5,550 research outputs found
Filamentary fragmentation in a turbulent medium
We present the results of smoothed particle hydrodynamic simulations
investigating the evolution and fragmentation of filaments that are accreting
from a turbulent medium. We show that the presence of turbulence, and the
resulting inhomogeneities in the accretion flow, play a significant role in the
fragmentation process. Filaments which experience a weakly turbulent accretion
flow fragment in a two-tier hierarchical fashion, similar to the fragmentation
pattern seen in the Orion Integral Shaped Filament. Increasing the energy in
the turbulent velocity field results in more sub-structure within the
filaments, and one sees a shift from gravity-dominated fragmentation to
turbulence-dominated fragmentation. The sub-structure formed in the filaments
is elongated and roughly parallel to the longitudinal axis of the filament,
similar to the fibres seen in observations of Taurus, and suggests that the
fray and fragment scenario is a possible mechanism for the production of
fibres. We show that the formation of these fibre-like structures is linked to
the vorticity of the velocity field inside the filament and the filament's
accretion from an inhomogeneous medium. Moreover, we find that accretion is
able to drive and sustain roughly sonic levels of turbulence inside the
filaments, but is not able to prevent radial collapse once the filaments become
supercritical. However, the supercritical filaments which contain fibre-like
structures do not collapse radially, suggesting that fibrous filaments may not
necessarily become radially unstable once they reach the critical line-density.Comment: (Accepted for publication in MNRAS
Vacuum Polarization by a Magnetic Flux Tube at Finite Temperature in the Cosmic String Spacetime
In this paper we analyse the effect produced by the temperature in the vacuum
polarization associated with charged massless scalar field in the presence of
magnetic flux tube in the cosmic string spacetime. Three different
configurations of magnetic fields are taken into account: a homogeneous
field inside the tube, a field proportional to and a
cylindrical shell with -function. In these three cases, the axis of the
infinitely long tube of radius coincides with the cosmic string. Because
the complexity of this analysis in the region inside the tube, we consider the
thermal effect in the region outside. In order to develop this analysis, we
construct the thermal Green function associated with this system for the three
above mentioned situations considering points in the region outside the tube.
We explicitly calculate in the high-temperature limit, the thermal average of
the field square and the energy-momentum tensor.Comment: 16 pages, 1 figur
First detection of CF+ towards a high-mass protostar
We report the first detection of the J = 1 - 0 (102.6 GHz) rotational lines
of CF+ (fluoromethylidynium ion) towards CygX-N63, a young and massive
protostar of the Cygnus X region. This detection occurred as part of an
unbiased spectral survey of this object in the 0.8-3 mm range, performed with
the IRAM 30m telescope. The data were analyzed using a local thermodynamical
equilibrium model (LTE model) and a population diagram in order to derive the
column density. The line velocity (-4 km s-1) and line width (1.6 km s-1)
indicate an origin from the collapsing envelope of the protostar.
We obtain a CF+ column density of 4.10e11 cm-2. The CF+ ion is thought to be
a good tracer for C+ and assuming a ratio of 10e-6 for CF+/C+, we derive a
total number of C+ of 1.2x10e53 within the beam. There is no evidence of carbon
ionization caused by an exterior source of UV photons suggesting that the
protostar itself is the source of ionization. Ionization from the protostellar
photosphere is not efficient enough. In contrast, X-ray ionization from the
accretion shock(s) and UV ionization from outflow shocks could provide a large
enough ionizing power to explain our CF+ detection.
Surprisingly, CF+ has been detected towards a cold, massive protostar with no
sign of an external photon dissociation region (PDR), which means that the only
possibility is the existence of a significant inner source of C+. This is an
important result that opens interesting perspectives to study the early
development of ionized regions and to approach the issue of the evolution of
the inner regions of collapsing envelopes of massive protostars. The existence
of high energy radiations early in the evolution of massive protostars also has
important implications for chemical evolution of dense collapsing gas and could
trigger peculiar chemistry and early formation of a hot core.Comment: 6 page
Structural Changes in Data Communication in Wireless Sensor Networks
Wireless sensor networks are an important technology for making distributed
autonomous measures in hostile or inaccessible environments. Among the
challenges they pose, the way data travel among them is a relevant issue since
their structure is quite dynamic. The operational topology of such devices can
often be described by complex networks. In this work, we assess the variation
of measures commonly employed in the complex networks literature applied to
wireless sensor networks. Four data communication strategies were considered:
geometric, random, small-world, and scale-free models, along with the shortest
path length measure. The sensitivity of this measure was analyzed with respect
to the following perturbations: insertion and removal of nodes in the geometric
strategy; and insertion, removal and rewiring of links in the other models. The
assessment was performed using the normalized Kullback-Leibler divergence and
Hellinger distance quantifiers, both deriving from the Information Theory
framework. The results reveal that the shortest path length is sensitive to
perturbations.Comment: 12 pages, 4 figures, Central European Journal of Physic
Pediatric Inflammatory Bowel Disease. Is it Still Increasing?
Introdução: A Doença de Crohn (DC), Colite Ulcerosa (CU) e Colite Indeterminada (CI),
habitualmente designadas por Doença InflamatĂłria Intestinal (DII), representam um grupo heterogĂ©neo de patologias crĂłnicas, de etiologia desconhecida e evolução variável, podendo manifestar-se, em idade pediátrica, em cerca de 25 a 30% dos casos. Estudos epidemiolĂłgicos internacionais comprovam o aumento exponencial da sua incidĂŞncia nos paĂses industrializados,
em particular da DC, nos Ăşltimos 50 anos. Objectivos: Caracterização da população pediátrica com o diagnĂłstico de DII, seguida na consulta de Gastrenterologia Infantil do Hospital de Dona Estefânia (HDE). Material e MĂ©todos: Estudo descritivo e retrospectivo, mediante consulta de processos clĂnicos, de doentes com o diagnĂłstico de DII, entre 1987 e 2009 (23 anos). Utilizaram-se critĂ©rios clĂnicos, radiolĂłgicos e histolĂłgicos para a definição de DII.
Foram estudadas as seguintes variáveis: caracterização da DII, sexo, antecedentes familiares, idade Ă data do diagnĂłstico, intervalo de tempo entre o inĂcio da sintomatologia e respectivo diagnĂłstico e apresentação clĂnica. Foram comparados quatro intervalos de tempo: 1987-1992, 1993-1998, 1999-2004 e 2005-2009. Resultados: Foram incluĂdas 100 crianças, 51
pertencentes ao sexo feminino, das quais 59% correspondem a DC, 38% a CU e 3% a CI.
Verificou-se a presença de antecedentes familiares de DII em sete casos, nĂŁo se verificando diferença significativa de sexo entre a CU e a DC. No perĂodo compreendido entre 2005 e 2009 foi registado o maior nĂşmero de novos casos (55 no total; mĂ©dia: 11 casos/ano) e entre 1987 e 1992 registou-se o menor nĂşmero de novos casos (9; 1,5 casos/ano). O intervalo de tempo que
decorreu entre o inĂcio dos sintomas e o diagnĂłstico de DII variou entre nove meses (1987-1992) e quatro meses (2005-2009). A idade no momento do diagnĂłstico variou entre os 14 meses e os 17 anos, com um valor mĂ©dio de 10,5 anos. A sintomatologia inaugural mais frequente foi a presença de dor abdominal, a diarreia e a hematoquĂ©zia. ConclusĂŁo: A DII engloba um grupo
heterogéneo de patologias, nem sempre fáceis de diagnosticar ou classificar, dada a ausência de critérios de diagnóstico uniformes.
Os resultados apresentados mostram o aumento do nĂşmero de novos casos, na consulta de
Gastrenterologia do HDE, nas Ăşltimas duas dĂ©cadas, nĂŁo se verificando diferença no que diz respeito ao sexo. O tempo que decorreu entre o inĂcio dos sintomas e o diagnĂłstico diminuiu ao longo dos anos, tendo permanecido inalterada a idade no momento do diagnĂłstico e a apresentação clĂnica
Massive 70 micron quiet clumps I: evidence of embedded low/intermediate-mass star formation activity
Massive clumps, prior to the formation of any visible protostars, are the
best candidates to search for the elusive massive starless cores. In this work
we investigate the dust and gas properties of massive clumps selected to be 70
micron quiet, therefore good starless candidates. Our sample of 18 clumps has
masses 300 < M < 3000 M_sun, radius 0.54 < R < 1.00 pc, surface densities Sigma
> 0.05 g cm^-2 and luminosity/mass ratio L/M < 0.3. We show that half of these
70 micron quiet clumps embed faint 24 micron sources. Comparison with GLIMPSE
counterparts shows that 5 clumps embed young stars of intermediate stellar mass
up to ~5.5 M_sun. We study the clump dynamics with observations of N2H+ (1-0),
HNC (1-0) and HCO+ (1-0) made with the IRAM 30m telescope. Seven clumps have
blue-shifted spectra compatible with infall signatures, for which we estimate a
mass accretion rate 0.04 < M_dot < 2.0 x 10^-3 M_sun yr^-1, comparable with
values found in high-mass protostellar regions, and free-fall time of the order
of t_ff = 3 x 10^5 yr. The only appreciable difference we find between objects
with and without embedded 24 micron sources is that the infall rate appears to
increase from 24 micron dark to 24 micron bright objects. We conclude that all
70 micron quiet objects have similar properties on clump scales, independently
of the presence of an embedded protostar. Based on our data we speculate that
the majority, if not all of these clumps may already embed faint, low-mass
protostellar cores. If these clumps are to form massive stars, this must occur
after the formation of these lower mass stars.Comment: 44 pages, 11 Figures. Accepted for publication in MNRA
Duration of Low Wage Employment: A Study Based on a Survival Model
This paper includes a survival analysis which attempts to explain the duration, as in the number of years a worker remains in a low wage situation. Explanatory variables take into account the characteristics of the employee, such as education, age, tenure with the company, gender and nationality, and the characteristics of the job and the company such as industry affiliation, number of employees, age of the company and location.low wage, survival, Portugal
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