419 research outputs found
Constant entropy hybrid stars: a first approximation of cooling evolution
We aim to study the possibility of a hadron-quark phase transition in the
interior of neutron stars, taking into account different schematic evolutionary
stages at finite temperature. We also discuss the strange quark matter
stability in the quark matter phase. Furthermore, we aim to analyze the
astrophysical properties of hot and cold hybrid stars, considering the
constraint on maximum mass given by the pulsars J1614-2230 and J0348+0432. We
have developed a computational code to construct semi-analytical hybrid
equations of state at fixed entropy per baryon and to obtain different families
of hybrid stars. An analytical approximation of the Field Correlator Method is
developed for the quark matter equation of state. For the hadronic equation of
state we use a table based on the relativistic mean field theory, without
hyperons. We solved the relativistic structure equations of hydrostatic
equilibrium and mass conservation for hybrid star configurations. For the
different equations of state obtained, we calculated the stability window for
the strange quark matter, lepton abundances, temperature profiles and contours
profiles for the maximum mass star depending on the parameters of the Field
Correlator Method. We also computed the mass-radius and gravitational
mass-baryonic mass relationships for different hybrid star families. We have
analyzed different stages of hot hybrid stars as a first approximation of the
cooling evolution of neutron stars with quark matter cores. We obtain cold
hybrid stars with maximum masses for different combinations of
the Field Correlator Method parameters. In addition, our study based on the
gravitational mass - baryonic mass plane shows a late phase transition between
hadronic and quark matter during the proto-hybrid star evolution, in contrast
with previous studies of proto-neutron stars.Comment: 12 pages, 14 figures, published in A&A 601, A21 (2017
Magnetars as Highly Magnetized Quark Stars: an analytical treatment
We present an analytical model of a magnetar as a high density magnetized
quark bag. The effect of strong magnetic fields (B > 5 x 10^{16} G) in the
equation of state is considered. An analytic expression for the Mass-Radius
relationship is found from the energy variational principle in general
relativity. Our results are compared with observational evidences of possible
quark and/or hybrid stars.Comment: 15 pages, 1 figure/Comments added and two references removed.
Accepted for publication in Astrophysical Journa
The Tropics as Reservoir of Otherwise Extinct Mammals: The Case of Rodents from a New Pliocene Faunal Assemblage from Northern Venezuela
We report a new vertebrate assemblage from the Pliocene Vergel Member of the San Gregorio Formation in northwestern Venezuela, which includes Crocodylia and Testudines indet., toxodonts, at least four species of xenarthrans of the Dasypodidae, Pampatheriidae, Glyptodontidae and Megatheriidae, and rodents. The last are Cardiatherium, cf. Caviodon (Hydrochoeridae), Neoepiblema (Neoepiblemidae), and what is here described as a new genus of a low-crowned octodontoid. cf. Caviodon is the first cardiomyine for northern South America. The rodent assemblage resembles in its ecological composition those of the late Miocene (Huayquerian) from the "Mesopotamian” of Argentina and the Acre region in Brazil, with partially overlapping systematic composition. The stratigraphic position of the San Gregorio Formation and mammals other than caviomorphs suggest a late Pliocene age for these sediments, implying the endurance of rodent taxa beyond their biochron in southern South Americ
First record of <i>Banderomys leanzai Kramarz</i>, 2005 (Rodentia, Caviomorpha) in Chubut Province, Patagonia (Argentina)
Cephalomyidae was established in the XIX century, and was considered by different authors as a heterogeneous group of hypsodont South American hystricognath rodents. Other authors corroborated this family as a natural group among caviomorphs and recognized some taxa recorded from the Deseadan-Colhuehuapian South American Land Mammals Ages (SALMAs). Among them, Banderomys leanzai (the only species of this genus) was, until now, characteristic and exclusive of the locality of Cerro Bandera (early Miocene, Colhuehuapian?, Neuquén Province, Argentina). The aim of this work is to report the first record of B. leanzai outside the type locality; the new specimens come from the early Miocene of central Chubut Province (Patagonia, Argentina). This new finding extends the geographical distribution of the species and increases the dental morphology knowledge of B. leanzai.Facultad de Ciencias Naturales y Muse
Testing theories that predict time variation of fundamental constants
We consider astronomical and local bounds on time variation of fundamental
constants to test some generic Kaluza-Klein-like models and some particular
cases of Beckenstein theory. Bounds on the free parameters of the different
theories are obtained. Furthermore, we find that none of the proposed models,
is able to explain recent results [Webb99,Webb00] claiming an observed
variation of the fine structure constant from quasar absorption systems at
redshifts .Comment: 18 pages, 2 figures, new data adde
Gravitational instabilities in Kerr space-times
In this paper we consider the possible existence of unstable axisymmetric
modes in Kerr space times, resulting from exponentially growing solutions of
the Teukolsky equation. We describe a transformation that casts the radial
equation that results upon separation of variables in the Teukolsky equation,
in the form of a Schr\"odinger equation, and combine the properties of the
solutions of this equations with some recent results on the asymptotic
behaviour of spin weighted spheroidal harmonics to prove the existence of an
infinite family of unstable modes. Thus we prove that the stationary region
beyond a Kerr black hole inner horizon is unstable under gravitational linear
perturbations. We also prove that Kerr space-time with angular momentum larger
than its square mass, which has a naked singularity, is unstable.Comment: 9 pages, 4 figures, comments, references and calculation details
added, asymptotic expansion typos fixe
Early Universe Constraints on Time Variation of Fundamental Constants
We study the time variation of fundamental constants in the early Universe.
Using data from primordial light nuclei abundances, CMB and the 2dFGRS power
spectrum, we put constraints on the time variation of the fine structure
constant , and the Higgs vacuum expectation value leads to a variation
in the electron mass, among other effects. Along the same line, we study the
variation of and the electron mass . In a purely phenomenological
fashion, we derive a relationship between both variations.Comment: 18 pages, 12 figures, accepted for publication in Physical Review
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