451 research outputs found
High-Contrast Observations in Optical and Infrared Astronomy
High-contrast observations in optical and infrared astronomy are defined as
any observation requiring a technique to reveal a celestial object of interest
that is in such close angular proximity to another source brighter by a factor
of at least 10^5 that optical effects hinder or prevent the collection of
photons directly from the target of observation. This is a relatively new type
of observation that enables research on previously obscured parts of the
Universe. In particular, it is most applicable to Comparative Planetary
Science, a field that directly attacks such questions as "how common are
planetary systems? What types of planets exist, and are there planets other
than Earth that are capable of supporting life as we know it?" We survey the
scientific motivations for high-contrast observations, provide an overview of
the techniques currently being used or developed, and discuss some ideas and
studies for future prospects.Comment: In press for Annual Review of Astronomy and Astrophysics (Vol. 47).
46 pages, 15 figure
Instability of black hole formation under small pressure perturbations
We investigate here the spectrum of gravitational collapse endstates when
arbitrarily small perfect fluid pressures are introduced in the classic black
hole formation scenario as described by Oppenheimer, Snyder and Datt (OSD) [1].
This extends a previous result on tangential pressures [2] to the more
physically realistic scenario of perfect fluid collapse. The existence of
classes of pressure perturbations is shown explicitly, which has the property
that injecting any smallest pressure changes the final fate of the dynamical
collapse from a black hole to a naked singularity. It is therefore seen that
any smallest neighborhood of the OSD model, in the space of initial data,
contains collapse evolutions that go to a naked singularity outcome. This gives
an intriguing insight on the nature of naked singularity formation in
gravitational collapse.Comment: 7 pages, 1 figure, several modifications to match published version
on GR
Large scale outflows from z ~ 0.7 starburst galaxies identified via ultra-strong MgII quasar absorption lines
(Abridged) Star formation-driven outflows are a critical phenomenon in
theoretical treatments of galaxy evolution, despite the limited ability of
observations to trace them across cosmological timescales. If the strongest
MgII absorption-line systems detected in the spectra of background quasars
arise in such outflows, "ultra-strong" MgII (USMgII) absorbers would identify
significant numbers of galactic winds over a huge baseline in cosmic time, in a
manner independent of the luminous properties of the galaxy. To this end, we
present the first detailed imaging and spectroscopic study of the fields of two
USMgII absorber systems culled from a statistical absorber catalog, with the
goal of understanding the physical processes leading to the large velocity
spreads that define such systems. Each field contains two bright emission-line
galaxies at similar redshift (dv < 300 km/s) to that of the absorption.
Lower-limits on their instantaneous star formation rates (SFR) from the
observed OII and Hb line fluxes, and stellar masses from spectral template
fitting indicate specific SFRs among the highest for their masses at z~0.7.
Additionally, their 4000A break and Balmer absorption strengths imply they have
undergone recent (~0.01 - 1 Gyr) starbursts. The concomitant presence of two
rare phenomena - starbursts and USMgII absorbers - strongly implies a causal
connection. We consider these data and USMgII absorbers in general in the
context of various popular models, and conclude that galactic outflows are
generally necessary to account for the velocity extent of the absorption. We
favour starburst driven outflows over tidally-stripped gas from a major
interaction which triggered the starburst as the energy source for the majority
of systems. Finally, we discuss the implications of these results and speculate
on the overall contribution of such systems to the global SFR density at z~0.7.Comment: 15 pages, 6 figure, accepted for publication by MNRA
The effect of feedback on the emission properties of the Warm-Hot Intergalactic Medium
At present, 30-40 per cent of the baryons in the local Universe is still
undetected. According to theoretical predictions, this gas should reside in
filaments filling the large-scale structure (LSS) in the form of a Warm-Hot
Intergalactic Medium (WHIM), at temperatures of 10^5 - 10^7 K, thus emitting in
the soft X-ray energies via free-free interaction and line emission from heavy
elements. In this work we characterize the properties of the X-ray emission of
the WHIM, and the LSS in general, focusing on the influence of different
physical mechanisms, namely galactic winds (GWs), black-hole feedback and
star-formation, and providing estimates of possible observational constraints.
To this purpose we use a set of cosmological hydrodynamical simulations that
include a self-consistent treatment of star-formation and chemical enrichment
of the intergalactic medium, that allows us to follow the evolution of
different metal species. We construct a set of simulated light-cones to make
predictions of the emission in the 0.3-10 keV energy range. We obtain that GWs
increase by a factor of 2 the emission of both galaxy clusters and WHIM. The
amount of oxygen at average temperature and, consequently, the amount of
expected bright Ovii and Oviii lines is increased by a factor of 3 due to GWs
and by 20 per cent when assuming a top-heavy IMF. We compare our results with
current observational constraints and find that the emission from faint groups
and WHIM should account from half to all of the unresolved X-ray background in
the 1-2 keV band.Comment: 15 pages, 8 figures, 4 tables. Accepted for publication in the MNRAS.
Minor changes after referee repor
Cosmological Black Holes as Seeds of Voids in Galaxy Distribution
Deep surveys indicate a bubbly structure of cosmological large scale which
should be the result of evolution of primordial density perturbations. Several
models have been proposed to explain origin and dynamics of such features but,
till now, no exhaustive and fully consistent theory has been found. We discuss
a model where cosmological black holes, deriving from primordial perturbations,
are the seeds for large-scale-structure voids. We give details of dynamics and
accretion of the system voids-cosmological black holes from the epochs
till now finding that void of of diameter and
under-density of -0.9 will fits the observations without conflicting with the
homogeneity and isotropy of cosmic microwave background radiation.Comment: to appear in Astronomy & Astrophysic
ESO Imaging Survey. The Stellar Catalogue in the Chandra Deep Field South
(abridged) Stellar catalogues in five passbands (UBVRI) over an area of
approximately 0.3 deg^2, comprising about 1200 objects, and in seven passbands
(UBVRIJK) over approximately 0.1 deg^2, comprising about 400 objects, in the
direction of the Chandra Deep Field South are presented.
The 90% completeness level of the number counts is reached at approximately U
= 23.8, B = 24.0, V = 23.5, R = 23.0, I = 21.0, J = 20.5, K = 19.0.
A scheme is presented to select point sources from these catalogues, by
combining the SExtractor parameter CLASS_STAR from all available passbands.
Probable QSOs and unresolved galaxies are identified by using the previously
developed \chi^2-technique (Hatziminaoglou et al 2002), that fits the overall
spectral energy distributions to template spectra and determines the best
fitting template.
The observed number counts, colour-magnitude diagrams, colour-colour diagrams
and colour distributions are presented and, to judge the quality of the data,
compared to simulations based on the predictions of a Galactic Model convolved
with the estimated completeness functions and the error model used to describe
the photometric errors of the data.
The resulting stellar catalogues and the objects identified as likely QSOs
and unresolved galaxies with coordinates, observed magnitudes with errors and
assigned spectral types by the -technique are presented and are
publicly available.Comment: Paper as it will appear in print. Complete figures and tables can be
obtained from: http://www.eso.org/science/eis/eis_pub/eis_pub.html. Astronomy
& Astrophysics, accepted for publicatio
PTHrP on MCF-7 breast cancer cells: a growth factor or an antimitogenic peptide?
Letter to the Edito
Warm stellar matter with deconfinement: application to compact stars
We investigate the properties of mixed stars formed by hadronic and quark
matter in -equilibrium described by appropriate equations of state (EOS)
in the framework of relativistic mean-field theory. We use the non- linear
Walecka model for the hadron matter and the MIT Bag and the Nambu-Jona-Lasinio
models for the quark matter. The phase transition to a deconfined quark phase
is investigated. In particular, we study the dependence of the onset of a mixed
phase and a pure quark phase on the hyperon couplings, quark model and
properties of the hadronic model. We calculate the strangeness fraction with
baryonic density for the different EOS. With the NJL model the strangeness
content in the mixed phase decreases. The calculations were performed for T=0
and for finite temperatures in order to describe neutron and proto-neutron
stars. The star properties are discussed. Both the Bag model and the NJL model
predict a mixed phase in the interior of the star. Maximum allowed masses for
proto-neutron stars are larger for the NJL model ( M)
than for the Bag model ( M).Comment: RevTeX,14 figures, accepted to publication in Physical Review
A Self-Consistent Model for Positronium Formation from Helium Atoms
The differential and total cross sections for electron capture by positrons
from helium atoms are calculated using a first-order distorted wave theory
satisfying the Coulomb boundary conditions. In this formalism a parametric
potential is used to describe the electron screening in a consistent and
realistic manner. The present procedure is self consistent because (i) it
satisfies the correct boundary conditions and post-prior symmetry, and (ii) the
potential and the electron binding energies appearing in the transition
amplitude are consistent with the wave functions describing the collision
system. The results are compared with the other theories and with the available
experimental measurements. At the considered range of collision energies, the
results agree reasonably well with recent experiments and theories.
[Note: This paper will be published on volume 42 of the Brazilian Journal of
Physics
Neutron Stars in Teleparallel Gravity
In this paper we deal with neutron stars, which are described by a perfect
fluid model, in the context of the teleparallel equivalent of general
relativity. We use numerical simulations to find the relationship between the
angular momentum of the field and the angular momentum of the source. Such a
relation was established for each stable star reached by the numerical
simulation once the code is fed with an equation of state, the central energy
density and the ratio between polar and equatorial radii. We also find a regime
where linear relation between gravitational angular momentum and moment of
inertia (as well as angular velocity of the fluid) is valid. We give the
spatial distribution of the gravitational energy and show that it has a linear
dependence with the squared angular velocity of the source.Comment: 19 pages, 14 figures. arXiv admin note: text overlap with
arXiv:1206.331
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