26,707 research outputs found
A Study of Cool White Dwarfs in the Sloan Digital Sky Survey Data Release 12
In this work we study white dwarfs where to compare the differences in the
cooling of DAs and non-DAs and their formation channels. Our final sample is
composed by nearly DAs and more than non-DAs that are
simultaneously in the SDSS DR12 spectroscopic database and in the \textit{Gaia}
survey DR2. We present the mass distribution for DAs, DBs and DCs, where it is
found that the DCs are more massive than DAs and
DBs on average. Also we present the photometric effective temperature
distribution for each spectral type and the distance distribution for DAs and
non-DAs. In addition, we study the ratio of non-DAs to DAs as a function of
effective temperature. We find that this ratio is around for
effective temperature above and increases by a factor
of five for effective temperature cooler than . If we assume
that the increase of non-DA stars between to
is due to convective dilution, per cent of
the DAs should turn into non-DAs to explain the observed ratio. Our
determination of the mass distribution of DCs also agrees with the theory that
convective dilution and mixing are more likely to occur in massive white
dwarfs, which supports evolutionary models and observations suggesting that
higher mass white dwarfs have thinner hydrogen layers.Comment: 9 pages, 10 figures, accepted by MNRA
Microquasar models for 3EG J1828+0142 and 3EG J1735-1500
Microquasars are promising candidates to emit high-energy gamma-rays.
Moreover, statistical studies show that variable EGRET sources at low galactic
latitudes could be associated with the inner spiral arms. The variable nature
and the location in the Galaxy of the high-mass microquasars, concentrated in
the galactic plane and within 55 degrees from the galactic center, give to
these objects the status of likely counterparts of the variable low-latitude
EGRET sources. We consider in this work the two most variable EGRET sources at
low-latitudes: 3EG J1828+0142 and 3EG J1735-1500, proposing a microquasar model
to explain the EGRET data in consistency with the observations at lower
energies (from radio frequencies to soft gamma-rays) within the EGRET error
box.Comment: (1)Universitat de Barcelona, (2)Instituto Argentino de
Radioastronomia (3) Facultad de Ciencias Astronomicas y Geofisicas
(4)Lawrence Livermore National Laboratory 6 pages, 2 figures. Presented as a
poster at the V Microquasar Workshop, Beijing, June 2004. Accepted for
publication in the Chinese Journal of Astronomy & Astrophysic
The seismic properties of low-mass He-core white dwarf stars
We present here a detailed pulsational study applied to low-mass He-core
white dwarfs, based on full evolutionary models representative of these
objects. The background stellar models on which our pulsational analysis was
carried out were derived by taking into account the complete evolutionary
history of the progenitor stars, with special emphasis on the diffusion
processes acting during the white dwarf cooling phase. We computed nonradial
-modes to assess the dependence of the pulsational properties of these
objects with stellar parameters such as the stellar mass and the effective
temperature, and also with element diffusion processes. We also performed a g-
and p-mode pulsational stability analysis on our models and found well-defined
blue edges of the instability domain, where these stars should start to exhibit
pulsations. We found substantial differences in the seismic properties of white
dwarfs with and the extremely low-mass (ELM) white
dwarfs (). Specifically, -mode pulsation modes
in ELM white dwarfs mainly probe the core regions and are not dramatically
affected by mode-trapping effects by the He/H interface, whereas the opposite
is true for more massive He-core white dwarfs. We found that element diffusion
processes substantially affects the shape of the He/H chemical transition
region, leading to non-negligible changes in the period spectrum of low-mass
white dwarfs. Our stability analysis successfully predicts the pulsations of
the only known variable low-mass white dwarf (SDSS J184037.78+642312.3), and
also predicts both - and -mode pulsational instabilities in a significant
number of known low-mass and ELM white dwarfs.Comment: 14 pages, 15 figures, 2 tables. To be published in Astronomy &
Astrophysic
The sdA problem - II. Photometric and Spectroscopic Follow-up
Subdwarf A star (sdA) is a spectral classification given to objects showing
H-rich spectra and sub-main sequence surface gravities, but effective
temperature lower than the zero-age horizontal branch. Their evolutionary
origin is an enigma. In this work, we discuss the results of follow-up
observations of selected sdAs. We obtained time resolved spectroscopy for 24
objects, and time-series photometry for another 19 objects. For two targets, we
report both spectroscopy and photometry observations. We confirm seven objects
to be new extremely-low mass white dwarfs (ELMs), one of which is a known
eclipsing star. We also find the eighth member of the pulsating ELM class.Comment: Accepted for publication in MNRAS. 19 pages, 30 figures, 6 table
An independent constraint on the secular rate of variation of the gravitational constant from pulsating white dwarfs
A secular variation of the gravitational constant modifies the structure and
evolutionary time scales of white dwarfs. Using an state-of-the-art stellar
evolutionary code and an up-to-date pulsational code we compute the effects of
a secularly varying on the pulsational properties of variable white dwarfs.
Comparing the the theoretical results obtained taking into account the effects
of a running with the observed periods and measured rates of change of the
periods of two well studied pulsating white dwarfs, G117--B15A and R548, we
place constraints on the rate of variation of Newton's constant. We derive an
upper bound yr using the variable
white dwarf G117--B15A, and yr using
R548. Although these upper limits are currently less restrictive than those
obtained using other techniques, they can be improved in a future measuring the
rate of change of the period of massive white dwarfs.Comment: 13 pages, 4 tables, 3 figures. To be published in the Journal of
Cosmology and Astroparticle Physic
Beyond mean-field bistability in driven-dissipative lattices: bunching-antibunching transition and quantum simulation
In the present work we investigate the existence of multiple nonequilibrium
steady states in a coherently driven XY lattice of dissipative two-level
systems. A commonly used mean-field ansatz, in which spatial correlations are
neglected, predicts a bistable behavior with a sharp shift between low- and
high-density states. In contrast one-dimensional matrix product methods reveal
these effects to be artifacts of the mean-field approach, with both
disappearing once correlations are taken fully into account. Instead, a
bunching-antibunching transition emerges. This indicates that alternative
approaches should be considered for higher spatial dimensions, where classical
simulations are currently infeasible. Thus we propose a circuit QED quantum
simulator implementable with current technology to enable an experimental
investigation of the model considered
Dynamics of Entanglement Transfer Through Multipartite Dissipative Systems
We study the dynamics of entanglement transfer in a system composed of two
initially correlated three-level atoms, each located in a cavity interacting
with its own reservoir. Instead of tracing out reservoir modes to describe the
dynamics using the master equation approach, we consider explicitly the
dynamics of the reservoirs. In this situation, we show that the entanglement is
completely transferred from atoms to reservoirs. Although the cavities mediate
this entanglement transfer, we show that under certain conditions, no
entanglement is found in cavities throughout the dynamics. Considering the
entanglement dynamics of interacting and non-interacting bipartite subsystems,
we found time windows where the entanglement can only flow through interacting
subsystems, depending on the system parameters.Comment: 8 pages, 11 figures, publishe in Physical Review
Chromaticity effects in microlensing by wormholes
Chromaticity effects introduced by the finite source size in microlensing
events by presumed natural wormholes are studied. It is shown that these
effects provide a specific signature that allow to discriminate between
ordinary and negative mass lenses through the spectral analysis of the
microlensing events. Both galactic and extragalactic situations are discussed.Comment: To appear in Modern Physics Letters A, 200
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