2,166 research outputs found
Gravitational Collapse in Constant Potential Bath
We analyse here the gravitational collapse of directed null radiation in a
background with a constant potential such as one produced by a star system like
galaxy in which the collapsing object is immersed. Both naked singularities and
black holes are shown to be developing as the final outcome of the collapse. An
interesting feature that emerges is that a part of the naked singularity
spectrum in collapsing Vaidya region gets covered in the corresponding
dual-Vaidya region, which corresponds to the Vaidya directed null radiation
sitting in constant potential bath. The implications of such a result towards
the issue of stability of naked singularities are discussed.Comment: LaTex 12 Pages, 2 figures. To appear in Physical Review
PowerAqua: fishing the semantic web
The Semantic Web (SW) offers an opportunity to develop novel, sophisticated forms of question answering (QA). Specifically, the availability of distributed semantic markup on a large scale opens the way to QA systems which can make use of such semantic information to provide precise, formally derived answers to questions. At the same time the distributed, heterogeneous, large-scale nature of the semantic information introduces significant challenges. In this paper we describe the design of a QA system, PowerAqua, designed to exploit semantic markup on the web to provide answers to questions posed in natural language. PowerAqua does not assume that the user has any prior information about the semantic resources. The system takes as input a natural language query, translates it into a set of logical queries, which are then answered by consulting and aggregating information derived from multiple heterogeneous semantic sources
Solar analogs with and without planets: T trends and galactic evolution
We explore a sample of 148 solar-like stars to search for a possible
correlation between the slopes of the abundance trends versus condensation
temperature (known as the Tc slope) both with stellar parameters and Galactic
orbital parameters in order to understand the nature of the peculiar chemical
signatures of these stars and the possible connection with planet formation. We
find that the Tc slope correlates at a significant level with the stellar age
and the stellar surface gravity. We also find tentative evidence that the Tc
slope correlates with the mean galactocentric distance of the stars (Rmean),
suggesting that stars that originated in the inner Galaxy have fewer refractory
elements relative to the volatile ones. We found that the chemical
peculiarities (small refractory-to-volatile ratio) of planet-hosting stars is
probably a reflection of their older age and their inner Galaxy origin. We
conclude that the stellar age and probably Galactic birth place are key to
establish the abundances of some specific elements.Comment: Proceedings of the GREAT-ITN conference: The Milky Way Unravelled by
Gaia. Will be published in the "EAS Publications Series
From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio
The main goal of this work is to study element ratios that are important for
the formation of planets of different masses. We study potential correlations
between the existence of planetary companions and the relative elemental
abundances of their host stars. We use a large sample of FGK-type dwarf stars
for which precise Mg, Si, and Fe abundances have been derived using HARPS
high-resolution and high-quality data. A first analysis of the data suggests
that low-mass planet host stars show higher [Mg/Si] ratios, while giant planet
hosts present [Mg/Si] that is lower than field stars. However, we found that
the [Mg/Si] ratio significantly depends on metallicity through Galactic
chemical evolution. After removing the Galactic evolution trend only the
difference in the [Mg/Si] elemental ratio between low-mass planet hosts and
non-hosts was present in a significant way. These results suggests that
low-mass planets are more prevalent around stars with high [Mg/Si]. Our results
demonstrate the importance of Galactic chemical evolution and indicate that it
may play an important role in the planetary internal structure and composition.Comment: Accepted by A&A (Letter to the Editor
Digital compensation of the side-band-rejection ratio in a fully analog 2SB sub-millimeter receiver
In observational radio astronomy, sideband-separating receivers are
preferred, particularly under high atmospheric noise, which is usually the case
in the sub-millimeter range. However, obtaining a good rejection ratio between
the two sidebands is difficult since, unavoidably, imbalances in the different
analog components appear. We describe a method to correct these imbalances
without making any change in the analog part of the sideband-separating
receiver, specifically, keeping the intermediate-frequency hybrid in place.
This opens the possibility of implementing the method in any existing receiver.
We have built hardware to demonstrate the validity of the method and tested it
on a fully analog receiver operating between 600 and 720GHz. We have tested the
stability of calibration and performance vs time and after full resets of the
receiver. We have performed an error analysis to compare the digital
compensation in two configurations of analog receivers, with and without
intermediate frequency (IF) hybrid. An average compensated sideband rejection
ratio of 46dB is obtained. Degradation of the compensated sideband rejection
ratio on time and after several resets of the receiver is minimal. A receiver
with an IF hybrid is more robust to systematic errors. Moreover, we have shown
that the intrinsic random errors in calibration have the same impact for
configuration without IF hybrid and for a configuration with IF hybrid with
analog rejection ratio better than 10dB. Compensated rejection ratios above
40dB are obtained even in the presence of high analog rejection. The method is
robust allowing its use under normal operational conditions at any telescope.
We also demonstrate that a full analog receiver is more robust against
systematic errors. Finally, the error bars associated to the compensated
rejection ratio are almost independent of whether IF hybrid is present or not
Using image analysis in the study of multiphase gas absorption
For the air–water–calcium alginate beads system, the effect of the presence of solids on the mass transfer characteristics in a bubble column was experimentally studied.
Volumetric liquid side mass transfer coefficient, kLa, specific interfacial area, a, and hence liquid side mass transfer coefficient, kL, were determined under different solid concentrations (0, 5, and 10 vol%), superficial gas velocities (up to 0.27 cm/s) and solid sizes (1.2 and 2.1mm diameter). The bubble characteristics, namely the interfacial area, were obtained using an image analysis technique.
This technique proved to be a suitable and practical method to characterize mass transfer phenomena in bubble columns for the range of operating conditions used. The solids affect negatively kLa, decreasing both a and kL, the effect being more pronounced for the smaller particles. For these particles the variation of kLa is due to the variation of its two components, while for larger particles kLa variation is due, essentially, to changes in kL as no significant differences in a were observed.Fundação para a Ciência e Tecnologia (FCT
WKB-type Approximation to Noncommutative Quantum Cosmology
In this work, we develop and apply the WKB approximation to several examples
of noncommutative quantum cosmology, obtaining the time evolution of the
noncommutative universe, this is done starting from a noncommutative quantum
formulation of cosmology where the noncommutativity is introduced by a
deformation on the minisuperspace variables. This procedure gives a
straightforward algorithm to incorporate noncommutativity to cosmology and
inflation.Comment: Revtex4, 6 pages, no figure
Orbital and physical properties of planets and their hosts: new insights on planet formation and evolution
We explore the relations between physical and orbital properties of planets
and properties of their host stars to identify the main observable signatures
of the formation and evolution processes of planetary systems. We use a large
sample of FGK dwarf planet hosts with stellar parameters derived in a
homogeneous way from the SWEET-Cat database to study the relation between
stellar metallicity and position of planets in the period-mass diagram. In the
second part we use all the RV-detected planets orbiting FGK stars to explore
the role of planet-disk and planet-planet interaction on the evolution of
orbital properties of planets with masses above 1MJup. We show that planets
orbiting metal-poor stars have longer periods than those in metal-rich systems.
This trend is valid for masses at least from 10MEarth to 4MJup. Earth-like
planets orbiting metal-rich stars always show shorter periods (fewer than 20
days) than those orbiting metal-poor stars. We also found statistically
significant evidence that very high mass giants have on average more eccentric
orbits than giant planets with lower mass.Finally, we show that the
eccentricity of planets with masses higher than 4MJup tends to be lower for
planets with shorter periods. Our results suggest that the planets in the P-MP
diagram are evolving differently because of a mechanism that operates over a
wide range of planetary masses. This mechanism is stronger or weaker depending
on the metallicity of the respective system. One possibility is that planets in
metal-poor disks form farther out from their central star and/or they form
later and do not have time to migrate as far as the planets in metal-rich
systems. The trends and dependencies obtained for very high mass planetary
systems suggest that planet-disk interaction is a very important and
orbit-shaping mechanism for planets in the high-mass domain. Shortened.Comment: 8 pages, 4 figures and 1 table. Accepted for publication in A&
Abundance trend with condensation temperature for stars with different Galactic birth places
During the past decade, several studies reported a correlation between
chemical abundances of stars and condensation temperature (also known as Tc
trend). However, the real astrophysical nature of this correlation is still
debated. The main goal of this work is to explore the possible dependence of
the Tc trend on stellar Galactocentric distances, Rmean. We used high-quality
spectra of about 40 stars observed with the HARPS and UVES spectrographs to
derive precise stellar parameters, chemical abundances, and stellar ages. A
differential line-by-line analysis was applied to achieve the highest possible
precision in the chemical abundances. We confirm previous results that [X/Fe]
abundance ratios depend on stellar age and that for a given age, some elements
also show a dependence on Rmean. When using the whole sample of stars, we
observe a weak hint that the Tc trend depends on Rmean. The observed dependence
is very complex and disappears when only stars with similar ages are
considered. To conclude on the possible dependence of the Tc trend on the
formation place of stars, a larger sample of stars with very similar
atmospheric parameters and stellar ages observed at different Galactocentric
distances is neededComment: Accepted by A&
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