215 research outputs found
A hydrodynamical analysis of the steady-state shock model
In this article some of the hydrodynamical (HD) aspects of steady shocks as
described by the steady-state shock model are reviewed and discussed. It is
found that, at least in some of the contexts in which the steady-state model is
used, the steady-state assumption cannot be satisfied. Moreover, the main
result of the present work is that even if the assumptions on steadiness and on
the geometry are fully satisfied, serious limitations in the application of the
model are found: (i) in the absence of down-stream boundary conditions the
model is not related to the physical process(es) that originate the shock, (ii)
matter shocked during the presumed phase of steadiness of the shock is not
hydrodynamically interacting with previously shocked matter, and (iii) the
steady-state model assumes that the flow is stable against perturbations.
Furthermore, even if boundary conditions were assumed, the link between the
steady model and the astrophysical context would not be strictly speaking the
correct HD link. Time-dependent HD computations in different astrophysical
contexts (e.g. SNRs and molecular shocks) show that the steady-state
approximation is inadequate to describe these post-shock structures. Based on
the HD limitations of the steady-state model, it is advised that the model be
used to describe post-shock structures only in those astrophysical contexts
where full time-dependent HD models have already positively tested the
steadiness of the flow. Alternatively, it is suggested to replace the
steady-state model either with time-dependent HD models, or with less
problematic approximations.Comment: 9 pages, to be published in The Open Astronomy Journal (
http://www.bentham.org/open/toaaj/index.htm
An automated system for lung nodule detection in low-dose computed tomography
A computer-aided detection (CAD) system for the identification of pulmonary
nodules in low-dose multi-detector helical Computed Tomography (CT) images was
developed in the framework of the MAGIC-5 Italian project. One of the main
goals of this project is to build a distributed database of lung CT scans in
order to enable automated image analysis through a data and cpu GRID
infrastructure. The basic modules of our lung-CAD system, a dot-enhancement
filter for nodule candidate selection and a neural classifier for
false-positive finding reduction, are described. The system was designed and
tested for both internal and sub-pleural nodules. The results obtained on the
collected database of low-dose thin-slice CT scans are shown in terms of free
response receiver operating characteristic (FROC) curves and discussed.Comment: 9 pages, 9 figures; Proceedings of the SPIE Medical Imaging
Conference, 17-22 February 2007, San Diego, California, USA, Vol. 6514,
65143
Computer-aided detection of pulmonary nodules in low-dose CT
A computer-aided detection (CAD) system for the identification of pulmonary
nodules in low-dose multi-detector helical CT images with 1.25 mm slice
thickness is being developed in the framework of the INFN-supported MAGIC-5
Italian project. The basic modules of our lung-CAD system, a dot enhancement
filter for nodule candidate selection and a voxel-based neural classifier for
false-positive finding reduction, are described. Preliminary results obtained
on the so-far collected database of lung CT scans are discussed.Comment: 3 pages, 4 figures; Proceedings of the CompIMAGE - International
Symposium on Computational Modelling of Objects Represented in Images:
Fundamentals, Methods and Applications, 20-21 Oct. 2006, Coimbra, Portuga
A scalable system for microcalcification cluster automated detection in a distributed mammographic database
A computer-aided detection (CADe) system for microcalcification cluster
identification in mammograms has been developed in the framework of the
EU-founded MammoGrid project. The CADe software is mainly based on wavelet
transforms and artificial neural networks. It is able to identify
microcalcifications in different datasets of mammograms (i.e. acquired with
different machines and settings, digitized with different pitch and bit depth
or direct digital ones). The CADe can be remotely run from GRID-connected
acquisition and annotation stations, supporting clinicians from geographically
distant locations in the interpretation of mammographic data. We report and
discuss the system performances on different datasets of mammograms and the
status of the GRID-enabled CADe analysis.Comment: 6 pages, 4 figures; Proceedings of the IEEE NNS and MIC Conference,
October 23-29, 2005, Puerto Ric
Recombination Line vs. Forbidden Line Abundances in Planetary Nebulae
Recombination lines (RLs) of C II, N II, and O II in planetary nebulae (PNs)
have been found to give abundances that are much larger in some cases than
abundances from collisionally-excited forbidden lines (CELs). The origins of
this abundance discrepancy are highly debated. We present new spectroscopic
observations of O II and C II recombination lines for six planetary nebulae.
With these data we compare the abundances derived from the optical
recombination lines with those determined from collisionally-excited lines.
Combining our new data with published results on RLs in other PNs, we examine
the discrepancy in abundances derived from RLs and CELs. We find that there is
a wide range in the measured abundance discrepancy Delta(O+2) = log O+2(RL) -
log O+2(CEL), ranging from approximately 0.1 dex up to 1.4 dex. Most RLs yield
similar abundances, with the notable exception of O II multiplet V15, known to
arise primarily from dielectronic recombination, which gives abundances
averaging 0.6 dex higher than other O II RLs. We compare Delta(O+2) against a
variety of physical properties of the PNs to look for clues as to the mechanism
responsible for the abundance discrepancy. The strongest correlations are found
with the nebula diameter and the Balmer surface brightness. An inverse
correlation of Delta(O+2) with nebular density is also seen. Similar results
are found for carbon in comparing C II RL abundances with ultraviolet
measurements of C III].Comment: 48 pages, 14 figures, accepted for publication in the Astrophysical
Journal Supplemen
IVOA Recommendation: An IVOA Standard for Unified Content Descriptors Version 1.1
This document describes the current understanding of the IVOA controlled
vocabulary for describing astronomical data quantities, called Unified Content
Descriptors (UCDs).
The present document defines a new standard (named UCD1+) improving the first
generation of UCDs (hereafter UCD1). The basic idea is to adopt a new syntax
and vocabulary requiring little effort for people to adapt softwares already
using UCD1.
This document also addresses the questions of maintenance and evolution of
the UCD1+. Examples of use cases within the VO, and tools for using UCD1+ are
also described
Italian center for Astronomical Archives publishing solution: modular and distributed
The Italian center for Astronomical Archives tries to provide astronomical
data resources as interoperable services based on IVOA standards. Its VO
expertise and knowledge comes from active participation within IVOA and VO at
European and international level, with a double-fold goal: learn from the
collaboration and provide inputs to the community. The first solution to build
an easy to configure and maintain resource publisher conformant to VO standards
proved to be too optimistic. For this reason it has been necessary to re-think
the architecture with a modular system built around the messaging concept,
where each modular component speaks to the other interested parties through a
system of broker-managed queues. The first implemented protocol, the Simple
Cone Search, shows the messaging task architecture connecting the parametric
HTTP interface to the database backend access module, the logging module, and
allows multiple cone search resources to be managed together through a
configuration manager module. Even if relatively young, it already proved the
flexibility required by the overall system when the database backend changed
from MySQL to PostgreSQL+PgSphere. Another implementation test has been made to
leverage task distribution over multiple servers to serve simultaneously: FITS
cubes direct linking, cubes cutout and cubes positional merging. Currently the
implementation of the SIA-2.0 standard protocol is ongoing while for TAP we
will be adapting the TAPlib library. Alongside these tools a first
administration tool (TASMAN) has been developed to ease the build up and
maintenance of TAP_SCHEMA-ta including also ObsCore maintenance capability.
Future work will be devoted at widening the range of VO protocols covered by
the set of available modules, improve the configuration management and develop
specific purpose modules common to all the service components.Comment: SPIE Astronomical Telescopes + Instrumentation 2018, Software and
Cyberinfrastructure for Astronomy V, pre-publishing draft proceeding (reduced
abstract
- âŠ