459 research outputs found
Magnetic activity cycles in solar-like stars: The cross-correlation technique of p-mode frequency shifts
Aims. We aim studying the use of cross-correlation techniques to infer the
frequency shifts induced by changing magnetic fields in the p-mode frequencies
and provide precise estimation of the error bars. Methods. This technique and
the calculation of the associated errors is first tested and validated on the
Sun where the p-mode magnetic behaviour is very well known. These validation
tests are performed on 6000-day time series of Sun-as-a-star observations
delivered by the SoHO spacecraft. Errors of the frequency shifts are quantified
through Monte Carlo simulations. The same methodology is then applied to three
solar-like oscillating stars: HD 49933, observed by CoRoT, as well as KIC
3733735 and KIC 7940546 observed by Kepler. Results. We first demonstrate the
reliability of the error bars computed with the Monte Carlo simulations using
the Sun. From the three analyzed stars we confirm the presence of a magnetic
activity cycle with this methodology in HD 49933 and we unveil seismic
signature of on going magnetic variations in KIC 3733735. Finally, the third
star, KIC 7940546, seems to be in a quiet regime.Comment: Paper accepted in A&A. 7 pages and 11 figure
TWO-PION EXCHANGE NUCLEAR POTENTIAL - CHIRAL CANCELLATIONS
We show that chiral symmetry is responsible for large cancellations in the
two-pion exchange nucleon-nucleon interaction, which are similar to those
occuring in free pion-nucleon scattering.Comment: REVTEX style, 5 pages, 3 PostScrip figures compressed, tarred and
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NN Scattering: Chiral Predictions for Asymptotic Observables
We assume that the nuclear potential for distances larger than 2.5 fm is
given just by the exchanges of one and two pions and, for the latter, we adopt
a model based on chiral symmetry and subthreshold pion-nucleon amplitudes,
which contains no free parameters. The predictions produced by this model for
nucleon-nucleon observables are calculated and shown to agree well with both
experiment and those due to phenomenological potentials.Comment: 16 pages, 12 PS figures included, to appear in Physical Review
Quark Condensate in the Deuteron
We study the changes produced by the deuteron on the QCD quark condensate by
means the Feynman-Hellmann theorem and find that the pion mass dependence of
the pion-nucleon coupling could play an important role. We also discuss the
relation between the many body effect of the condensate and the meson exchange
currents, as seen by photons and pions. For pion probes, the many-body term in
the physical amplitude differs significantly from that of soft pions, the one
linked to the condensate. Thus no information about the many-body term of the
condensate can be extracted from the pion-deuteron scattering length. On the
other hand, in the Compton amplitude, the relationship with the condensate is a
more direct one.Comment: to appear in Physics Review C (19 pages, 3 figures
Visibilities and bolometric corrections for stellar oscillation modes observed by Kepler
Kepler produces a large amount of data used for asteroseismological analyses,
particularly of solar-like stars and red giants. The mode amplitudes observed
in the Kepler spectral band have to be converted into bolometric amplitudes to
be compared to models. We give a simple bolometric correction for the
amplitudes of radial modes observed with Kepler, as well as the relative
visibilities of non-radial modes. We numerically compute the bolometric
correction c_{K-bol} and mode visibilities for different effective temperatures
Teff within the range 4000-7500 K, using a similar approach to a recent one
from the literature (Michel et al. 2009, A&A 495, 979). We derive a law for the
correction to bolometric values: c_{K-bol} = 1 + a_1 (Teff-To) + a_2
(Teff-To)^2, with To = 5934 K, a_1 = 1.349e-4 K^{-1}, and a_2 = -3.120e-9
K^{-2} or, alternatively, as the power law c_{K-bol} = (Teff/To)^alpha with
alpha = 0.80. We give tabulated values for the mode visibilities based on
limb-darkening (LD), computed from ATLAS9 model atmospheres for Teff in
[4000,7500] K, log g in [2.5,4.5], and [M/H] in [-1.0,+1.0]. We show that using
LD profiles already integrated over the spectral band provides quick and good
approximations for visibilities. We point out the limits of these classical
visibility estimations.Comment: 5 pages, 4 figures, 1 table, minor language edition. Published in A&
PERSYST, a model for ex ante assessment of cropping systems performances. Adaptation to organic farming in the Ile-de-France region
PERSYST is a web software for ex ante assessment of crop yield that takes explicitely into account the cropping system (i.e. crop rotation and crop management) perspective. Environmental and economic indicators are calculated at crop rotation scale to complete the previous yield assessment. In 2012, a research program enabled the model adaptation to organic systems, taking into account weed management and organic inputs supply. This program also allowed to parameterize the web software in the Ile-de-France region for 8 soil types and 18crops. Parameterization has been validated for the most common situations. Validation remains to be done for less common ones. When completed, validation will make it possible to test the software in promising situations of use, such as supporting organic farmers analyzing their current cropping systems, or supporting farmers thinking about converting their farms to organic farming
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