40,225 research outputs found
Quantum Phase Transitions in the Interacting Boson Model: Integrability, level repulsion and level crossing
We study the quantum phase transition mechanisms that arise in the
Interacting Boson Model. We show that the second-order nature of the phase
transition from U(5) to O(6) may be attributed to quantum integrability,
whereas all the first-order phase transitions of the model are due to level
repulsion with one singular point of level crossing. We propose a model
Hamiltonian with a true first-order phase transition for finite systems due to
level crossings.Comment: Accepted in PR
High-precision acoustic helium signatures in 18 low-mass low-luminosity red giants. Analysis from more than four years of Kepler observations
High-precision frequencies of acoustic modes in red giant stars are now
available thanks to the long observing length and high-quality of the light
curves provided by the NASA Kepler mission, thus allowing to probe the interior
of evolved cool low-mass stars with unprecedented level of detail. We
characterize the acoustic signature of the helium second ionization zone in a
sample of 18 low-mass low-luminosity red giants by exploiting new mode
frequency measurements derived from more than four years of Kepler
observations. We analyze the second frequency differences of radial acoustic
modes in all the stars of the sample by using the Bayesian code Diamonds. We
find clear acoustic glitches due to the signature of helium second ionization
in all the stars of the sample. We measure the acoustic depth and the
characteristic width of the acoustic glitches with a precision level on average
around 2% and 8%, respectively. We find good agreement with
theoretical predictions and existing measurements from the literature. Lastly,
we derive the amplitude of the glitch signal at for the
second differences and for the frequencies with an average precision of
6%, obtaining values in the range 0.14-0.24 Hz, and 0.08-0.33
Hz, respectively, which can be used to investigate the helium abundance in
the stars.Comment: 12 pages, 19 figures, 3 tables. Accepted for publication in A&
Bayesian peak bagging analysis of 19 low-mass low-luminosity red giants observed with Kepler
The currently available Kepler light curves contain an outstanding amount of
information but a detailed analysis of the individual oscillation modes in the
observed power spectra, also known as peak bagging, is computationally
demanding and challenging to perform on a large number of targets. Our intent
is to perform for the first time a peak bagging analysis on a sample of 19
low-mass low-luminosity red giants observed by Kepler for more than four years.
This allows us to provide high-quality asteroseismic measurements that can be
exploited for an intensive testing of the physics used in stellar structure
models, stellar evolution and pulsation codes, as well as for refining existing
asteroseismic scaling relations in the red giant branch regime. For this
purpose, powerful and sophisticated analysis tools are needed. We exploit the
Bayesian code Diamonds, using an efficient nested sampling Monte Carlo
algorithm, to perform both a fast fitting of the individual oscillation modes
and a peak detection test based on the Bayesian evidence. We find good
agreement for the parameters estimated in the background fitting phase with
those given in the literature. We extract and characterize a total of 1618
oscillation modes, providing the largest set of detailed asteroseismic mode
measurements ever published. We report on the evidence of a change in regime
observed in the relation between linewidths and effective temperatures of the
stars occurring at the bottom of the RGB. We show the presence of a linewidth
depression or plateau around for all the red giants of the
sample. Lastly, we show a good agreement between our measurements of maximum
mode amplitudes and existing maximum amplitudes from global analyses provided
in the literature, useful as empirical tools to improve and simplify the future
peak bagging analysis on a larger sample of evolved stars.Comment: 78 pages, 46 figures, 22 tables. Accepted for publication in A&
A population synthesis study of the luminosity function of hot white dwarfs
We present a coherent and detailed Monte Carlo simulation of the population
of hot white dwarfs. We assess the statistical significance of the hot end of
the white dwarf luminosity function and the role played by the bolometric
corrections of hydrogen-rich white dwarfs at high effective temperatures. We
use the most up-to-date stellar evolutionary models and implement a full
description of the observational selection biases to obtain realistic
simulations of the observed white dwarf population. Our theoretical results are
compared with the luminosity function of hot white dwarfs obtained from the
Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. We find
that the theoretical results are in excellent agreement with the observational
data for the population of white dwarfs with hydrogen deficient atmospheres
(non-DA white dwarfs). For the population of white dwarfs with hydrogen-rich
atmospheres (white dwarfs of the DA class), our simulations show some
discrepancies with the observations for the brightest luminosity bins. These
discrepancies can be attributed to the way in which the masses of the white
dwarfs contributing to this luminosity bin have been computed, as most of them
have masses smaller than the theoretical lower limit for carbon-oxygen white
dwarfs. We conclude that the way in which the observational luminosity function
of hot white dwarfs is obtained is very sensitive to the particular
implementation of the method used to derive the masses of the sample. We also
provide a revised luminosity function for hot white dwarfs with hydrogen-rich
atmospheres.Comment: 6 pages, 5 figures, accepted for publication in A&
Routh reduction and the class of magnetic Lagrangian systems
In this paper, some new aspects related to Routh reduction of Lagrangian
systems with symmetry are discussed. The main result of this paper is the
introduction of a new concept of transformation that is applicable to systems
obtained after Routh reduction of Lagrangian systems with symmetry, so-called
magnetic Lagrangian systems. We use these transformations in order to show
that, under suitable conditions, the reduction with respect to a (full)
semi-direct product group is equivalent to the reduction with respect to an
Abelian normal subgroup. The results in this paper are closely related to the
more general theory of Routh reduction by stages.Comment: 23 page
Time-dependent Photoionization of Gaseous Nebulae: the Pure Hydrogen Case
We study the problem of time-dependent photoionization of low density gaseous
nebulae subjected to sudden changes in the intensity of ionizing radiation. To
this end, we write a computer code that solves the full time-dependent energy
balance, ionization balance, and radiation transfer equations in a
self-consistent fashion for a simplified pure hydrogen case. It is shown that
changes in the ionizing radiation yield ionization/thermal fronts that
propagate through the cloud, but the propagation times and response times to
such fronts vary widely and non-linearly from the illuminated face of the cloud
to the ionization front (IF). Ionization/thermal fronts are often supersonic,
and in slabs initially in pressure equilibrium such fronts yield large pressure
imbalances that are likely to produce important dynamical effects in the cloud.
Further, we studied the case of periodic variations in the ionizing flux. It
is found that the physical conditions of the plasma have complex behaviors that
differ from any steady-state solutions. Moreover, even the time average
ionization and temperature is different from any steady-state case. This time
average is characterized by over-ionization and a broader IF with respect to
the steady-state solution for a mean value of the radiation flux. Around the
time average of physical conditions there is large dispersion in instantaneous
conditions, particularly across the IF, which increases with the period of
radiation flux variations. Moreover, the variations in physical conditions are
asynchronous along the slab due to the combination of non-linear propagation
times for thermal/ionization fronts and equilibration times.Comment: Accepted for publication in ApJ. 36 pages, 12 figure
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