43,832 research outputs found
Evolution of Vacuum Bubbles Embeded in Inhomogeneous Spacetimes
We study the propagation of bubbles of new vacuum in a radially inhomogeneous
background filled with dust or radiation, and including a cosmological
constant, as a first step in the analysis of the influence of inhomogeneities
in the evolution of an inflating region. We also compare the cases with dust
and radiation backgrounds and show that the evolution of the bubble in
radiation environments is notably different from that in the corresponding dust
cases, both for homogeneous and inhomogeneous ambients, leading to appreciable
differences in the evolution of the proper radius of the bubble.Comment: 18 pages, 15 figures, accepted for publication in Journal of
Cosmology and Astroparticle Physics (new version with a few cosmetic changes
w.r.t. the published one
Analysis of the acoustic cut-off frequency and HIPs in six Kepler stars with stochastically excited pulsations
Gravito-acoustic modes in the Sun and other stars propagate in resonant
cavities with a frequency below a given limit known as the cut-off frequency.
At higher frequencies, waves are no longer trapped in the stellar interior and
become traveller waves. In this article we study six pulsating solar-like stars
at different evolutionary stages observed by the NASA Kepler mission. These
high signal-to-noise targets show a peak structure that extends at very high
frequencies and are good candidates for studying the transition region between
the modes and the interference peaks or pseudo-modes. Following the same
methodology successfully applied on Sun-as-a-star measurements, we uncover the
existence of pseudo-modes in these stars with one or two dominant interference
patterns depending on the evolutionary stage of the star. We also infer their
cut-off frequency as the midpoint between the last eigenmode and the first peak
of the interference patterns. By using ray theory we show that, while the
period of one of the interference pattern is very close to half the large
separation the other, one depends on the time phase of mixed waves, thus
carrying additional information on the stellar structure and evolution.Comment: Accepted for publication in A&A. 14 pages, 28 figure
Structure of Compact Stars in R-squared Palatini Gravity
We analyse configurations of compact stars in the so-called R-squared gravity
in the Palatini formalism. Using a realistic equation of state we show that the
mass-radius configurations are lighter than their counterparts in General
Relativity. We also obtain the internal profiles, which run in strong
correlation with the derivatives of the equation of state, leading to regions
where the mass parameter decreases with the radial coordinate in a
counter-intuitive way. In order to analyse such correlation, we introduce a
parametrisation of the equation of state given by multiple polytropes, which
allows us to explicitly control its derivatives. We show that, even in a
limiting case where hard phase transitions in matter are allowed, the internal
profile of the mass parameter still presents strange features and the
calculated M-R configurations also yield NSs lighter than those obtained in
General Relativity.Comment: 9 pages, 5 figures. Accepted for publication in General Relativity
and Gravitatio
Adaptive Transmission Techniques for Mobile Satellite Links
Adapting the transmission rate in an LMS channel is a challenging task
because of the relatively fast time variations, of the long delays involved,
and of the difficulty in mapping the parameters of a time-varying channel into
communication performance. In this paper, we propose two strategies for dealing
with these impairments, namely, multi-layer coding (MLC) in the forward link,
and open-loop adaptation in the return link. Both strategies rely on
physical-layer abstraction tools for predicting the link performance. We will
show that, in both cases, it is possible to increase the average spectral
efficiency while at the same time keeping the outage probability under a given
threshold. To do so, the forward link strategy will rely on introducing some
latency in the data stream by using retransmissions. The return link, on the
other hand, will rely on a statistical characterization of a physical-layer
abstraction measure.Comment: Presented at the 30th AIAA International Communications Satellite
Systems Conference (ICSSC), Ottawa, Canada, 2012. Best Professional Paper
Awar
HD 169142 in the eyes of ZIMPOL/SPHERE
We present new data of the protoplanetary disc surrounding the Herbig Ae/Be
star HD 169142 obtained in the very broad-band (VBB) with the Zurich imaging
polarimeter (ZIMPOL), a sub-system of the Spectro-Polarimetric High-contrast
Exoplanet REsearch instrument (SPHERE) at the Very Large Telescope (VLT). Our
Polarimetric Differential Imaging (PDI) observations probe the disc as close as
0.03" (3.5au) to the star and are able to trace the disc out to ~1.08"
(~126au). We find an inner hole, a bright ring bearing substructures around
0.18" (21au), and an elliptically shaped gap stretching from 0.25" to 0.47"
(29-55au). Outside of 0.47", the surface brightness drops off, discontinued
only by a narrow annular brightness minimum at ~0.63"-0.74" (74-87au). These
observations confirm features found in less-well resolved data as well as
reveal yet undetected indications for planet-disc interactions, such as
small-scale structures, star-disk offsets, and potentially moving shadows.Comment: Accepted for publication in MNRA
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