9,423 research outputs found
Testing excitation models of rapidly oscillating Ap stars with interferometry
Rapidly oscillating Ap stars are unique objects in the potential they offer
to study the interplay between a number of important physical phenomena, in
particular, pulsations, magnetic fields, diffusion, and convection.
Nevertheless, the simple understanding of how the observed pulsations are
excited in these stars is still in progress. In this work we perform a test to
what is possibly the most widely accepted excitation theory for this class of
stellar pulsators. The test is based on the study of a subset of members of
this class for which stringent data on the fundamental parameters are available
thanks to interferometry. For three out of the four stars considered in this
study, we find that linear, non-adiabatic models with envelope convection
suppressed around the magnetic poles can reproduce well the frequency region
where oscillations are observed. For the fourth star in our sample no agreement
is found, indicating that a new excitation mechanism must be considered. For
the three stars whose observed frequencies can be explained by the excitation
models under discussion, we derive the minimum angular extent of the region
where convection must be suppressed. Finally, we find that the frequency
regions where modes are expected to be excited in these models is very
sensitive to the stellar radius. This opens the interesting possibility of
determining this quantity and related ones, such as the effective temperature
or luminosity, from comparison between model predictions and observations, in
other targets for which these parameters are not well determined.Comment: Accepted for publication in the MNRA
Probing tiny convective cores with the acoustic modes of lowest degree
Solar-like oscillations are expected to be excited in stars of up to about
1.6 solar masses. Most of these stars will have convective cores during their
Main-sequence evolution. At the edges of these convective cores there is a
rapid variation in the sound speed which influences the frequencies of acoustic
oscillations. In this paper we build on earlier work by Cunha and Metcalfe, to
investigate further the impact that these rapid structural variations have on
different p-mode frequency combinations, involving modes of low degree. In
particular, we adopt a different expression to describe the sound speed
variation at the edge of the core, which we show to reproduce more closely the
profiles derived from the equilibrium models. We analyse the impact of this
change on the frequency perturbation derived for radial modes. Moreover, we
consider three different small frequency separations involving, respectively,
modes of degree l = 0, 1, 2, 3; l = 0, 1; and l = 0, 2, and show that they are
all significantly affected by the sharp sound speed variation at the edge of
the core. In particular, we confirm that the frequency derivative of the
diagnostic tool that combines modes of degree up to 3 can potentially be used
to infer directly the amplitude of the relative sound speed variation at the
edge of the core. Concerning the other two diagnostic tools, we show that at
high frequencies they can be up to a few microhertzs smaller than what would be
expected in the absence of the rapid structural variation at the edge of the
core. Also, we show that the absolute values of their frequency derivatives are
significantly increased, in a manner that is strongly dependent on stellar age.Comment: 7 pages. submitted to A&
Topological mass generation to antisymmetric tensor matter field
We propose a mechanism to give mass to tensor matter field which preserve the
U(1) symmetry. We introduce a complex vector field that couples with the tensor
in a topological term. We also analyze the influence of the kinetic terms of
the complex vector in our mechanism.Comment: 5 pages, to appear in Europhysics Letter
Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures
The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure
Measuring the extent of convective cores in low-mass stars using Kepler data: towards a calibration of core overshooting
Our poor understanding of the boundaries of convective cores generates large
uncertainties on the extent of these cores and thus on stellar ages. Our aim is
to use asteroseismology to consistently measure the extent of convective cores
in a sample of main-sequence stars whose masses lie around the mass-limit for
having a convective core. We first test and validate a seismic diagnostic that
was proposed to probe in a model-dependent way the extent of convective cores
using the so-called ratios, which are built with and
modes. We apply this procedure to 24 low-mass stars chosen among Kepler targets
to optimize the efficiency of this diagnostic. For this purpose, we compute
grids of stellar models with both the CESAM2k and MESA evolution codes, where
the extensions of convective cores are modeled either by an instantaneous
mixing or as a diffusion process. Among the selected targets, we are able to
unambiguously detect convective cores in eight stars and we obtain seismic
measurements of the extent of the mixed core in these targets with a good
agreement between the CESAM2k and MESA codes. By performing optimizations using
the Levenberg-Marquardt algorithm, we then obtain estimates of the amount of
extra-mixing beyond the core that is required in CESAM2k to reproduce seismic
observations for these eight stars and we show that this can be used to propose
a calibration of this quantity. This calibration depends on the prescription
chosen for the extra-mixing, but we find that it should be valid also for the
code MESA, provided the same prescription is used. This study constitutes a
first step towards the calibration of the extension of convective cores in
low-mass stars, which will help reduce the uncertainties on the ages of these
stars.Comment: 27 pages, 15 figures, accepted in A&
Fluorine in AGB Carbon Stars Revisited
A reanalysis of the fluorine abundance in three Galactic AGB carbon stars (TX
Psc, AQ Sgr and R Scl) has been performed from the molecular HF (1-0) R9 line
at 2.3358 m. High-resolution (R) and high signal to noise
spectra obtained with the CRIRES spectrograph and the VLT telescope or from the
NOAO archive (for TX Psc) have been used. Our abundance analysis uses the
latest generation of MARCS model atmospheres for cool carbon rich stars. Using
spectral synthesis in LTE we derive for these stars fluorine abundances that
are systematically lower by dex in average with respect to the sole
previous estimates by Jorissen, Smith & Lambert (1992). The possible reasons of
this discrepancy are explored. We conclude that the difference may rely on the
blending with C-bearing molecules (CN and C) that were not properly taken
into account in the former study. The new F abundances are in better agreement
with the prediction of full network stellar models of low mass AGB stars. These
models also reproduce the -process elements distribution in the sampled
stars. This result, if confirmed in a larger sample of AGB stars, might
alleviate the current difficulty to explain the largest [F/O] ratios found by
Jorissen et al. In particular, it may not be necessary to search for
alternative nuclear chains affecting the production of F in AGB stars.Comment: 25 pages, 3 figures. to be appear in The Astrophysical Journal (Jan
2009 issue
Construção de uma biblioteca metagenômica de expressão da microbiota de rúmen de caprinos.
A criação de bibliotecas metagenômicas oferece a oportunidade para a bioprospecção de genes de interesse biotecnológico de microrganismos não-cultiváveis. O rúmen de caprinos é um ambiente anaeróbico ou microaerófilo onde ocorre a degradação de material lignocelulósico pela ação de microrganismos. Assim, a microbiota do rúmen de caprinos foi identificada como potencial fonte de enzimas, genes e de novos produtos para aplicações no desenvolvimento industrial do setor sucroalcooleiro. Por meio da extração direta do DNA total dos microrganismos do rúmen de caprinos foi possÃvel construir uma biblioteca metagenômica de expressão de pequenos insertos, na faixa de 3 a 8 kb, com aproximadamente 50.000 clones. Foi realizada a validação da biblioteca por meio de restrição enzimática de clones aleatórios e da clonagem e sequenciamento de clones do gene 16S rDNA para verificar a diversidade bacteriana representada na biblioteca. Esta biblioteca será utilizada para triagem de clones com diversas atividades enzimáticas tais como celulases, xilanases e amilases.bitstream/item/17904/1/cot_02-1.pd
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