magellanic Cloud
(abridged) We present a comprehensive study of stellar stratification in
young star clusters in the Large Magellanic Cloud (LMC). We apply our recently
developed effective radius method for the assessment of stellar stratification
on imaging data obtained with the Advanced Camera for Surveys of three young
LMC clusters to characterize the phenomenon and develop a comparative scheme
for its assessment in such clusters. The clusters of our sample, NGC 1983, NGC
2002 and NGC 2010, are selected on the basis of their youthfulness, and their
variety in appearance, structure, stellar content, and surrounding stellar
ambient. Our photometry is complete for magnitudes down to m_814 ~ 23 mag,
allowing the calculation of the structural parameters of the clusters, the
estimation of their ages and the determination of their stellar content. Our
study shows that each cluster in our sample demonstrates stellar stratification
in a quite different manner and at different degree from the others.
Specifically, NGC 1983 shows to be partially segregated only for the faintest
stars of the cluster, NGC 2002 shows evidence of strong stellar stratification
for both bright and faint stars, and NGC 2010 is found not to be segregated.
For the parametrization of the phenomenon of stellar stratification and its
quantitative comparison among these clusters, we propose the slope derived from
the change in the effective radius over the corresponding magnitude range as
indicative parameter of the degree of stratification in the clusters. A
positive value of this slope indicates mass segregation in the cluster, while a
negative or zero value signifies the lack of the phenomenon.Comment: To appear in the Astrophysical Journal Vol. 709 (2010), pp. 263-277
Version with low-Resolution gray-scaled figures. Version with full resolution
color figures available from
http://rapidshare.com/files/328406139/Gouliermis_2010.ApJ.709.pd
Clues about the Star Formation History of the M31 Disk from WFPC2 Photometry
Over the past several years, the Hubble Space Telescope (HST) has acquired
many broad-band images of various regions in the M31 disk. I have obtained 27
such fields from the HST data archive in order to produce color-magnitude
diagrams (CMDs) of the stellar populations contained within these areas of the
disk. I have attempted to reproduce these CMDs using theoretical stellar
evolution models in conjunction with statistical tools for determining the star
formation history which best fits the observations. The wide range of
extinction values within any given field makes the data difficult to accurately
reproduce; nevertheless, I have managed to find star formation histories which
roughly reproduce the data. These statistically determined star formation
histories reveal that, like the disk of the Galaxy, the disk of M31 contains
very few old metal-poor stars. The histories also suggest that the star
formation rate of the disk as a whole has been low over the past ~1 Gyr.Comment: 60 pages, 2 tables, 37 figures, accepted for publication in MNRA
Age Determination of Six Intermediate-age SMC Star Clusters with HST/ACS
We present a photometric analysis of the star clusters Lindsay 1, Kron 3,
NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud (SMC),
observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in
the F555W and F814W filters. Our color magnitude diagrams (CMDs) extend ~3.5
mag deeper than the main-sequence turnoff points, deeper than any previous
data. Cluster ages were derived using three different isochrone models: Padova,
Teramo, and Dartmouth, which are all available in the ACS photometric system.
Fitting observed ridgelines for each cluster, we provide a homogeneous and
unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs
are best approximated by the Dartmouth isochrones for all clusters, except for
NGC419 where the Padova isochrones provided the best fit. The CMD of NGC419
shows several main-sequence turn-offs, which belong to the cluster and to the
SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC419.
Interestingly, our intermediate-age star clusters have a metallicity spread of
~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic
age-metallicity relation. We find an indication for centrally concentrated blue
straggler star candidates in NGC416, while for the other clusters these are not
present. Using the red clump magnitudes, we find that the closest cluster,
NGC419 (~50kpc), and the farthest cluster, Lindsay 38 (~67kpc), have a relative
distance of ~17kpc, which confirms the large depth of the SMC.Comment: 25 pages, 45 Figure
The Star Formation History of the Large Magellanic Cloud
We present the first-ever global, spatially-resolved reconstruction of the
star formation history (SFH) of the Large Magellanic Cloud (LMC), based on the
application of our StarFISH analysis software to the multiband photometry of
twenty million of its stars from the Magellanic Clouds Photometric Survey. The
general outlines of our results are consistent with previously published
results: following an initial burst of star formation, there was a quiescent
epoch from approximately 12 to 5 Gyr ago. Star formation then resumed and has
proceeded until the current time at an average rate of roughly 0.2 solar
masses/yr, with temporal variations at the factor-of-two level. The re-ignition
of star formation about 5 Gyr ago, in both the LMC and SMC, is suggestive of a
dramatic event at that time in the Magellanic system. Among the global
variations in the recent star formation rate are peaks at roughly 2 Gyr, 500
Myr, 100 Myr and 12 Myr. The peaks at 500 Myr and 2 Gyr are nearly coincident
with similar peaks in the SFH of the Small Magellanic Cloud, suggesting a joint
history for these galaxies extending back at least several Gyr. The chemical
enrichment history recovered from our StarFISH analysis is in broad agreement
with that inferred from the LMC's star cluster population, although our
constraints on the ancient chemical enrichment history are weak. We conclude
from the concordance between the star formation and chemical enrichment
histories of the field and cluster populations that the field and cluster star
formation modes are tightly coupled.Comment: 20 pages, with color figures. Accepted for publication in A
Targeting of Chimeric G-Alpha(I) Proteins to Specific Membrane Domains
Heterotrimeric guanine nucleotide-regulatory (G) proteins are associated with a variety of intracellular membranes and specific plasma membrane domains. In polarized epithelial LLC-PK1 cells we have shown previously that endogenous Galpha(i-2) is localized on the basolateral plasma membrane, whereas Galpha(i-3) is localized on Golgi membranes. The targeting of these highly homologous Galpha(i) proteins to distinct membrane domains was studied by the transfection and expression of chimeric Galpha(i) proteins in LLC-PK1 cells. Chimeric cDNAs were constructed from the cDNAs for Galpha(i-3) and Galpha(i-2) and introduced into a pMXX eukaryotic expression vector containing a mouse metaltothionein-I promotor. Stably transfected cell lines were produced that expressed either Galpha(i-2/3) or Galpha(i-3/2) chimeric proteins. Chimeric and endogenous Galpha(i) proteins were detected in cells using specific carboxy-terminal peptide antibodies. Immunofluorescence staining was used to localize endogenous and chimeric Galpha(i) proteins in LLC-PK1 cells. The staining of chimeric proteins was detected as an increased intensity of staining on membranes containing endogenous Galpha(i) proteins. Using confocal microscopy and image analysis we localized Galpha(i-2) to a specific sub-domain of the lateral membrane of polarized cells, the chimeric Galpha(i-3/2) protein was then shown to colocalize with endognenous Galpha(i-2) in the same lateral plasma membrane domain. The chimeric Galpha(i-2/3) protein colocalized with endogenous Galpha(i-3) on Golgi membranes in LLC-PK1 cells. These results show that chimeric Galpha(i) proteins were targeted to the same membrane domains as endogenous Galpha(i) proteins and the specificity of their membrane targeting was conferred by the carboxy-terminal end of the proteins. These data provide the first evidence for specific targeting information contained in the carboxy termini of Galpha(i) proteins, which appears to be independent of amino-terminal membrane attachment sites in these proteins
Resolving Stellar Populations outside the Local Group: MAD observations of UKS2323-326
We present a study aimed at deriving constraints on star formation at
intermediate ages from the evolved stellar populations in the dwarf irregular
galaxy UKS2323-326. These observations were also intended to demonstrate the
scientific capabilities of the multi-conjugated adaptive optics demonstrator
(MAD) implemented at the ESO Very Large Telescope as a test-bench of adaptive
optics (AO) techniques. We perform accurate, deep photometry of the field using
J and Ks band AO images of the central region of the galaxy. The near-infrared
(IR) colour-magnitude diagrams clearly show the sequences of asymptotic giant
branch (AGB) stars, red supergiants, and red giant branch (RGB) stars down to
~1 mag below the RGB tip. Optical-near-IR diagrams, obtained by combining our
data with Hubble Space Telescope observations, provide the best separation of
stars in the various evolutionary stages. The counts of AGB stars brighter than
the RGB tip allow us to estimate the star formation at intermediate ages.
Assuming a Salpeter initial mass function, we find that the star formation
episode at intermediate ages produced ~6x10^5 M_sun of stars in the observed
region.Comment: 4 pages, 4 figures, Accepted for publication in A&A Letter
The Hubble Constant from Observations of the Brightest Red Giant Stars in a Virgo-Cluster Galaxy
The Virgo and Fornax clusters of galaxies play central roles in determining
the Hubble constant H_0. A powerful and direct way of establishing distances
for elliptical galaxies is to use the luminosities of the brightest red-giant
stars (the TRGB luminosity, at M_I = -4.2). Here we report the direct
observation of the TRGB stars in a dwarf elliptical galaxy in the Virgo
cluster. We find its distance to be 15.7 +- 1.5 Megaparsecs, from which we
estimate a Hubble constant of H_0 = 77 +- 8 km/s/Mpc. Under the assumption of a
low-density Universe with the simplest cosmology, the age of the Universe is no
more than 12-13 billion years.Comment: 12 pages, LaTeX, with 2 postscript figures; in press for Nature, July
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Discourse Understanding and Factual Consistency in Abstractive Summarization
We introduce a general framework for abstractive summarization with factual consistency and distinct modeling of the narrative flow in an output summary. Our work addresses current limitations of models for abstractive summarization that often hallucinate information or generate summaries with coherence issues.
To generate abstractive summaries with fac- tual consistency and narrative flow, we propose Cooperative Generator – Discriminator Networks (Co-opNet), a novel transformer-based framework where a generator works with a discriminator architecture to compose coherent long-form summaries. We explore four different discriminator objectives which each capture a different aspect of coherence, including whether salient spans of generated abstracts are hallucinated or appear in the input context, and the likelihood of sentence adjacency in generated abstracts.
We measure the ability of Co-opNet to learn these objectives with arXiv scientific papers, using the abstracts as a proxy for gold long-form scientific article summaries. Empirical results from automatic and human evaluations demonstrate that Co-opNet learns to summarize with considerably improved global coherence compared to competitive baselines
Newly Identified Star Clusters in M33. III. Structural Parameters
We present the morphological properties of 161 star clusters in M33 using the
Advanced Camera For Surveys Wide Field Channel onboard the Hubble Space
Telescope using observations with the F606W and F814W filters. We obtain, for
the first time, ellipticities, position angles, and surface brightness profiles
for a significant number of clusters. On average, M33 clusters are more
flattened than those of the Milky Way and M31, and more similar to clusters in
the Small Magellanic Cloud. The ellipticities do not show any correlation with
age or mass, suggesting that rotation is not the main cause of elongation in
the M33 clusters. The position angles of the clusters show a bimodality with a
strong peak perpendicular to the position angle of the galaxy major axis. These
results support the notion that tidal forces are the reason for the cluster
flattening. We fit King and EFF models to the surface brightness profiles and
derive structural parameters including core radii, concentration, half-light
radii and central surface brightness for both filters. The surface brightness
profiles of a significant number of clusters show irregularities such as bumps
and dips. Young clusters (Log age < 8) are notably better fitted by models with
no radial truncation (EFF models), while older clusters show no significant
differences between King or EFF fits. M33 star clusters seem to have smaller
sizes, smaller concentrations, and smaller central surface brightness as
compared to clusters in the MW, M31, LMC and SMC. Analysis of the structural
parameters presents a age-radius relation also detected in other star cluster
systems. The overall analysis shows differences in the structural evolution
between the M33 cluster system and cluster systems in nearby galaxies. These
differences could have been caused by the strong differences in these various
environments.Comment: 30 pages, 12 figures, accepted for publication in MNRA
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