3,143 research outputs found
Carbon burning in intermediate mass primordial stars
The evolution of a zero metallicity 9 M_s star is computed, analyzed and
compared with that of a solar metallicity star of identical ZAMS mass. Our
computations range from the main sequence until the formation of a massive
oxygen-neon white dwarf. Special attention has been payed to carbon burning in
conditions of partial degeneracy as well as to the subsequent thermally pulsing
Super-AGB phase. The latter develops in a fashion very similar to that of a
solar metallicity 9 M_s star, as a consequence of the significant enrichment in
metals of the stellar envelope that ensues due to the so-called third dredge-up
episode. The abundances in mass of the main isotopes in the final ONe core
resulting from the evolution are X(^{16}O) approx 0.59, X(^{20}Ne) approx 0.28
and X(^{24}Mg) approx 0.05. This core is surrounded by a 0.05 M_s buffer mainly
composed of carbon and oxygen, and on top of it a He envelope of mass 10^{-4}
M_sComment: 11 pages, 11 figures, accepted for publication in A&
Evolution and CNO yields of Z=10^-5 stars and possible effects on CEMP production
Our main goals are to get a deeper insight into the evolution and final fates
of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to
investigate their C, N, and O yields. Using the Monash University Stellar
Evolution code we computed and analysed the evolution of stars of metallicity Z
= 10^-5 and masses between 4 and 9 M_sun, from their main sequence until the
late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Our
model stars experience a strong C, N, and O envelope enrichment either due to
the second dredge-up, the dredge-out phenomenon, or the third dredge-up early
during the TP-(S)AGB phase. Their late evolution is therefore similar to that
of higher metallicity objects. When using a standard prescription for the mass
loss rates during the TP-(S)AGB phase, the computed stars lose most of their
envelopes before their cores reach the Chandrasekhar mass, so our standard
models do not predict the occurrence of SNI1/2 for Z = 10^-5 stars. However, we
find that the reduction of only one order of magnitude in the mass-loss rates,
which are particularly uncertain at this metallicity, would prevent the
complete ejection of the envelope, allowing the stars to either explode as an
SNI1/2 or become an electron-capture SN. Our calculations stop due to an
instability near the base of the convective envelope that hampers further
convergence and leaves remnant envelope masses between 0.25 M_sun for our 4
M_sun model and 1.5 M_sun for our 9 M_sun model. We present two sets of C, N,
and O yields derived from our full calculations and computed under two
different assumptions, namely, that the instability causes a practically
instant loss of the remnant envelope or that the stars recover and proceed with
further thermal pulses. Our results have implications for the early chemical
evolution of the Universe.Comment: 12 pages, 13 figures, accepted for publication in A&
Bipolar querying of valid-time intervals subject to uncertainty
Databases model parts of reality by containing data representing properties of real-world objects or concepts. Often, some of these properties are time-related. Thus, databases often contain data representing time-related information. However, as they may be produced by humans, such data or information may contain imperfections like uncertainties. An important purpose of databases is to allow their data to be queried, to allow access to the information these data represent. Users may do this using queries, in which they describe their preferences concerning the data they are (not) interested in. Because users may have both positive and negative such preferences, they may want to query databases in a bipolar way. Such preferences may also have a temporal nature, but, traditionally, temporal query conditions are handled specifically. In this paper, a novel technique is presented to query a valid-time relation containing uncertain valid-time data in a bipolar way, which allows the query to have a single bipolar temporal query condition
TransverseDiff gravity is to scalar-tensor as unimodular gravity is to General Relativity
Transverse Diffeomorphism (TDiff) theories are well-motivated theories of
gravity from the quantum perspective, which are based upon a gauge symmetry
principle. The main contribution of this work is to firmly establish a
correspondence between TransverseDiff and the better-known scalar-tensor
gravity --- in its more general form ---, a relation which is completely
analogous to that between unimodular gravity and General Relativity. We then
comment on observational aspects of TDiff. In connection with this proof, we
derive a very general rule that determines under what conditions the procedure
of fixing a gauge symmetry can be equivalently applied before the variational
principle leading to the equations of motion, as opposed to the standard
procedure, which takes place afterwards; this rule applies to gauge-fixing
terms without derivatives.Comment: 10 pages; amsart style; v3: version as appeared in JCAP, redaction
improve
Observables, gauge invariance, and the role of the observers in the limit from general relativity to special relativity
Some conceptual issues concerning general invariant theories, with special
emphasis on general relativity, are analyzed. The common assertion that
observables must be required to be gauge invariant is examined in the light of
the role played by a system of observers. Some features of the reduction of the
gauge group are discussed, including the fact that in the process of a partial
gauge fixing the reduction at the level of the gauge group and the reduction at
the level of the variational principle do not commute. Distinctions between the
mathematical and the physical concept of gauge symmetry are discussed and
illustrated with examples. The limit from general relativity to special
relativity is considered as an example of a gauge group reduction that is
allowed in some specific physical circumstances. Whether and when the
Poincar\'e group must be considered as a residual gauge group will come out as
a result of our analysis, that applies, in particular, to asymptotically flat
spaces.Comment: 17 page
Neutrino Opacities in Neutron Stars with Kaon Condensates
The neutrino mean free paths in hot neutron-star matter are obtained in the
presence of kaon condensates. The kaon-induced neutrino absorption process,
which is allowed only in the presence of kaon condensates, is considered for
both nondegenerate and degenerate neutrinos. The neutrino mean free path due to
this process is compared with that for the neutrino-nucleon scattering. While
the mean free path for the kaon-induced neutrino absorption process is shown to
be shorter than the ordinary two-nucleon absorption process by several orders
of magnitude when temperature is not very high, the neutrino-nucleon scattering
process has still a dominant contribution to the neutrino opacity. Thus, the
kaon-induced neutrino absorption process has a minor effect on the thermal and
dynamical evolution of protoneutron stars.Comment: 35 pages, 4 figure
Color-Neutral Superconducting Quark Matter
We investigate the consequences of enforcing local color neutrality on the
color superconducting phases of quark matter by utilizing the
Nambu-Jona-Lasinio model supplemented by diquark and the t'Hooft six-fermion
interactions. In neutrino free matter at zero temperature, color neutrality
guarantees that the number densities of u, d, and s quarks in the
Color-Flavor-Locked (CFL) phase will be equal even with physical current quark
masses. Electric charge neutrality follows as a consequence and without the
presence of electrons. In contrast, electric charge neutrality in the less
symmetric 2-flavor superconducting (2SC) phase with ud pairing requires more
electrons than the normal quark phase. The free energy density cost of
enforcing color and electric charge neutrality in the CFL phase is lower than
that in the 2SC phase, which favors the formation of the CFL phase. With
increasing temperature and neutrino content, an unlocking transition occurs
from the CFL phase to the 2SC phase with the order of the transition depending
on the temperature, the quark and lepton number chemical potentials. The
astrophysical implications of this rich structure in the phase diagram,
including estimates of the effects from Goldstone bosons in the CFL phase, are
discussed.Comment: 20 pages, 4 figures; version to appear in Phys. Rev.
{SelfPose}: {3D} Egocentric Pose Estimation from a Headset Mounted Camera
We present a solution to egocentric 3D body pose estimation from monocular
images captured from downward looking fish-eye cameras installed on the rim of
a head mounted VR device. This unusual viewpoint leads to images with unique
visual appearance, with severe self-occlusions and perspective distortions that
result in drastic differences in resolution between lower and upper body. We
propose an encoder-decoder architecture with a novel multi-branch decoder
designed to account for the varying uncertainty in 2D predictions. The
quantitative evaluation, on synthetic and real-world datasets, shows that our
strategy leads to substantial improvements in accuracy over state of the art
egocentric approaches. To tackle the lack of labelled data we also introduced a
large photo-realistic synthetic dataset. xR-EgoPose offers high quality
renderings of people with diverse skintones, body shapes and clothing,
performing a range of actions. Our experiments show that the high variability
in our new synthetic training corpus leads to good generalization to real world
footage and to state of theart results on real world datasets with ground
truth. Moreover, an evaluation on the Human3.6M benchmark shows that the
performance of our method is on par with top performing approaches on the more
classic problem of 3D human pose from a third person viewpoint.Comment: 14 pages. arXiv admin note: substantial text overlap with
arXiv:1907.1004
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