160 research outputs found
Medical imaging analysis with artificial neural networks
Given that neural networks have been widely reported in the research community of medical imaging, we provide a focused literature survey on recent neural network developments in computer-aided diagnosis, medical image segmentation and edge detection towards visual content analysis, and medical image registration for its pre-processing and post-processing, with the aims of increasing awareness of how neural networks can be applied to these areas and to provide a foundation for further research and practical development. Representative techniques and algorithms are explained in detail to provide inspiring examples illustrating: (i) how a known neural network with fixed structure and training procedure could be applied to resolve a medical imaging problem; (ii) how medical images could be analysed, processed, and characterised by neural networks; and (iii) how neural networks could be expanded further to resolve problems relevant to medical imaging. In the concluding section, a highlight of comparisons among many neural network applications is included to provide a global view on computational intelligence with neural networks in medical imaging
Iron abundances from optical Fe III absorption lines in B-type stellar spectra
The role of optical Fe III absorption lines in B-type stars as iron abundance
diagnostics is considered. To date, ultraviolet Fe lines have been widely used
in B-type stars, although line blending can severely hinder their diagnostic
power. Using optical spectra, covering a wavelength range ~ 3560 - 9200 A, a
sample of Galactic B-type main-sequence and supergiant stars of spectral types
B0.5 to B7 are investigated. A comparison of the observed Fe III spectra of
supergiants, and those predicted from the model atmosphere codes TLUSTY
(plane-parallel, non-LTE), with spectra generated using SYNSPEC (LTE), and
CMFGEN (spherical, non-LTE), reveal that non-LTE effects appear small. In
addition, a sample of main-sequence and supergiant objects, observed with
FEROS, reveal LTE abundance estimates consistent with the Galactic environment
and previous optical studies. Based on the present study, we list a number of
Fe III transitions which we recommend for estimating the iron abundance from
early B-type stellar spectra.Comment: 3 figures and 8 tables. Table 3 is to be published online only
(included here on last page). Accepted for publication in MNRA
The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars
We have previously analysed the spectra of 135 early B-type stars in the LMC
and found several groups of stars that have chemical compositions that conflict
with the theory of rotational mixing. Here we extend this study to Galactic and
SMC metallicities with the analysis of ~50 Galactic and ~100 SMC early B-type
stars with rotational velocities up to ~300km/s. The surface nitrogen
abundances are utilised as a probe of the mixing process.
In the SMC, we find a population of slowly rotating nitrogen-rich stars
amongst the early B type core-hydrogen burning stars, similar to the LMC. In
the Galactic sample we find no significant enrichment amongst the core
hydrogen-burning stars, which appears to be in contrast with the expectation
from both rotating single-star and close binary evolution models. However, only
a small number of the rapidly rotating stars have evolved enough to produce a
significant nitrogen enrichment, and these may be analogous to the non-enriched
rapid rotators previously found in the LMC sample. Finally, in each metallicity
regime, a population of highly enriched supergiants is observed, which cannot
be the immediate descendants of core-hydrogen burning stars. Their abundances
are, however, compatible with them having gone through a previous red
supergiant phase. Together, these observations paint a complex picture of the
nitrogen enrichment in massive main sequence and supergiant stellar
atmospheres, where age and binarity cause crucial effects. Whether rotational
mixing is required to understand our results remains an open question at this
time, but could be answered by identifying the true binary fraction in those
groups of stars that do not agree with single-star evolutionary models
(abridged).Comment: Accepted paper - 86 pages with tables and figure
The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution
Rotation has become an important element in evolutionary models of massive
stars, specifically via the prediction of rotational mixing. Here, we study a
sample of stars, including rapid rotators, to constrain such models and use
nitrogen enrichments as a probe of the mixing process. Chemical compositions
(C, N, O, Mg and Si) have been estimated for 135 early B-type stars in the
Large Magellanic Cloud with projected rotational velocities up to ~300km/s
using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including
rotational mixing, have been generated attempting to reproduce these
observations by adjusting the overshooting and rotational mixing parameters and
produce reasonable agreement with 60% of our core hydrogen burning sample. We
find (excluding known binaries) a significant population of highly nitrogen
enriched intrinsic slow rotators vsini less than 50km/s incompatible with our
models ~20% of the sample). Furthermore, while we find fast rotators with
enrichments in agreement with the models, the observation of evolved (log g
less than 3.7dex) fast rotators that are relatively unenriched (a further ~20%
of the sample) challenges the concept of rotational mixing. We also find that
70% of our blue supergiant sample cannot have evolved directly from the
hydrogen burning main-sequence. We are left with a picture where invoking
binarity and perhaps fossil magnetic fields are required to understand the
surface properties of a population of massive main sequence stars.Comment: ApJL. 10 pages, 1 figure. Updated to match accepted versio
B-type supergiants in the SMC: Rotational velocities and implications for evolutionary models
High-resolution spectra for 24 SMC and Galactic B-type supergiants have been
analysed to estimate the contributions of both macroturbulence and rotation to
the broadening of their metal lines. Two different methodologies are
considered, viz. goodness-of-fit comparisons between observed and theoretical
line profiles and identifying zeros in the Fourier transforms of the observed
profiles. The advantages and limitations of the two methods are briefly
discussed with the latter techniques being adopted for estimated projected
rotational velocities (\vsini) but the former being used to estimate
macroturbulent velocities. Only one SMC supergiant, SK 191, shows a significant
degree of rotational broadening (\vsini 90 \kms). For the remaining
targets, the distribution of projected rotational velocities are similar in
both our Galactic and SMC samples with larger values being found at earlier
spectral types. There is marginal evidence for the projected rotational
velocities in the SMC being higher than those in the Galactic targets but any
differences are only of the order of 5-10 \kms, whilst evolutionary models
predict differences in this effective temperature range of typically 20 to 70
\kms. The combined sample is consistent with a linear variation of projected
rotational velocity with effective temperature, which would imply rotational
velocities for supergiants of 70 \kms at an effective temperature of 28 000 K
(approximately B0 spectral type) decreasing to 32 \kms at 12 000 K (B8 spectral
type). For all targets, the macroturbulent broadening would appear to be
consistent with a Gaussian distribution (although other distributions cannot be
discounted) with an half-width varying from approximately 20 \kms
at B8 to 60 \kms at B0 spectral types.Comment: 4 figures, 8 pages, submitted to Astronomy and Astrophysic
The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
The quantitative study of the physical properties and chemical abundances of
large samples of massive blue stars at different metallicities is a powerful
tool to understand the nature and evolution of these objects. Their analysis
beyond the Milky Way is challenging, nonetheless it is doable and the best way
to investigate their behavior in different environments. Fulfilling this task
in an objective way requires the implementation of automatic analysis
techniques that can perform the analyses systematically, minimizing at the same
time any possible bias.
As part of the ARAUCARIA project we carry out the first quantitative
spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55
at 1.94 Mpc away. By applying the methodology developed in this work, we derive
their stellar parameters, chemical abundances and provide a characterization of
the present-day metallicity of their host galaxy.
Based on the characteristics of the stellar atmosphere/line formation code
FASTWIND, we designed and created a grid of models for the analysis of massive
blue supergiant stars. Along with this new grid, we implemented a spectral
analysis algorithm. Both tools were specially developed to perform fully
consistent quantitative spectroscopic analyses of low spectral resolution of
B-type supergiants in a fast and objective way.
We present the main characteristics of our FASTWIND model grid and perform a
number of tests to investigate the reliability of our methodology. The
automatic tool is applied afterward to a sample of 12 B-type supergiant stars
in NGC55, deriving the stellar parameters and abundances. The results indicate
that our stars are part of a young population evolving towards a red supergiant
phase. The derived chemical composition hints to an average metallicity similar
to the one of the Large Magellanic Cloud, with no indication of a spatial trend
across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&
SN 2005 gj: Evidence for LBV supernovae progenitors?
There has been mounting observational evidence in favour of Luminous Blue
Variables (LBVs) being the direct progenitors of supernovae. Here we present
possibly the most convincing evidence yet for such progenitors. We find
multiple absorption component P-Cygni profiles of hydrogen and helium in the
spectrum of SN 2005gj, which we interpret as being an imprint of the
progenitors mass-loss history. Such profiles have previously only been detected
in Luminous Blue Variables. This striking resemblance of the profiles, along
with wind velocities and periods consistent with LBV's leads us to connect SN
2005gj to an LBV progenitor.Comment: Accepted as a letter to A&A, 4 pages,3 figure
Testing the predicted mass-loss bi-stability jump at radio wavelengths
In this study, we test the theoretically predicted mass-loss behaviour as a
function of stellar effective temperature across the so-called `bi-stability'
jump (BSJ). We gathered radio observations of 30 OB supergiants (O8-B3). We
derived the radio mass-loss rates and wind efficiencies, and compared our
results with Halpha mass-loss rates and predictions based on radiation-driven
wind models. he wind efficiency shows the possible presence of a local maximum
around an effective temperature of 21~000 K -- in qualitative agreement with
predictions. We also find that the radio mass-loss rates show good agreement
with empirical Halpha rates. However, the empirical mass-loss rates are larger
than the predicted rates from radiation-driven wind theory for objects above
the BSJ temperature, whilst they are smaller for the rest. A new wind
momenta-luminosity relation for O8-B0 stars has been derived.Comment: 13 pages, 13 figures, A&
Atmospheric parameters and rotational velocities for a sample of Galactic B-type supergiants
High resolution optical spectra of 57 Galactic B-type supergiant stars have
been analyzed to determine their rotational and macroturbulent velocities. In
addition, their atmospheric parameters (effective temperature, surface gravity
and microturbulent velocity) and surface nitrogen abundances have been
estimated using a non-LTE grid of model atmospheres. Comparisons of the
projected rotational velocities have been made with the predictions of stellar
evolutionary models and in general good agreement was found. However for a
small number of targets, their observed rotational velocities were
significantly larger than predicted, although their nitrogen abundances were
consistent with the rest of the sample. We conclude that binarity may have
played a role in generating their large rotational velocities. No correlation
was found between nitrogen abundances and the current projected rotational
velocities. However a correlation was found with the inferred projected
rotational velocities of the main sequence precursors of our supergiant sample.
This correlation is again in agreement with the predictions of single star
evolutionary models that incorporate rotational mixing. The origin of the
macroturbulent and microturbulent velocity fields is discussed and our results
support previous theoretical studies that link the former to sub-photospheric
convection and the latter to non-radial gravity mode oscillations. In addition,
we have attempted to identify differential rotation in our most rapidly
rotating targets.Comment: Submitted to MNRAS, 16 page
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