3,525 research outputs found
On the occurrence and detectability of Bose-Einstein condensation in helium white dwarfs
It has been recently proposed that helium white dwarfs may provide promising
conditions for the occurrence of the Bose-Einstein condensation. The argument
supporting this expectation is that in some conditions attained in the core of
these objects, the typical De Broglie wavelength associated with helium nuclei
is of the order of the mean distance between neighboring nuclei. In these
conditions the system should depart from classical behavior showing quantum
effects. As helium nuclei are bosons, they are expected to condense.
In order to explore the possibility of detecting the Bose-Einstein
condensation in the evolution of helium white dwarfs we have computed a set of
models for a variety of stellar masses and values of the condensation
temperature. We do not perform a detailed treatment of the condensation process
but mimic it by suppressing the nuclei contribution to the equation of state by
applying an adequate function. As the cooling of white dwarfs depends on
average properties of the whole stellar interior, this procedure should be
suitable for exploring the departure of the cooling process from that predicted
by the standard treatment.
We find that the Bose-Einstein condensation has noticeable, but not dramatic
effects on the cooling process only for the most massive white dwarfs
compatible with a helium dominated interior (\approx 0.50 M_\odot) and very low
luminosities (say, Log(L/L_\odot) < -4.0). These facts lead us to conclude that
it seems extremely difficult to find observable signals of the Bose-Einstein
condensation.
Recently, it has been suggested that the population of helium white dwarfs
detected in the globular cluster NGC 6397 is a good candidate for detecting
signals of the Bose-Einstein condensation. We find that these stars have masses
too low and are too bright to have an already condensed interior.Comment: 13 pages, 3 figures, 2 tables, accepted for publication in the
Journal of Cosmology and Astroparticle Physics (JCAP
An evolutionary model for the gamma-ray system PSR J1311-3430 and its companion
The most recent member of the millisecond pulsar with very low-mass
companions and short orbital periods class, PSR J1311-3430 (Pletsch et al.
2012) is a remarkable object in various senses. Besides being the first
discovered in gamma-rays, its measured features include the very low or absent
hydrogen content. We show in this Letter that this important piece of
information leads to a very restricted range of initial periods for a given
donor mass. For that purpose, we calculate in detail the evolution of the
binary system self-consistently, including mass transfer and evaporation,
finding the features of the new evolutionary path leading to the observed
configuration. It is also important to remark that the detailed evolutionary
history of the system naturally leads to a high final pulsar mass, as it seems
to be demanded by observations.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS
Letter
The Quasi-Roche lobe overflow state in the evolution of Close Binary Systems containing a radio pulsar
We study the evolution of close binary systems formed by a normal (solar
composition), intermediate mass donor star together with a neutron star. We
consider models including irradiation feedback and evaporation. These
non-standard ingredients deeply modify the mass transfer stages of these
binaries. While models that neglect irradiation feedback undergo continuous,
long standing mass transfer episodes, models including these effect suffer a
number cycles of mass transfer and detachment. During mass transfer the systems
should reveal themselves as low-mass X-ray binaries (LMXBs), whereas when
detached they behave as a binary radio pulsars. We show that at these stages
irradiated models are in a Roche lobe overflow (RLOF) state or in a quasi-RLOF
state. Quasi-RLOF stars have a radius slightly smaller than its Roche lobe.
Remarkably, these conditions are attained for orbital period and donor mass
values in the range corresponding to a family of binary radio pulsars known as
"redbacks". Thus, redback companions should be quasi-RLOF stars. We show that
the characteristics of the redback system PSR J1723-2837 are accounted for by
these models.
In each mass transfer cycle these systems should switch from LMXB to binary
radio pulsar states with a timescale of \sim million years. However, there is
recent and fast growing evidence of systems switching on far shorter, human
timescales. This should be related to instabilities in the accretion disc
surrounding the neutron star and/or radio ejection, still to be included in the
model having the quasi-RLOF state as a general condition.Comment: 27 pages, 7 figures. Accepted for publication in The Astrophysical
Journa
Positive operator valued measures covariant with respect to an irreducible representation
Given an irreducible representation of a group G, we show that all the
covariant positive operator valued measures based on G/Z, where Z is a central
subgroup, are described by trace class, trace one positive operators.Comment: 9 pages, Latex2
Group Theoretical Quantum Tomography
The paper is devoted to the mathematical foundation of the quantum tomography
using the theory of square-integrable representations of unimodular Lie groups.Comment: 13 pages, no figure, Latex2e. Submitted to J.Math.Phy
PREDICTION MODELS FOR FREESTYLE PERFORMANCE TIMES IN MASTER SWIMMERS
This study was designed to define the most important factors to predict freestyle performance times in 135 elite master swimmers by prediction models which include age, anthropometric and strength variables. To cross validate these equations found for Elite swimmers, we used a group composed by 126 lower technical level age - and experience - matched master swimmers. Results demonstrated that age, height and hand grip strength were the best predictors in short events, whereas age and height predict middle and long events. The corresponding coefficients of determination (R2) of performance times were 0.84 in 50m, 0.73 in 100m, 0.75 in 200m, 0.66 in 400m and 0.63 in the 800m events. A good correlation have been found when these models have been applied in 126 non-elite master swimmers demonstrating to be useful in all Master swimmers
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