3,228 research outputs found
Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities
The availability of asteroseismic constraints for a large sample of stars
from the missions CoRoT and Kepler paves the way for various statistical
studies of the seismic properties of stellar populations. In this paper, we
evaluate the impact of rotation-induced mixing and thermohaline instability on
the global asteroseismic parameters at different stages of the stellar
evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic
Giant Branch to distinguish stellar populations. We present a grid of stellar
evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and
0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed
either with standard prescriptions or including both thermohaline convection
and rotation-induced mixing. For the whole grid we provide the usual stellar
parameters (luminosity, effective temperature, lifetimes, ...), together with
the global seismic parameters, i.e. the large frequency separation and
asymptotic relations, the frequency corresponding to the maximum oscillation
power {\nu}_{max}, the maximal amplitude A_{max}, the asymptotic period spacing
of g-modes, and different acoustic radii. We discuss the signature of
rotation-induced mixing on the global asteroseismic quantities, that can be
detected observationally. Thermohaline mixing whose effects can be identified
by spectroscopic studies cannot be caracterized with the global seismic
parameters studied here. But it is not excluded that individual mode
frequencies or other well chosen asteroseismic quantities might help
constraining this mixing.Comment: 15 pages, 11 figures, accepted for publication in A&
Carrier and polarization dynamics in monolayer MoS2
In monolayer MoS2 optical transitions across the direct bandgap are governed
by chiral selection rules, allowing optical valley initialization. In time
resolved photoluminescence (PL) experiments we find that both the polarization
and emission dynamics do not change from 4K to 300K within our time resolution.
We measure a high polarization and show that under pulsed excitation the
emission polarization significantly decreases with increasing laser power. We
find a fast exciton emission decay time on the order of 4ps. The absence of a
clear PL polarization decay within our time resolution suggests that the
initially injected polarization dominates the steady state PL polarization. The
observed decrease of the initial polarization with increasing pump photon
energy hints at a possible ultrafast intervalley relaxation beyond the
experimental ps time resolution. By compensating the temperature induced change
in bandgap energy with the excitation laser energy an emission polarization of
40% is recovered at 300K, close to the maximum emission polarization for this
sample at 4K.Comment: 7 pages, 7 figures including supplementary materia
Spin and recombination dynamics of excitons and free electrons in p-type GaAs : effect of carrier density
Carrier and spin recombination are investigated in p-type GaAs of acceptor
concentration NA = 1.5 x 10^(17) cm^(-3) using time-resolved photoluminescence
spectroscopy at 15 K. At low pho- tocarrier concentration, acceptors are mostly
neutral and photoelectrons can either recombine with holes bound to acceptors
(e-A0 line) or form excitons which are mostly trapped on neutral acceptors
forming the (A0X) complex. It is found that the spin lifetime is shorter for
electrons that recombine through the e-A0 transition due to spin relaxation
generated by the exchange scattering of free electrons with either trapped or
free holes, whereas spin flip processes are less likely to occur once the
electron forms with a free hole an exciton bound to a neutral acceptor. An
increase of exci- tation power induces a cross-over to a regime where the
bimolecular band-to-band (b-b) emission becomes more favorable due to screening
of the electron-hole Coulomb interaction and ionization of excitonic complexes
and free excitons. Then, the formation of excitons is no longer possible, the
carrier recombination lifetime increases and the spin lifetime is found to
decrease dramatically with concentration due to fast spin relaxation with free
photoholes. In this high density regime, both the electrons that recombine
through the e-A0 transition and through the b-b transition have the same spin
relaxation time.Comment: 4 pages, 5 figure
Lunar laser ranging in infrfared at hte Grasse laser station
For many years, lunar laser ranging (LLR) observations using a green
wavelength have suffered an inhomogeneity problem both temporally and
spatially. This paper reports on the implementation of a new infrared detection
at the Grasse LLR station and describes how infrared telemetry improves this
situation. Our first results show that infrared detection permits us to densify
the observations and allows measurements during the new and the full Moon
periods. The link budget improvement leads to homogeneous telemetric
measurements on each lunar retro-reflector. Finally, a surprising result is
obtained on the Lunokhod 2 array which attains the same efficiency as Lunokhod
1 with an infrared laser link, although those two targets exhibit a
differential efficiency of six with a green laser link
Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
A precise characterisation of the red giants in the seismology fields of the
CoRoT satellite is a prerequisite for further in-depth seismic modelling.
High-resolution FEROS and HARPS spectra were obtained as part of the
ground-based follow-up campaigns for 19 targets holding great asteroseismic
potential. These data are used to accurately estimate their fundamental
parameters and the abundances of 16 chemical species in a self-consistent
manner. Some powerful probes of mixing are investigated (the Li and CNO
abundances, as well as the carbon isotopic ratio in a few cases). The
information provided by the spectroscopic and seismic data is combined to
provide more accurate physical parameters and abundances. The stars in our
sample follow the general abundance trends as a function of the metallicity
observed in stars of the Galactic disk. After an allowance is made for the
chemical evolution of the interstellar medium, the observational signature of
internal mixing phenomena is revealed through the detection at the stellar
surface of the products of the CN cycle. A contamination by NeNa-cycled
material in the most massive stars is also discussed. With the asteroseismic
constraints, these data will pave the way for a detailed theoretical
investigation of the physical processes responsible for the transport of
chemical elements in evolved, low- and intermediate-mass stars.Comment: Accepted for publication in A&A, 25 pages, 13 colour figures (revised
version after language editing
Delay and distortion of slow light pulses by excitons in ZnO
Light pulses propagating through ZnO undergo distortions caused by both bound
and free excitons. Numerous lines of bound excitons dissect the pulse and
induce slowing of light around them, to the extend dependent on their nature.
Exciton-polariton resonances determine the overall pulse delay and attenuation.
The delay time of the higher-energy edge of a strongly curved light stripe
approaches 1.6 ns at 3.374 eV with a 0.3 mm propagation length. Modelling the
data of cw and time-of-flight spectroscopies has enabled us to determine the
excitonic parameters, inherent for bulk ZnO. We reveal the restrictions on
these parameters induced by the light attenuation, as well as a discrepancy
between the parameters characterizing the surface and internal regions of the
crystal.Comment: 4 pages, 4 figure
The Magnetic Fields at the Surface of Active Single G-K Giants
We investigate the magnetic field at the surface of 48 red giants selected as
promising for detection of Stokes V Zeeman signatures in their spectral lines.
We use the spectropolarimeters Narval and ESPaDOnS to detect circular
polarization within the photospheric absorption lines of our targets and use
the least-squares deconvolution (LSD) method. We also measure the classical
S-index activity indicator, and the stellar radial velocity. To infer the
evolutionary status of our giants and to interpret our results, we use
state-of-the-art stellar evolutionary models with predictions of convective
turnover times. We unambiguously detect magnetic fields via Zeeman signatures
in 29 of the 48 red giants in our sample. Zeeman signatures are found in all
but one of the 24 red giants exhibiting signs of activity, as well as 6 out of
17 bright giant stars.The majority of the magnetically detected giants are
either in the first dredge up phase or at the beginning of core He burning,
i.e. phases when the convective turnover time is at a maximum: this corresponds
to a 'magnetic strip' for red giants in the Hertzsprung-Russell diagram. A
close study of the 16 giants with known rotational periods shows that the
measured magnetic field strength is tightly correlated with the rotational
properties, namely to the rotational period and to the Rossby number Ro. Our
results show that the magnetic fields of these giants are produced by a dynamo.
Four stars for which the magnetic field is measured to be outstandingly strong
with respect to that expected from the rotational period/magnetic field
relation or their evolutionary status are interpreted as being probable
descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4
bright giants (Aldebaran, Alphard, Arcturus, eta Psc) are detected with
magnetic field strength at the sub-gauss level.Comment: 34 pages, 22 Figures, accepted for publication in Astronomy &
Astrophysic
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