4,251 research outputs found
Atmospheric turbulence in phase-referenced and wide-field interferometric images: Application to the SKA
Phase referencing is a standard calibration procedure in radio
interferometry. It allows to detect weak sources by using quasi-simultaneous
observations of closeby sources acting as calibrators. Therefore, it is assumed
that, for each antenna, the optical paths of the signals from both sources are
similar. However, atmospheric turbulence may introduce strong differences in
the optical paths of the signals and affect, or even waste, phase referencing
for cases of relatively large calibrator-to-target separations and/or bad
weather. The situation is similar in wide-field observations, since the random
deformations of the images, mostly caused by atmospheric turbulence, have
essentially the same origin as the random astrometric variations of
phase-referenced sources with respect to the phase center of their calibrators.
In this paper, we present the results of a Monte Carlo study of the astrometric
precision and sensitivity of an interferometric array (a realization of the
Square Kilometre Array, SKA) in phase-referenced and wide-field observations.
These simulations can be extrapolated to other arrays by applying the
corresponding corrections. We consider several effects from the turbulent
atmosphere (i.e., ionosphere and wet component of the troposphere) and also
from the antenna receivers. We study the changes in dynamic range and
astrometric precision as a function of observing frequency, source separation,
and strength of the turbulence. We find that, for frequencies between 1 and 10
GHz, it is possible to obtain images with high fidelity, although the
atmosphere strongly limits the sensitivity of the instrument compared to the
case with no atmosphere. Outside this frequency window, the dynamic range of
the images and the accuracy of the source positions decrease. [...] (Incomplete
abstract. Please read manuscript.)Comment: 9 pages, 11 figures. Accepted for publication in A&A
VLBI observations of SN2011dh: imaging of the youngest radio supernova
We report on the VLBI detection of supernova SN2011dh at 22GHz using a subset
of the EVN array. The observations took place 14 days after the discovery of
the supernova, thus resulting in a VLBI image of the youngest radio-loud
supernova ever. We provide revised coordinates for the supernova with
milli-arcsecond precision, linked to the ICRF. The recovered flux density is a
factor 2 below the EVLA flux density reported by other authors at the same
frequency and epoch of our observations. This discrepancy could be due to
extended emission detected with the EVLA or to calibration problems in the VLBI
and/or EVLA observations.Comment: Letter. Accepted in A&
Chaotic Diffusion in the Gliese-876 Planetary System
Chaotic diffusion is supposed to be responsible for orbital instabilities in
planetary systems after the dissipation of the protoplanetary disk, and a
natural consequence of irregular motion. In this paper we show that resonant
multi-planetary systems, despite being highly chaotic, not necessarily exhibit
significant diffusion in phase space, and may still survive virtually unchanged
over timescales comparable to their age.Using the GJ-876 system as an example,
we analyze the chaotic diffusion of the outermost (and less massive) planet. We
construct a set of stability maps in the surrounding regions of the Laplace
resonance. We numerically integrate ensembles of close initial conditions,
compute Poincar\'e maps and estimate the chaotic diffusion present in this
system. Our results show that, the Laplace resonance contains two different
regions: an inner domain characterized by low chaoticity and slow diffusion,
and an outer one displaying larger values of dynamical indicators. In the outer
resonant domain, the stochastic borders of the Laplace resonance seem to
prevent the complete destruction of the system. We characterize the diffusion
for small ensembles along the parameters of the outermost planet. Finally, we
perform a stability analysis of the inherent chaotic, albeit stable Laplace
resonance, by linking the behavior of the resonant variables of the
configurations to the different sub-structures inside the three-body resonance.Comment: 13 pages, 7 figures, 2 tables. Accepted for publication in MNRA
Identification of the optical and near-infrared counterpart of GRS 1758-258
Context. Understood to be a microquasar in the Galactic center region, GRS
1758-258 has not yet been unambiguously identified to have an
optical/near-infrared counterpart, mainly because of the high absorption and
the historic lack of suitable astrometric stars, which led to the use of
secondary astrometric solutions. Although it is considered with 1E 1740.7-2942
as the prototypical microquasar in the Galactic center region, the Galactic
origin of both sources has not yet been confirmed. Aims. We attempt to improve
previous astrometry to identify a candidate counterpart to GRS 1758-258. We
present observations with the Gran Telescopio de Canarias (GTC), in which we
try to detect any powerful emission lines that would infer an extragalactic
origin of this source. Methods. We use modern star catalogues to reanalyze
archival images of the GRS 1758-258 field in the optical and near-infrared
wavelengths, and compute a new astrometric solution. We also reanalyzed
archival radio data of GRS 1758-258 to determine a new and more accurate radio
position. Results. Our improved astrometric solution for the GRS 1758-258 field
represents a significant advancement on previous works and allows us to
identify a single optical/near-infrared source, which we propose as the
counterpart of GRS 1758-258. The GTC spectrum of this source is however of low
signal-to-noise ratio and does not rule out a Galactic origin. Hence, new
spectral observations are required to confirm or discard a Galactic nature.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic
Elucidating mysteries of phase-segregated membranes: Mobile-lipid recruitment facilitates pores' passage to the fluid phase
Phase segregation of multicomponent lipid bilayers leads to, under phase-coexistence conditions, domain formation, featuring delimitation by essentially one-dimensional borders. (Micro-)phase segregation of bilayers is proposed to influence the physiological behaviour of cell membranes and provides the driving force for lipid-raft formation. Experiments show a maximum in the electrical-conductivity of membranes at the phase-transition point, which has been conjectured to arise from border-nucleated transmembrane-conducting defects or pores. However, recent electroporation experiments on phase-segregated bilayers demonstrate electro-pore detection in the liquid disordered phase (Ld), wherein they diffuse over macroscopic periods without absorption into the liquid ordered phase (Lo). Here, we scrutinise transmembrane-pore formation via molecular dynamics simulations on a multicomponent phase-segregated bilayer. We find that pores created in Lo domains always migrate spontaneously to the Ld phase, via 'recruitment' of unsaturated lipids to the pore's rim to transport the pore to the fluid phase under a large stress-field driving force. Once in Ld domains, pores migrate towards their centre, never returning or pinning to Lo. These findings are explained by thermodynamics. By comparing the free-energy cost for creating pores in the bulk of Ld and Lo membranes, and in the phase-segregated system, we show that it is always more energetically tractable to create pores in Ld domains, independent of the pore size.Fil: LĂłpez MartĂ, JesĂşs MarĂa. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: English, Niall J.. University College Dublin; IrlandaFil: del Popolo, Mario Gabriel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Mendoza; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentin
DiagnĂłstico y tratamiento de la espondilomielopatĂs caudal cervical (sĂndrome del wobbler) en el perro
La espondilomielopatĂa caudal cervical es un sindrome de comprension cervical medular muy comun en la clinica diaria de pequeños animales y de compleja etiologia y patogenesis. Este articulo pretende exponer una revision actualizada de sus aspectos clinicos mas relevantes, su diagnostico y sus diversas formas de tratamiento conservador y quirurgico
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