160 research outputs found
Seismic diagnostics for transport of angular momentum in stars 2. Interpreting observed rotational splittings of slowly-rotating red giant stars
Asteroseismology with the space-borne missions CoRoT and Kepler provides a
powerful mean of testing the modeling of transport processes in stars.
Rotational splittings are currently measured for a large number of red giant
stars and can provide stringent constraints on the rotation profiles. The aim
of this paper is to obtain a theoretical framework for understanding the
properties of the observed rotational splittings of red giant stars with slowly
rotating cores. This allows us to establish appropriate seismic diagnostics for
rotation of these evolved stars. Rotational splittings for stochastically
excited dipolar modes are computed adopting a first-order perturbative approach
for two benchmark models assuming slowly rotating cores. For red
giant stars with slowly rotating cores, we show that the variation of the
rotational splittings of modes with frequency depends only on the
large frequency separation, the g-mode period spacing, and the ratio of the
average envelope to core rotation rates (). This leds us to propose a
way to infer directly from the observations. This method is
validated using the Kepler red giant star KIC 5356201. Finally, we provide a
theoretical support for the use of a Lorentzian profile to measure the observed
splittings for red giant stars.Comment: 15 pages, 15 figures, accepted for publication in A&
The {\gamma} Dor stars as revealed by Kepler : A key to reveal deep-layer rotation in A and F stars
The {\gamma} Dor pulsating stars present high-order gravity modes, which make
them important targets in the intermediate-and low-mass main-sequence region of
the Hertzsprung-Russell diagram. Whilst we have only access to rotation in the
envelope of the Sun, the g modes of {\gamma} Dor stars can in principle deliver
us constraints on the inner layers. With the puzzling discovery of unexpectedly
low rotation rates in the core of red giants, the {\gamma} Dor stars appear now
as unique targets to explore internal angular momentum transport in the
progenitors of red giants. Yet, the {\gamma} Dor pulsations remain hard to
detect from the ground for their periods are close to 1 day. While the CoRoT
space mission first revealed intriguing frequency spectra, the almost
uninterrupted 4-year photometry from the Kepler mission eventually shed a new
light on them. It revealed regularities in the spectra, expected to bear
signature of physical processes, including rotation, in the shear layers close
to the convective core. We present here the first results of our effort to
derive exploitable seismic diagnosis for mid- to fast rotators among {\gamma}
Dor stars. We confirm their potential to explore the rotation history of this
early phase of stellar evolution.Comment: 4 pages, 1 figure, proceedings of the 22nd Los Alamos Stellar
Pulsation Conference, "Wide-field variability surveys: a 21st-century
perspective" held in San Pedro de Atacama, Chile, Nov. 28-Dec. 2, 201
PLATO: PSF modelling using a microscanning technique
The PLATO space mission is designed to detect telluric planets in the
habitable zone of solar type stars, and simultaneously characterise the host
star using ultra high precision photometry. The photometry will be performed on
board using weighted masks. However, to reach the required precision,
corrections will have to be performed by the ground segment and will rely on
precise knowledge of the instrument PSF (Point Spread Function). We here
propose to model the PSF using a microscanning method.Comment: 2 pages, conference proceedings of the CoRoT Symposium 3, KASC 7,
appears in EPJ conference 201
Angular momentum redistribution by mixed modes in evolved low-mass stars. I. Theoretical formalism
Seismic observations by the space-borne mission \emph{Kepler} have shown that
the core of red giant stars slows down while evolving, requiring an efficient
physical mechanism to extract angular momentum from the inner layers. Current
stellar evolution codes fail to reproduce the observed rotation rates by
several orders of magnitude, and predict a drastic spin-up of red giant cores
instead. New efficient mechanisms of angular momentum transport are thus
required.
In this framework, our aim is to investigate the possibility that mixed modes
extract angular momentum from the inner radiative regions of evolved low-mass
stars. To this end, we consider the Transformed Eulerian Mean (TEM) formalism,
introduced by Andrews \& McIntyre (1978), that allows us to consider the
combined effect of both the wave momentum flux in the mean angular momentum
equation and the wave heat flux in the mean entropy equation as well as their
interplay with the meridional circulation.
In radiative layers of evolved low-mass stars, the quasi-adiabatic
approximation, the limit of slow rotation, and the asymptotic regime can be
applied for mixed modes and enable us to establish a prescription for the wave
fluxes in the mean equations. The formalism is finally applied to a benchmark model, representative of observed CoRoT and \emph{Kepler}
oscillating evolved stars.
We show that the influence of the wave heat flux on the mean angular momentum
is not negligible and that the overall effect of mixed modes is to extract
angular momentum from the innermost region of the star. A quantitative and
accurate estimate requires realistic values of mode amplitudes. This is
provided in a companion paper.Comment: Accepted in A&A, 11 pages, and 6 figure
Angular momentum redistribution by mixed modes in evolved low-mass stars. II. Spin-down of the core of red giants induced by mixed modes
The detection of mixed modes in subgiants and red giants by the CoRoT and
\emph{Kepler} space-borne missions allows us to investigate the internal
structure of evolved low-mass stars. In particular, the measurement of the mean
core rotation rate as a function of the evolution places stringent constraints
on the physical mechanisms responsible for the angular momentum redistribution
in stars. It showed that the current stellar evolution codes including the
modelling of rotation fail to reproduce the observations. An additional
physical process that efficiently extracts angular momentum from the core is
thus necessary.
Our aim is to assess the ability of mixed modes to do this. To this end, we
developed a formalism that provides a modelling of the wave fluxes in both the
mean angular momentum and the mean energy equations in a companion paper. In
this article, mode amplitudes are modelled based on recent asteroseismic
observations, and a quantitative estimate of the angular momentum transfer is
obtained. This is performed for a benchmark model of 1.3 at three
evolutionary stages, representative of the evolved pulsating stars observed by
CoRoT and Kepler.
We show that mixed modes extract angular momentum from the innermost regions
of subgiants and red giants. However, this transport of angular momentum from
the core is unlikely to counterbalance the effect of the core contraction in
subgiants and early red giants. In contrast, for more evolved red giants, mixed
modes are found efficient enough to balance and exceed the effect of the core
contraction, in particular in the hydrogen-burning shell. Our results thus
indicate that mixed modes are a promising candidate to explain the observed
spin-down of the core of evolved red giants, but that an other mechanism is to
be invoked for subgiants and early red giants.Comment: Accepted in A&A, 7 pages, 8 figure
Rotational splittings for slow to moderate rotators: Latitudinal dependency or higher order effects in \Omega?
Information about the rotation rate is contained in the low frequency part of
power spectra, where signatures of nonuniform surface rotation are expected, as
well as in the frequency splittings induced by the internal rotation rate. We
wish to figure out whether the differences between the seismic rotation period
as determined by a mean rotational splitting, and the rotation period measured
from the low frequency peak in the Fourier spectrum (observed for some of
CoRoT's targets) can provide constraints on the rotation profile. For uniform
moderate rotators,perturbative corrections to second and third order in terms
of the rotation angular velocity \Omega, may mimic differential rotation. We
apply our perturbation method to evaluate mode frequencies accurate up to
\Omega^3 for uniform rotation. Effects of latitudinal dependence are calculated
in the linear approximation. In \beta Cephei pulsators models, third order
effects become comparable to that of a horizontal shear similar to the solar
one at rotation rates well below the breakup values. We show how a clean
signature of the latitudinal shear may be extracted. Our models of two CoRoT
target HD 181906 and HD 181420, represent lower main sequence objects. These
are slow rotators and nonlinear effects in splittings are accordingly small. We
use data on one low frequency peak and one splitting of a dipolar mode to
constrain the rotation profile in HD 181420 and HD 181906. The relative
influence of the two effects strongly depends on the type of the oscillation
modes in the star and on the magnitude of the rotation rate. Given mean
rotational splitting and the frequency of a spot signature, it is possible to
distinguish between the two hypothesis, and in the case of differential
rotation in latitude, we propose a method to determine the type of rotation
profile and a range of values for the shear.Comment: 17 pages, 12 figures, A&A accepte
Helminthosporium bicolor, un pathogène foliaire du riz et de Stenotaphrum secundatum au Maroc
Helminthosporium bicolor est isolé pour la première fois au Maroc à partir des lésions foliaires d’Oryza sativa et de Stenotaphrum secundatum. Les plantes de ces deux espèces hôtes inoculées avec deux isolats d’Helminthosporium bicolor ont développé des lésions sporulantes. Les réisolements du pathogène à partir des ces lésions étaient positifs. Les indices de sévérité sur les deux variétés du riz varient entre 43,62% et 52,87%, par contre sur Stenotaphrum secundatum, ils atteignent 65,65% sur les plantes inoculées avec l’isolat R1. La sporulation est maximale sur S. secundatum, elle est de 8,4.105 conidies/cm2 pour l’isolat R1 et varie entre 3,28. 105 et 4,32.105 conidies /cm2 sur les feuilles de deux variétés du riz. © 2013 International Formulae Group. All rights reserved.Mots clés: Helminthosporium bicolor, Oryza sativa, Stenotaphrum secundatum, inoculation, sporulation
Comparative pathogenicity of Colletotrichum spp. against different varieties of strawberry plants (Fragaria ananassa) widely grown in Morocco
The evolution of anthracnose symptoms on the aerial part (leaves, stems and strawberries) of three varieties Fortuna, Camarosa and Festival of strawberry plants inoculated with the conidial suspensions of Colletotrichum acutatum and Colletotrichum gloeosporioides isolates was followed. The severity index and infection coefficients increased in function of time. Seven days after inoculation they were low not exceeding 13.43% and 43.33, but they increased four weeks after inoculation, respectively, to 37.96% and 99 on strawberry plants of the Camarosa variety, 54.44% and 105 on those of Fortuna and 51.12% and 85 on those of Festival. At the sixth week, the severity index and infection coefficients became very high, reaching respectively 100% and 408 on Fortuna plants inoculated with C. gloeosporioides isolate (Coll3) followed by Coll2 (89.28% – 300), Coll1 (86.66% – 378) and Coll4 (80.45% – 198) of C. acutatum species. Similarly, the isolate Coll3 caused fruit rot; the percentage of rotten strawberries was 100% on Fortuna variety, 83.33% on Festival and 70.25% on Camarosa. A positive re-isolation of the tested Colletotrichum isolates has been noted from leaves of strawberry varieties and negative from crowns or the roots. A significant to moderate reduction in fresh and dry weights of the aerial part and roots was noted in inoculated strawberry plants compared to the control
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