855 research outputs found
The ACS LCID Project: data reduction strategy
During Cycle 14 a total of 113 HST orbits were secured to observe five
isolated dwarf galaxies, namely Tucana, LGS3, LeoA, IC1613, and Cetus. The aim
of the project is a full characterization of the stellar content of these
galaxies, in term of their SFH, radial distributions, halo populations and
variable stars. Deep (V~29) F475W, F814W data allowed us to fully sample all
the evolutionary phases from the tip of the Red Giant Branch (RGB) to well
below the old Main Sequence Turnoff (MSTO). Here we describe the observational
design, and the reduction and calibration strategy adopted. A comparison of the
results obtained using two different packages, ALLFRAME and Dolphot, is
presented.Comment: 2 pages, 1 figure. To appear in the Proceedings of IAU Symposium 241:
"Stellar Populations as Building Blocks of Galaxies", 10-16 December, 2006 at
La Palma, Canary Islands, Spai
The ACS LCID project VII: the blue stragglers population in the isolated dSph galaxies Cetus and Tucana
We present the first investigation of the Blue Straggler star (BSS)
population in two isolated dwarf spheroidal galaxies of the Local Group, Cetus
and Tucana. Deep HST/ACS photometry allowed us to identify samples of 940 and
1214 candidates, respectively. The analysis of the star formation histories of
the two galaxies suggests that both host a population of BSSs. Specifically, if
the BSS candidates are interpreted as young main sequence stars, they do not
conform to their galaxy's age-metallicity relationship. The analysis of the
luminosity function and the radial distributions support this conclusion, and
suggest a non-collisional mechanism for the BSS formation, from the evolution
of primordial binaries. This scenario is also supported by the results of new
dynamical simulations presented here. Both galaxies coincide with the
relationship between the BSS frequency and the absolute visual magnitude Mv
found by Momany et al (2007). If this relationship is confirmed by larger
sample, then it could be a valuable tool to discriminate between the presence
of BSSs and galaxies hosting truly young populations.Comment: Accepted for publication on ApJ. 15 pages, 3 tables, 13 figures. A
version with high resolution figure can be downloaded from
http://rialto.ll.iac.es/proyecto/LCID/?p=publication
Variable Stars in the Cetus dSph Galaxy: Population Gradients and Connections with the Star Formation History
We investigate the variable star content of the isolated, Local Group, dwarf
spheroidal galaxy (dSph) Cetus. Multi-epoch, wide-field images collected with
the VLT/VIMOS camera allowed us to detect 638 variable stars (630 RR Lyrae
stars and 8 Anomalous Cepheids), 475 of which are new detections. We present a
full catalogue of periods, amplitudes, and mean magnitudes. Motivated by the
recent discovery that the pulsational properties of the RR Lyrae stars in the
Tucana dSph revealed the presence of a metallicity gradient within the oldest
(>10 Gyr old) stellar populations, we investigated the possibility of an
analogous effect in Cetus. We found that, despite the obvious radial gradient
in the Horizontal Branch (HB) and Red Giant Branch (RGB) morphologies, both
becoming bluer on average for increasing distance from the center of Cetus, the
properties of the RR Lyrae stars are homogeneous within the investigated area
(out to r~15'), with no significant evidence of a radial gradient. We discuss
this in connection with the star formation history (SFH) previously derived for
the two galaxies. The observed differences between these two systems show that
even systems this small show a variety of early evolutionary histories. These
differences could be due to different merger or accretion histories.Comment: Accepted for publication on MNRAS. The complete set of light curves
and finding charts, together with the full table of the pulsational
properties of all variable stars will be available in the on-line edition of
the pape
Distance and reddening of the Local Group dwarf irregular galaxy NGC 6822
On the basis of a new photometric analysis of the Local Group dwarf irregular
galaxy NCG 6822 based on observations obtained with the Advanced Camera for
Surveys onboard the Hubble Space Telescope, we have obtained a new estimate of
the extinction of two fields located in the southeast region of the galaxy.
Because of significant differences in the distance estimates to NGC 6822
available in literature, we decided to provide an independent determination of
the distance to this galaxy based on an updated and self-consistent theoretical
calibration of the tip of the red giant branch brightness. As a result we newly
determined the distance to NGC 6822 to be equal to , and compared our measurement with the most recent determinations of this
distance.Comment: 5 pages, 5 figures, Astronomy & Astrophysics (Research Note), in
press (proof correction included
Artificial Intelligence and Deep Learning-Based System Design for Diabetic Retinopathy Classification
One of the biggest causes of avoidable blindness throughout the world is diabetic retinopathy (DR). There is a significant unmet need to test all diabetes patients for DR, and many instances of DR go undetected and untreated. In order to automate DR screening, this research aimed to create reliable diagnostic technologies. In order to reduce the pace of vision loss, it is important to refer eyes suspected of having DR to an ophthalmologist for further assessment and treatment. The primary goal of this research is to improve the classification accuracy for Diabetic Retinopathy (DR). In this script, we present a new neural network model for DR forecasting. The suggested model's accuracy in identifying DR phases was measured against that of regular and ensemble-based models. Various benchmark datasets, including MESSIDOR, IDRID, and APTOS, are used in the studies. The suggested DRPNN algorithm outperformed the competition in experiments assessed using industry-standard criteria
The origin of the LMC stellar bar: clues from the SFH of the bar and inner disk
We discuss the origin of the LMC stellar bar by comparing the star formation
histories (SFH) obtained from deep color-magnitude diagrams (CMDs) in the bar
and in a number of fields in different directions within the inner disk. The
CMDs, reaching the oldest main sequence turnoffs in these very crowded fields,
have been obtained with VIMOS on the VLT in service mode, under very good
seeing conditions. We show that the SFHs of all fields share the same patterns,
with consistent variations of the star formation rate as a function of time in
all of them. We therefore conclude that no specific event of star formation can
be identified with the formation of the LMC bar, which instead likely formed
from a redistribution of disk material that occurred when the LMC disk became
bar unstable, and shared a common SFH with the inner disk thereafter. The
strong similarity between the SFH of the center and edge of the bar rules out
significant spatial variations of the SFH across the bar, which are predicted
by scenarios of classic bar formation through buckling mechanisms.Comment: MNRAS Letters, accepte
On the central helium-burning variable stars of the LeoI dwarf spheroidal galaxy
We present a study of short period, central helium-burning variable stars in
the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and
51 Cepheids. So far, this is the largest sample of Cepheids and the largest
Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with
other Local Group dwarf spheroidals, Carina and Fornax, shows that the period
distribution of RR Lyrae stars is quite similar, suggesting similar properties
of the parent populations, whereas the Cepheid period distribution in LeoI
peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a
broader range, from 0.5 to 1.78d.
Evolutionary and pulsation predictions indicate, assuming a mean metallicity
peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids
traces a unique mix of Anomalous Cepheids (blue extent of the red--clump,
partially electron degenerate central helium-burning stars) and short-period
classical Cepheids (blue-loop, quiescent central helium-burning stars). Current
evolutionary prescriptions also indicate that the transition mass between the
two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars
metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different
implications of the current findings on the star formation history of LeoI.Comment: 5 Pages, 4 Figures, ApJ letter, accepte
The ACS LCID project IV: detection of the RGB bump in isolated galaxies of the Local Group
We report the detection and analysis of the red giant branch luminosity
function bump in a sample of isolated dwarf galaxies in the Local Group. We
have designed a new analysis approach comparing the observed color-magnitude
diagrams with theoretical best-fit color-magnitude diagrams derived from
precise estimates of the star formation histories of each galaxy. This analysis
is based on studying the difference between the V-magnitude of the RGB bump and
the horizontal branch at the level of the RR Lyrae instability strip
(Delta_vhbb) and we discuss here a technique for reliably measuring this
quantity in complex stellar systems. By using this approach, we find that the
difference between the observed and predicted values of Delta_vhbb is +0.13 +/-
0.14 mag. This is smaller, by about a factor of two, than the well-known
discrepancy between theory and observation at low metallicity commonly derived
for Galactic globular clusters. This result is confirmed by a comparison
between the adopted theoretical framework and empirical estimates of the
Delta_vhbb parameter for both a large database of Galactic globular clusters
and for four other dSph galaxies for which this estimate is available in the
literature. We also investigate the strength of the red giant branch bump
feature (R_bump), and find very good agreement between the observed and
theoretically predicted R_bump values. This agreement supports the reliability
of the evolutionary lifetimes predicted by theoretical models of the evolution
of low-mass stars.Comment: Accepted for publication on Ap
Bayesian inference of kinematic earthquake rupture parameters through fitting of strong motion data
Due to uncertainties in data and in forward modelling, the inherent limitations in data coverage and the non-linearity of the governing equation, earthquake source imaging is a problem with multiple solutions. The multiplicity of solutions can be conveniently expressed using a Bayesian approach, which allow to state inferences on model parameters in terms of probability density functions. The estimation of the posterior state of information, expressing the combination of the a priori knowledge on model parameters with the information contained in the data, is achieved in two steps. First, we explore the model space using an evolutionary algorithm to identify good data fitting regions. Secondly, using a neighbourhood algorithm and considering the entire ensemble of models found during the search stage, we compute a geometric approximation of the true posterior that is used to generate a second ensemble of models from which Bayesian inference can be performed. We apply this methodology to infer kinematic parameters of a synthetic fault rupture through fitting of strong motion data. We show how multiple rupture models are able to reproduce the observed waveforms within the same level of fit, suggesting therefore that the solution of the inversion cannot be expressed in terms of a single model but rather as a set of models which show certain statistical properties. For all model parameters we compute the posterior marginal distribution. We show how for some parameters the posterior do not follow a Gaussian distribution rendering the usual characterization in terms of mean value and standard deviation not correct. We compare the posterior marginal distributions with the ‘raw' marginal distributions computed from the ensemble of models generated by the evolutionary algorithm. We show how they are systematically different proving therefore that the search algorithm we adopt cannot be directly used to estimate uncertainties. We also analyse the stability of our inferences comparing the posterior marginals computed by different independent ensembles. The solutions provided by independent explorations are similar but not identical because each ensemble searches the model space differently resulting in different reconstructed posteriors. Our study illustrates how uncertainty estimates derive from the topology of the objective function, and how accurate and reliable resolution analysis is limited by the intrinsic difficulty of mapping the ‘true' structure of the objective functio
- …