5,092 research outputs found
Stellar Scattering and the Formation of Hot-Jupiters in Binary Systems
Hot Jupiters (HJs) are usually defined as giant Jovian-size planets with
orbital periods days. Although they lie close to the star, several
have finite eccentricities and significant misalignment angle with respect to
the stellar equator.
Two mechanisms have been proposed to explain the excited and misaligned
sub-population of HJs: Lidov-Kozai migration and planet-planet scattering.
Although both are based on completely different dynamical phenomena, they
appear to be equally effective in generating hot planets. Nevertheless, there
has been no detailed analysis comparing the predictions of both mechanisms.
In this paper we present numerical simulations of Lidov-Kozai trapping of
single planets in compact binary systems. Both the planet and the binary are
initially placed in coplanar orbits, although the inclination of the impactor
is assumed random. After the passage of the third star, we follow the orbital
and spin evolution of the planet using analytical models based on the octupole
expansion of the secular Hamiltonian.
The present work aims at the comparison of the two mechanisms, as an
explanation for the excited and inclined HJs in binary systems. We compare the
results obtained through this paper with results in Beaug\'e & Nesvorn\'y 2012,
where the authors analyze how the planet-planet scattering mechanisms works.
Several of the orbital characteristics of the simulated HJs are caused by
tidal trapping from quasi-parabolic orbits, independent of the driving
mechanism. These include both the 3-day pile-up and the distribution in the
eccentricity vs semimajor axis plane. However, the distribution of the
inclinations shows significant differences. While Lidov-Kozai trapping favors a
more random distribution, planet-planet scattering shows a large portion of
bodies nearly aligned with the equator of the central star.Comment: 12 pages, 6 figures. Accepted for publication at IJAB (internation
journal of astrobiology
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
Assessment of a high-resolution central scheme for the solution of the relativistic hydrodynamics equations
We assess the suitability of a recent high-resolution central scheme
developed by Kurganov & Tadmor (2000) for the solution of the relativistic
hydrodynamics equations. The novelty of this approach relies on the absence of
Riemann solvers in the solution procedure. The computations we present are
performed in one and two spatial dimensions in Minkowski spacetime. Standard
numerical experiments such as shock tubes and the relativistic flat-faced step
test are performed. As an astrophysical application the article includes
two-dimensional simulations of the propagation of relativistic jets using both
Cartesian and cylindrical coordinates. The simulations reported clearly show
the capabilities of the numerical scheme to yield satisfactory results, with an
accuracy comparable to that obtained by the so-called high-resolution
shock-capturing schemes based upon Riemann solvers (Godunov-type schemes), even
well inside the ultrarelativistic regime. Such central scheme can be
straightforwardly applied to hyperbolic systems of conservation laws for which
the characteristic structure is not explicitly known, or in cases where the
exact solution of the Riemann problem is prohibitively expensive to compute
numerically. Finally, we present comparisons with results obtained using
various Godunov-type schemes as well as with those obtained using other
high-resolution central schemes which have recently been reported in the
literature.Comment: 14 pages, 12 figures, to appear in 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
Wavelet transforms for non-uniform speech recognition
An algorithm for nonuniform speech segmentation and its application in speech recognition systems is presented. A method based on the Modulated Gaussian Wavelet Transform based Speech Analyser (MGWTSA) and the subsequent parametrization block is used to transform a uniform signal into a set of nonuniformly separated frames, with the accurate information being fed into a speech recognition system. The algorithm needs a frame characterizing the signal where necessary, trying to reduce the number of frames per signal as much as possible, without an appreciable reduction in the recognition rate of the system.Peer ReviewedPostprint (published version
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
Optical spectroscopy of microquasar candidates at low galactic latitudes
We report optical spectroscopic observations of a sample of 6 low-galactic
latitude microquasar candidates selected by cross-identification of X-ray and
radio point source catalogs for |b|<5 degrees. Two objects resulted to be of
clear extragalactic origin, as an obvious cosmologic redshift has been measured
from their emission lines. For the rest, none exhibits a clear stellar-like
spectrum as would be expected for genuine Galactic microquasars. Their
featureless spectra are consistent with being extragalactic in origin although
two of them could be also highly reddened stars. The apparent non-confirmation
of our candidates suggests that the population of persistent microquasar
systems in the Galaxy is more rare than previously believed. If none of them is
galactic, the upper limit to the space density of new Cygnus X-3-like
microquasars within 15 kpc would be 1.1\times10^{-12} per cubic pc. A similar
upper limit for new LS 5039-like systems within 4 kpc is estimated to be
5.6\times10^{-11} per cubic pc.Comment: 7 pages, 7 figures. Published in A&A, see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004A%26A...413..309
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