115 research outputs found
A Survey of Hydroxyl Toward Supernova Remnants: Evidence for Extended 1720 MHz Maser Emission
We present the results of GBT observations of all four ground-state hydroxyl
(OH) transitions toward 15 supernova remnants (SNRs) which show OH(1720 MHz)
maser emission. This species of maser is well established as an excellent
tracer of an ongoing interaction between the SNR and dense molecular material.
For the majority of these objects we detect significantly higher flux densities
with a single dish than has been reported with interferometric observations. We
infer that spatially extended, low level maser emission is a common phenomenon
that traces the large-scale interaction in maser-emitting SNRs. Additionally we
use a collisional pumping model to fit the physical conditions under which OH
is excited behind the SNR shock front. We find the observed OH gas associated
with the SNR interaction having columns less than approximately 10^17 per
square cm, temperatures of 20 to 125 K, and densities 10^5 per cubic cm.Comment: 24 pages, 23 figures, Accepted to ApJ, March 26, 2008; v2 - added
Figure 6, minor clarifications to text in Sections 3 and
XMM-Newton observations of the supernova remnant IC443: I. soft X-ray emission from shocked interstellar medium
The shocked interstellar medium around IC443 produces strong X-ray emission
in the soft energy band (E<1.5 keV). We present an analysis of such emission as
observed with the EPIC MOS cameras on board the XMM-Newotn observatory, with
the purpose to find clear signatures of the interactions with the interstellar
medium (ISM) in the X-ray band, which may complement results obtained in other
wavelenghts. We found that the giant molecular cloud mapped in CO emission is
located in the foreground and gives an evident signature in the absorption of
X-rays. This cloud may have a torus shape and the part of torus interacting
with the IC443 shock gives rise to 2MASS-K emission in the southeast. The
measured density of emitting X-ray shocked plasma increases toward the
northeastern limb, where the remnant is interacting with an atomic cloud. We
found an excellent correlation between emission in the 0.3-0.5 keV band and
bright optical/radio filament on large spatial scales. The partial shell
structure seen in this band therefore traces the encounter with the atomic
cloud.Comment: 10 pages, 10 figures, accepted for publication in ApJ (20 September
2006, v649). For hi-res figures, see
http://www.astropa.unipa.it/Library/OAPA_preprints/ic443ele1.ps.g
GBT Observations of IC 443: the Nature of OH(1720 MHz) Masers and OH Absorption
We present results of spectral line observations of the ground state
transitions of hydroxyl(OH) toward supernova remnant IC 443 carried out with
the Green Bank Telescope. At a spatial resolution of 7.2 arcminutes we detect
weak, extended OH(1720 MHz) maser emission with OH(1667/5,1612 MHz) absorption
along the southern extent of the remnant, where no bright compact maser sources
have been observed previously. These newly detected SNR-type masers are
coincident with known molecular clumps and a ridge of shocked molecular
hydrogen indicative of the SNR shock front interacting with the adjacent
molecular cloud. Simultaneous observation of all four ground-state transitions
of OH permits us to fit physical conditions of the shocked gas at the
interaction site. A simple two-component model for the line profiles yields the
physical parameters for detected regions of maser emission including excitation
temperature, OH column density and filling factor. Observed line profiles
suggest the shock is largely propagating toward the line-of-sight in the region
of these newly identified weak masers. The implications of shock geometry and
physical parameters in producing extended OH maser emission in SNRs are
explored. We also present VLA radio continuum observations at 330 MHz for
comparison with OH line observations of the remnant.Comment: 20 pages, 4 figures, 3 tables, submitted to Ap
The Relation Between the Surface Brightness and the Diameter for Galactic Supernova Remnants
In this work, we have constructed a relation between the surface brightness
() and diameter (D) of Galactic C- and S-type supernova remnants
(SNRs). In order to calibrate the -D dependence, we have carefully
examined some intrinsic (e.g. explosion energy) and extrinsic (e.g. density of
the ambient medium) properties of the remnants and, taking into account also
the distance values given in the literature, we have adopted distances for some
of the SNRs which have relatively more reliable distance values. These
calibrator SNRs are all C- and S-type SNRs, i.e. F-type SNRs (and S-type SNR
Cas A which has an exceptionally high surface brightness) are excluded. The
Sigma-D relation has 2 slopes with a turning point at D=36.5 pc: (at 1
GHz)=8.4 D
WmHzster (for
WmHzster and D36.5 pc) and (at 1
GHz)=2.7 10 D
WmHzster (for
WmHzster and D36.5 pc). We discussed the theoretical
basis for the -D dependence and particularly the reasons for the change
in slope of the relation were stated. Added to this, we have shown the
dependence between the radio luminosity and the diameter which seems to have a
slope close to zero up to about D=36.5 pc. We have also adopted distance and
diameter values for all of the observed Galactic SNRs by examining all the
available distance values presented in the literature together with the
distances found from our -D relation.Comment: 45 pages, 2 figures, accepted for publication in Astronomical and
Astrophysical Transaction
Machine Learning in Automated Text Categorization
The automated categorization (or classification) of texts into predefined
categories has witnessed a booming interest in the last ten years, due to the
increased availability of documents in digital form and the ensuing need to
organize them. In the research community the dominant approach to this problem
is based on machine learning techniques: a general inductive process
automatically builds a classifier by learning, from a set of preclassified
documents, the characteristics of the categories. The advantages of this
approach over the knowledge engineering approach (consisting in the manual
definition of a classifier by domain experts) are a very good effectiveness,
considerable savings in terms of expert manpower, and straightforward
portability to different domains. This survey discusses the main approaches to
text categorization that fall within the machine learning paradigm. We will
discuss in detail issues pertaining to three different problems, namely
document representation, classifier construction, and classifier evaluation.Comment: Accepted for publication on ACM Computing Survey
SRAO CO Observation of 11 Supernova Remnants in l = 70 to 190 deg
We present the results of 12CO J = 1-0 line observations of eleven Galactic
supernova remnants (SNRs) obtained using the Seoul Radio Astronomy Observatory
(SRAO) 6-m radio telescope. The observation was made as a part of the SRAO CO
survey of SNRs between l = 70 and 190 deg, which is intended to identify SNRs
interacting with molecular clouds. The mapping areas for the individual SNRs
are determined to cover their full extent in the radio continuum. We used
halfbeam grid spacing (60") for 9 SNRs and full-beam grid spacing (120") for
the rest. We detected CO emission towards most of the remnants. In six SNRs,
molecular clouds showed a good spatial relation with their radio morphology,
although no direct evidence for the interaction was detected. Two SNRs are
particularly interesting: G85.4+0.7, where there is a filamentary molecular
cloud along the radio shell, and 3C434.1, where a large molecular cloud appears
to block the western half of the remnant. We briefly summarize the results
obtained for individual SNRs.Comment: Accepted for publication in Astrophysics & Space Science. 12 pages,
12 figures, and 3 table
A Sino-German 6\ cm polarization survey of the Galactic plane. V. Large supernova remnants
Observations of large supernova remnants (SNRs) at high frequencies are rare,
but provide valuable information about their physical properties. The total
intensity and polarization properties of 16 large SNRs in the Galactic plane
were investigated based on observations of the Urumqi 6\ cm
polarization survey of the Galactic plane with an angular resolution of
9\farcm5. We extracted total intensity and linear polarization maps of large
SNRs from the Urumqi 6\ cm survey, obtained their integrated flux
densities, and derived the radio spectra in context with previously published
flux densities at various frequencies. In particular, Effelsberg 11\
cm and 21\ cm survey data were used for calculating integrated flux
densities. The 6\ cm polarization data also delineate the magnetic
field structures of the SNRs. We present the first total intensity maps at
6\ cm for SNRs G106.3+2.7, G114.3+0.3, G116.5+1.1, G166.0+4.3 (VRO
42.05.01), G205.5+0.5 (Monoceros Nebula) and G206.9+2.3 (PKS 0646+06) and the
first polarization measurements at 6\ cm for SNRs G82.2+5.3 (W63),
G106.3+2.7, G114.3+0.3, G116.5+1.1, G166.0+4.3 (VRO 42.05.01), G205.5+0.5
(Monoceros Nebula) and G206.9+2.3 (PKS 0646+06). Most of the newly derived
integrated radio spectra are consistent with previous results. The new flux
densities obtained from the Urumqi 6\ cm, Effelsberg 11\ cm
and 21\ cm surveys are crucial to determine the spectra of SNR
G65.1+0.6, G69.0+2.7 (CTB 80), G93.7-0.2 and G114.3+0.3. We find that
G192.81.1 (PKS 0607+17) consists of background sources, \ion{H}{II} regions
and the extended diffuse emission of thermal nature, and conclude that
G192.81.1 is not a SNR.Comment: 15 pages, 10 figures, accepted by A&\amp;A. Language improved. For
the version with high resolution figures, please go to:
http://zmtt.bao.ac.cn/6cm/paper/gxy_largeSNR.pd
Reduced uptake of the proliferation-seeking radiotracer technetium-99m-labelled pentavalent dimercaptosuccinic acid in a 47-year-old woman with severe breast epithelial hyperplasia taking ibuprofen: a case report
<p>Abstract</p> <p>Introduction</p> <p>Recent studies have reported a risk reduction in the progression of benign breast disease to breast carcinoma through COX-2 pathways.</p> <p>Case presentation</p> <p>We present a case of severe epithelial hyperplasia in a 47-year-old woman with increased breast density submitted to scintimammography by the proliferation-imaging tracer Technetium-99m-labelled pentavalent dimercaptosuccinic acid, before and after an oral ibuprofen treatment for 4 weeks. The radiotracer uptake after ibuprofen intake was significantly reduced, both visually and by semi-quantitative analysis, based on a calculation of lesion-to-background ratios.</p> <p>Conclusion</p> <p>In proliferating breast lesions, scintigraphically displayed reduction in Technetium-99m-labelled pentavalent dimercaptosuccinic acid uptake may indicate inhibition by ibuprofen in the pathway of malignant epithelial cell transformation.</p
Phylogenetic Constraints Do Not Explain the Rarity of Nitrogen-Fixing Trees in Late-Successional Temperate Forests
Symbiotic nitrogen (N)-fixing trees are rare in late-successional temperate forests, even though these forests are often N limited. Two hypotheses could explain this paradox. The 'phylogenetic constraints hypothesis' states that no late-successional tree taxa in temperate forests belong to clades that are predisposed to N fixation. Conversely, the 'selective constraints hypothesis' states that such taxa are present, but N-fixing symbioses would lower their fitness. Here we test the phylogenetic constraints hypothesis.Using U.S. forest inventory data, we derived successional indices related to shade tolerance and stand age for N-fixing trees, non-fixing trees in the 'potentially N-fixing clade' (smallest angiosperm clade that includes all N fixers), and non-fixing trees outside this clade. We then used phylogenetically independent contrasts (PICs) to test for associations between these successional indices and N fixation. Four results stand out from our analysis of U.S. trees. First, N fixers are less shade-tolerant than non-fixers both inside and outside of the potentially N-fixing clade. Second, N fixers tend to occur in younger stands in a given geographical region than non-fixers both inside and outside of the potentially N-fixing clade. Third, the potentially N-fixing clade contains numerous late-successional non-fixers. Fourth, although the N fixation trait is evolutionarily conserved, the successional traits are relatively labile.These results suggest that selective constraints, not phylogenetic constraints, explain the rarity of late-successional N-fixing trees in temperate forests. Because N-fixing trees could overcome N limitation to net primary production if they were abundant, this study helps to understand the maintenance of N limitation in temperate forests, and therefore the capacity of this biome to sequester carbon
- …