557 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
Non-adiabatic oscillations of fast-rotating stars: the example of Rasalhague
Early-type stars generally tend to be fast rotators. In these stars, mode
identification is very challenging as the effects of rotation are not well
known. We consider here the example of Ophiuchi, for which dozens of
oscillation frequencies have been measured. We model the star using the
two-dimensional structure code ESTER, and we compute both adiabatic and
non-adiabatic oscillations using the TOP code. Both calculations yield very
complex spectra, and we used various diagnostic tools to try and identify the
observed pulsations. While we have not reached a satisfactory mode-to-mode
identification, this paper presents promising early results.Comment: 4 pages, 3 figures. SF2A 2017 proceeding
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
uuencode
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
Asteroseismology of solar-type stars
Until the last few decades, investigations of stellar interiors had been
restricted to theoretical studies only constrained by observations of their
global properties and external characteristics. However, in the last thirty
years the field has been revolutionized by the ability to perform seismic
investigations of stellar interiors. This revolution begun with the Sun, where
helioseismology has been yielding information competing with what can be
inferred about the Earth's interior from geoseismology. The last two decades
have witnessed the advent of asteroseismology of solar-like stars, thanks to a
dramatic development of new observing facilities providing the first reliable
results on the interiors of distant stars. The coming years will see a huge
development in this field. In this review we focus on solar-type stars, i.e.,
cool main-sequence stars where oscillations are stochastically excited by
surface convection. After a short introduction and a historical overview of the
discipline, we review the observational techniques generally used, and we
describe the theory behind stellar oscillations in cool main-sequence stars. We
continue with a complete description of the normal mode analyses through which
it is possible to extract the physical information about the structure and
dynamics of the stars. We then summarize the lessons that we have learned and
discuss unsolved issues and questions that are still unanswered.Comment: Invited review article for Living Reviews in Solar Physics. 91 pages
and 46 figure
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