351 research outputs found
Seismic evidence for a weak radial differential rotation in intermediate-mass core helium burning stars
The detection of mixed modes that are split by rotation in Kepler red giants
has made it possible to probe the internal rotation profiles of these stars,
which brings new constraints on the transport of angular momentum in stars.
Mosser et al. (2012) have measured the rotation rates in the central regions of
intermediate-mass core helium burning stars (secondary clump stars). Our aim
was to exploit& the rotational splittings of mixed modes to estimate the amount
of radial differential rotation in the interior of secondary clump stars using
Kepler data, in order to place constraints on angular momentum transport in
intermediate-mass stars. We selected a subsample of Kepler secondary clump
stars with mixed modes that are clearly rotationally split. By applying a
thorough statistical analysis, we showed that the splittings of both
gravity-dominated modes (trapped in central regions) and p-dominated modes
(trapped in the envelope) can be measured. We then used these splittings to
estimate the amount of differential rotation by using inversion techniques and
by applying a simplified approach based on asymptotic theory (Goupil et al.
2013). We obtained evidence for a weak radial differential rotation for six of
the seven targets that were selected, with the central regions rotating
to times faster than the envelope. The last target was
found to be consistent with a solid-body rotation. This demonstrates that an
efficient redistribution of angular momentum occurs after the end of the main
sequence in the interior of intermediate-mass stars, either during the
short-lived subgiant phase, or once He-burning has started in the core. In
either case, this should bring constraints on the angular momentum transport
mechanisms that are at work.Comment: 16 pages, 8 figures, accepted in A&
The connection between stellar granulation and oscillation as seen by the Kepler mission
The long and almost continuous observations by Kepler show clear evidence of
a granulation background signal in a large sample of stars, which is
interpreted as the surface manifestation of convection. It has been shown that
its characteristic timescale and rms intensity fluctuation scale with the peak
frequency (\nu_{max}) of the solar-like oscillations. Various attempts have
been made to quantify the observed signal, to determine scaling relations, and
to compare them to theoretical predictions. We use a probabilistic method to
compare different approaches to extracting the granulation signal. We fit the
power density spectra of a large set of Kepler targets, determine the
granulation and global oscillation parameter, and quantify scaling relations
between them. We establish that a depression in power at about \nu_{max}/2,
known from the Sun and a few other main-sequence stars, is also statistically
significant in red giants and that a super-Lorentzian function with two
components is best suited to reproducing the granulation signal in the broader
vicinity of the pulsation power excess. We also establish that the specific
choice of the background model can affect the determination of \nu_{max},
introducing systematic uncertainties that can significantly exceed the random
uncertainties. We find the characteristic background frequency and amplitude to
tightly scale with \nu_{max} for a wide variety of stars, and quantify a mass
dependency of the latter. To enable comparison with theoretical predictions, we
computed effective timescales and intensity fluctuations and found them to
approximately scale as \tau_{eff} \propto g^{-0.85}\,T^{-0.4} and A_{gran}
\propto (g^2M)^{-1/4}, respectively. Similarly, the bolometric pulsation
amplitude scales approximately as A_{puls} \propto (g^2M)^{-1/3}, which
implicitly verifies a separate mass and luminosity dependence of A_{puls}.Comment: 18 pages, 12 figures, accepted for A&
Calculation of the Alpha--Particle Ground State within the Hyperspherical Harmonic Basis
The problem of calculating the four--nucleon bound state properties for the
case of realistic two- and three-body nuclear potentials is studied using the
hyperspherical harmonic (HH) approach. A careful analysis of the convergence of
different classes of HH functions has been performed. A restricted basis is
chosen to allow for accurate estimates of the binding energy and other
properties of the 4He ground state. Results for various modern two-nucleon and
two- plus three-nucleon interactions are presented. The 4He asymptotic
normalization constants for separation in 2+2 and 1+3 clusters are also
computed.Comment: 29 pages, 4 figures, 11 tables, revtex
Solar-like oscillations with low amplitude in the CoRoT target HD 181906
Context: The F8 star HD 181906 (effective temperature ~6300K) was observed
for 156 days by the CoRoT satellite during the first long run in the centre
direction. Analysis of the data reveals a spectrum of solar-like acoustic
oscillations. However, the faintness of the target (m_v=7.65) means the
signal-to-noise (S/N) in the acoustic modes is quite low, and this low S/N
leads to complications in the analysis. Aims: To extract global variables of
the star as well as key parameters of the p modes observed in the power
spectrum of the lightcurve. Methods: The power spectrum of the lightcurve, a
wavelet transform and spot fitting have been used to obtain the average
rotation rate of the star and its inclination angle. Then, the autocorrelation
of the power spectrum and the power spectrum of the power spectrum were used to
properly determine the large separation. Finally, estimations of the mode
parameters have been done by maximizing the likelihood of a global fit, where
several modes were fit simultaneously. Results: We have been able to infer the
mean surface rotation rate of the star (~4 microHz) with indications of the
presence of surface differential rotation, the large separation of the p modes
(~87 microHz), and therefore also the ridges corresponding to overtones of the
acoustic modes.Comment: Paper Accepted to be published in A&A. 10 Pages, 12 figure
Asteroseismology from multi-month Kepler photometry: the evolved Sun-like stars KIC 10273246 and KIC 10920273
The evolved main-sequence Sun-like stars KIC 10273246 (F-type) and KIC
10920273 (G-type) were observed with the NASA Kepler satellite for
approximately ten months with a duty cycle in excess of 90%. Such continuous
and long observations are unprecedented for solar-type stars other than the
Sun.
We aimed mainly at extracting estimates of p-mode frequencies - as well as of
other individual mode parameters - from the power spectra of the light curves
of both stars, thus providing scope for a full seismic characterization.
The light curves were corrected for instrumental effects in a manner
independent of the Kepler Science Pipeline. Estimation of individual mode
parameters was based both on the maximization of the likelihood of a model
describing the power spectrum and on a classic prewhitening method. Finally, we
employed a procedure for selecting frequency lists to be used in stellar
modeling.
A total of 30 and 21 modes of degree l=0,1,2 - spanning at least eight radial
orders - have been identified for KIC 10273246 and KIC 10920273, respectively.
Two avoided crossings (l=1 ridge) have been identified for KIC 10273246,
whereas one avoided crossing plus another likely one have been identified for
KIC 10920273. Good agreement is found between observed and predicted mode
amplitudes for the F-type star KIC 10273246, based on a revised scaling
relation. Estimates are given of the rotational periods, the parameters
describing stellar granulation and the global asteroseismic parameters
and .Comment: 15 pages, 15 figures, to be published in Astronomy & Astrophysic
The solar-like CoRoT target HD 170987: spectroscopic and seismic observations
The CoRoT mission is in its third year of observation and the data from the
second long run in the galactic centre direction are being analysed. The
solar-like oscillating stars that have been observed up to now have given some
interesting results, specially concerning the amplitudes that are lower than
predicted. We present here the results from the analysis of the star HD
170987.The goal of this research work is to characterise the global parameters
of HD 170987. We look for global seismic parameters such as the mean large
separation, maximum amplitude of the modes, and surface rotation because the
signal-to-noise ratio in the observations do not allow us to measure individual
modes. We also want to retrieve the stellar parameters of the star and its
chemical composition.We have studied the chemical composition of the star using
ground-based observations performed with the NARVAL spectrograph. We have used
several methods to calculate the global parameters from the acoustic
oscillations based on CoRoT data. The light curve of the star has been
interpolated using inpainting algorithms to reduce the effect of data gaps. We
find power excess related to p modes in the range [400 - 1200]muHz with a mean
large separation of 55.2+-0.8muHz with a probability above 95% that increases
to 55.9 +-0.2muHz in a higher frequency range [500 - 1250] muHz and a rejection
level of 1%. A hint of the variation of this quantity with frequency is also
found. The rotation period of the star is estimated to be around 4.3 days with
an inclination axis of i=50 deg +20/-13. We measure a bolometric amplitude per
radial mode in a range [2.4 - 2.9] ppm around 1000 muHz. Finally, using a grid
of models, we estimate the stellar mass, M=1.43+-0.05 Msun, the radius,
R=1.96+-0.046 Rsun, and the age ~2.4 Gyr.Comment: 12 pages, 15 figures, accepted for publication in A&
The underlying physical meaning of the relation
Asteroseismology of stars that exhibit solar-like oscillations are enjoying a
growing interest with the wealth of observational results obtained with the
CoRoT and Kepler missions. In this framework, scaling laws between
asteroseismic quantities and stellar parameters are becoming essential tools to
study a rich variety of stars. However, the physical underlying mechanisms of
those scaling laws are still poorly known. Our objective is to provide a
theoretical basis for the scaling between the frequency of the maximum in the
power spectrum () of solar-like oscillations and the cut-off
frequency (). Using the SoHO GOLF observations together with
theoretical considerations, we first confirm that the maximum of the height in
oscillation power spectrum is determined by the so-called \emph{plateau} of the
damping rates. The physical origin of the plateau can be traced to the
destabilizing effect of the Lagrangian perturbation of entropy in the
upper-most layers which becomes important when the modal period and the local
thermal relaxation time-scale are comparable. Based on this analysis, we then
find a linear relation between and , with a
coefficient that depends on the ratio of the Mach number of the exciting
turbulence to the third power to the mixing-length parameter.Comment: 8 pages, 11 figures. Accepted in A&
Short-lived spots in solar-like stars as observed by CoRoT
Context. CoRoT light curves have an unprecedented photometric quality, having
simultaneously a high signal-to-noise ratio, a long time span and a nearly
continuous duty-cycle. Aims. We analyse the light-curves of four bright targets
observed in the seismology field and study short-lived small spots in
solar-like stars. Methods. We present a simple spot modeling by iterative
analysis. Its ability to extract relevant parameters is ensured by implementing
relaxation steps to avoid convergence to local minima of the sum of the
residuals between observations and modeling. The use of Monte-Carlo simulations
allows us to estimate the performance of the fits. Results. Our starspot
modeling gives a representation of the spots on these stars in agreement with
other well tested methods. Within this framework, parameters such as rigid-body
rotation and spot lifetimes seem to be precisely determined. Then, the
lifetime/rotation period ratios are in the range 0.5 - 2, and there is clear
evidence for differential rotation.Comment: 11 pages Accepted in A&
CaractĂ©risation physico-chimique des sols en vue de lâamĂ©lioration de la productivitĂ© du manioc (Manihot esculenta Crantz) dans la rĂ©gion de Damara au centre-sud de Centrafrique
La prĂ©sente Ă©tude vise Ă amĂ©liorer la productivitĂ© du manioc (Manihot esculenta), aliment de base des centrafricains. Plus spĂ©cifiquement, il sâagit de (i) caractĂ©riser les propriĂ©tĂ©s physico-chimiques du sol et (ii) dĂ©terminer les indicateurs de fertilitĂ© des sols Ă©tudiĂ©s. La collecte des donnĂ©es a consistĂ© Ă prĂ©lever le sol de la couche 0-20 cm, sur les champs de manioc du site expĂ©rimental de LaSBAD. Les sols ont Ă©tĂ© prĂ©levĂ©s suivant un dispositif de randomisation, en vue de constituer un Ă©chantillon composite de 1 kg par champs. Ils sont sĂ©chĂ©s, tamisĂ©s et analysĂ©s au laboratoire de sol du CIRAD-Montpellier-France. Les rĂ©sultats analytiques de sol ont Ă©tĂ© comparĂ©s Ă des valeurs de rĂ©fĂ©rences. Les sols Ă©tudiĂ©s prĂ©sentent une texture loameuse et un pH optimal de 5,5, favorable Ă la culture du manioc, et sont trĂšs pourvus en matiĂšre organique. Le bilan des Ă©lĂ©ments nutritifs rĂ©vĂšle des carences en calcium et en Ă©changeable. Lâanalyse en composantes principales a montrĂ© une corrĂ©lation entre les indicateurs de la fertilitĂ© des sols. Lâutilisation des lĂ©gumineuses et la fertilisation organo-minĂ©rale Ă une dose adĂ©quate sont des solutions envisageables pour amĂ©liorer la productivitĂ© du manioc en Centrafrique.Mots clĂ©s: indicateur de fertilitĂ©, productivitĂ©, manioc, propriĂ©tĂ© physico-chimique, CentrafriqueEnglish Title: Characterization physicochemical soils to improve productivity of cassava (Manihot esculenta Crantz) in the region of Damara in south-central of Central African RepublicEnglish AbstractThe present study is a contribution to the improvement of cassava (Manihot esculenta) tubersâ productivity in the Republic of Central Africa. Especially it aims to characterize the physicochemical properties of soil and (ii) assess fertility indicators of soil. Data collection was consisted to sample the soil from horizon 0-20 cm in cassava fields of producers and the agricultural site of the LaSBAD. Soils were sampled randomly to form a composite sample of 1 kg per field. Soil samples were dried, sieved, and analysed within the laboratory of soil of CIRAD in Montpellier-France. The analytical results of soils were compared to reference values. The studied soils are in their surface organic loamy texture and optimal of 5.5 pH suitable for the cultivation of cassava. They are well supplied with organic matter. The nutrient balance showed that the most significant deficiencies include calcium and magnesium exchangeable. The principal component analysis showed a best correlation relationship between indicators of soil fertility. The use of legumes, and fertilization organicmineral at one dose adequate were possible conceivable solutions to improve cassava in Central African Republic.Keywords: Diagnosis of deficiency, soilsâ fertility, cassava, savannah, Central African Republi
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