455 research outputs found
Effects of rotational mixing on the asteroseismic properties of solar-type stars
The influence of rotational mixing on the evolution and asteroseismic
properties of solar-type stars is studied. Rotational mixing changes the global
properties of a solar-type star with a significant increase of the effective
temperature resulting in a shift of the evolutionary track to the blue part of
the HR diagram. These differences are related to changes of the chemical
composition, because rotational mixing counteracts the effects of atomic
diffusion leading to larger helium surface abundances for rotating models than
for non-rotating ones. Higher values of the large frequency separation are then
found for rotating models than for non-rotating ones at the same evolutionary
stage, because the increase of the effective temperature leads to a smaller
radius and hence to an increase of the stellar mean density. Rotational mixing
also has a considerable impact on the structure and chemical composition of the
central stellar layers by bringing fresh hydrogen fuel to the core, thereby
enhancing the main-sequence lifetime. The increase of the central hydrogen
abundance together with the change of the chemical profiles in the central
layers result in a significant increase of the values of the small frequency
separations and of the ratio of the small to large separations for models
including shellular rotation. This increase is clearly seen for models with the
same age sharing the same initial parameters except for the inclusion of
rotation as well as for models with the same global stellar parameters and in
particular the same location in the HR diagram. By computing rotating models of
solar-type stars including the effects of a dynamo that possibly occurs in the
radiative zone, we find that the efficiency of rotational mixing is strongly
reduced when the effects of magnetic fields are taken into account, in contrast
to what happens in massive stars.Comment: 11 pages, 15 figures, accepted for publication in A&
A dam assessment support system based on physical measurements, sensory evaluations and expert judgements
In engineering system control, human beings can play various key roles, in particular concerning measurement, global assessment and decision. This paper focuses on methods that allow the representation and aggregation of heterogeneous data (sensory evaluations, physical measurements, outputs of mathematical models, etc.) used in a global dam assessment process. It is acknowledged that in such complex systems many of the variables involved are evaluated with uncertainty. We propose a possibility theory-based approach to deal with all the different uncertain pieces of information and propagate them in aggregation models for global dam assessment. Finally, decision-making and communication applications relating to dam safety are presented
WIYN/Hydra Detection of Lithium Depletion in F Stars of the Young Open Cluster M35 and Implications for the Development of the Lithium Gap
We report discovery of significant depletion of Li on the surfaces of F dwarf
stars in the 150-Myr-old open cluster M35, analagous to a feature in the
700-Myr-old Hyades cluster that has been referred to as the ``Li gap.'' We have
caught the gap in the act of forming: using high resolution, high S/N,
WIYN/Hydra observations, we detect Li in all but a few M35 F stars; the maximum
depletion lies at least 0.6-0.8 dex below minimally depleted (or undepleted)
stars. The M35 Li depletion region, a) is quite wide, with clear depletion seen
from 6000K to 6700K or hotter; b) shows a significant dispersion in Li
abundance at all T_eff, even with stars of the same T_eff; and c) contains
undepleted stars (as well as depleted ones) in the (narrow) classical Hyades
gap region, which itself shows no undepleted stars. All of these M35 Li
depletion properties support rotationally-induced slow mixing as the primary
physical mechanism that forms the gap, and argues against other proposed
mechanisms, particularly diffusion and steady main sequence mass loss. When
viewed in the context of the M35 Li depletion properties, the Hyades Li gap may
well be wider than is usually recognized.Comment: 14 Pages, 3 figures. Accepted to ApJ Letter
Citrus Genomics
Citrus is one of the most widespread fruit crops globally, with great economic and health value. It is among the most difficult plants to improve through traditional breeding approaches. Currently, there is risk of devastation by diseases threatening to limit production and future availability to the human population. As technologies rapidly advance in genomic science, they are quickly adapted to address the biological challenges of the citrus plant system and the world's industries. The historical developments of linkage mapping, markers and breeding, EST projects, physical mapping, an international citrus genome sequencing project, and critical functional analysis are described. Despite the challenges of working with citrus, there has been substantial progress. Citrus researchers engaged in international collaborations provide optimism about future productivity and contributions to the benefit of citrus industries worldwide and to the human population who can rely on future widespread availability of this health-promoting and aesthetically pleasing fruit crop
3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
We examine the stability and observational consequences of mixing induced by
3He burning in the envelopes of first ascent red giants. We demonstrate that
there are two unstable modes: a rapid, nearly adiabatic mode that we cannot
identify with an underlying physical mechanism, and a slow, nearly radiative
mode that can be identified with thermohaline convection. We present
observational constraints that make the operation of the rapid mode unlikely to
occur in real stars. Thermohaline convection turns out to be fast enough only
if fluid elements have finger-like structures with a length to diameter ratio
l/d > 10. We identify some potentially serious obstacles for thermohaline
convection as the predominant mixing mechanism for giants. We show that
rotation-induced horizontal turbulent diffusion may suppress the 3He-driven
thermohaline convection. Another potentially serious problem for it is to
explain observational evidence of enhanced extra mixing. The 3He exhaustion in
stars approaching the red giant branch (RGB) tip should make the 3He mixing
inefficient on the asymptotic giant branch (AGB). In spite of this, there are
observational data indicating the presence of extra mixing in low-mass AGB
stars similar to that operating on the RGB. Overmixing may also occur in
carbon-enhanced metal-poor stars.Comment: 25 pages, 6 figures, modified version, accepted by Ap
Aide à la décision pour l'expertise des barrages
National audienceUn modèle d'évaluation des performances des barrages vis-à-vis de leurs principaux modes de rupture et de dégradation a été développé (Curt, 2008). Les données d'entrée (indicateurs) et de sortie (performance du barrage) de ce modèle sont des distributions de possibilité. Cette communication est axée sur la problématique de la prise de décision associée à ce résultat possibiliste : comment prioriser les actions de maintenance à entreprendre sur le barrage et comment transmettre l'information aux gestionnaires ? Nous proposons une analyse comparative des méthodes de défuzzification afin de sélectionner les méthodes répondant le mieux à cette problématique d'aide à l'expertise des barrages. / An assessment model of dam performances as regards their main failure modes and degradation modes was developed (Curt, 2008). The input data (indicators) and the ouput data (dam performance) of this model are possibility distributions. This paper focuses on the problematic of decision making associated to this possibility result: how to classify maintenance actions that have to be made on that dam and how to convey this result to dam managers? A comparative analysis of defuzzyfication methods is provided; those methods allow to best answer to this problematic of dam expertise
Persistent time intervals between features in solar flare hard X-ray emission
Several solar hard X-ray events (greater than 100 keV) were observed simultaneously with identical instruments on the Venera 11, 12, 13, 14, and Prognoz spacecraft. High time resolution (= 2 ms) data were stored in memory when a trigger occurred. The observations of modulation are presented with a period of 1.6 s for the event on December 3, 1978. Evidence is also presented for fast time fluctuations from an event on November 6, 1979, observed from Venera 12 and another on September 6, 1981, observed from the Solar Maximum Mission. Power spectrum analysis, epoch folding, and Monte Carlo simulation were used to evaluate the statistical significance of persistent time delays between features. The results are discussed in light of the MHD model proposed by Zaitsev and Stepanov
Stellar evolution with rotation and magnetic fields II: General equations for the transport by Tayler--Spruit dynamo
We further develop the Tayler--Spruit dynamo theory, based on the most
efficient instability for generating magnetic fields in radiative layers of
differentially rotating stars. We avoid the simplifying assumptions that either
the -- or the --gradient dominates, but we treat the general case and
we also account for the nonadiabatic effects, which favour the growth of the
magnetic field. Stars with a magnetic field rotate almost as a solid body.
Several of their properties (size of the core, MS lifetimes, tracks,
abundances) are closer to those of models without rotation than with rotation
only. In particular, the observed N/C or N/H excesses in OB stars are better
explained by our previous models with rotation only than by the present models
with magnetic fields that predict no nitrogen excesses. We show that there is a
complex feedback loop between the magnetic instability and the thermal
instability driving meridional circulation. This opens the possibility for
further magnetic models, but at this stage we do not know the relative
importance of the magnetic fields due to the Tayler instability in stellar
interiors.Comment: 14 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
The Geneva stellar evolution code
This paper presents the Geneva stellar evolution code with special emphasis on the modeling of solar-type stars. The basic input physics used in the Geneva code as well as the modeling of atomic diffusion is first discussed. The physical description of rotation is then presented. Finally, the modeling of magnetic instabilities and transport of angular momentum by internal gravity waves is briefly summarize
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