566 research outputs found
Solutions of the bound state Faddeev-Yakubovsky equations in three dimensions by using NN and 3N potential models
A recently developed three-dimensional approach (without partial-wave
decomposition) is considered to investigate solutions of Faddeev-Yakubovsky
integral equations in momentum space for three- and four-body bound states,
with the inclusion of three-body forces. In the calculations of the binding
energies, spin-dependent nucleon-nucleon (NN) potential models (named, S,
MT-I/III, YS-type and PGL) are considered along with the scalar
two-meson exchange three-body potential. Good agreement of the presently
reported results with the ones obtained by other techniques are obtained,
demonstrating the advantage of an approach in which the formalism is much more
simplified and easy to manage for direct computation.Comment: 16 pages, 1 figure and 6 tables; to appear in Physical review
Procyon-A and Eta-Bootis: Observational Frequencies Analyzed by the Local-Wave Formalism
In the present analysis of Procyon-A and Eta-Bootis, we use the local-wave
formalism which, despite its lack of precision inherent to any semi-analytical
method, uses directly the model profile without any modification when
calculating the acoustic mode eigenfrequencies. These two solar-like stars
present steep variations toward the center due to the convective core
stratification, and toward the surface due to the very thin convective zone.
Based on different boundary conditions, the frequencies obtained with this
formalism are different from that of the classical numerical calculation. We
point out that (1) the frequencies calculated with the local-wave formalism
seem to agree better with observational ones. All the frequencies detected with
a good confident level including those classified as 'noise' find an
identification, (2) some frequencies can be clearly identified here as
indications of the core limit.Comment: SOHO 18 / GONG 2006 / HELAS I Meetin
Measuring the extent of convective cores in low-mass stars using Kepler data: towards a calibration of core overshooting
Our poor understanding of the boundaries of convective cores generates large
uncertainties on the extent of these cores and thus on stellar ages. Our aim is
to use asteroseismology to consistently measure the extent of convective cores
in a sample of main-sequence stars whose masses lie around the mass-limit for
having a convective core. We first test and validate a seismic diagnostic that
was proposed to probe in a model-dependent way the extent of convective cores
using the so-called ratios, which are built with and
modes. We apply this procedure to 24 low-mass stars chosen among Kepler targets
to optimize the efficiency of this diagnostic. For this purpose, we compute
grids of stellar models with both the CESAM2k and MESA evolution codes, where
the extensions of convective cores are modeled either by an instantaneous
mixing or as a diffusion process. Among the selected targets, we are able to
unambiguously detect convective cores in eight stars and we obtain seismic
measurements of the extent of the mixed core in these targets with a good
agreement between the CESAM2k and MESA codes. By performing optimizations using
the Levenberg-Marquardt algorithm, we then obtain estimates of the amount of
extra-mixing beyond the core that is required in CESAM2k to reproduce seismic
observations for these eight stars and we show that this can be used to propose
a calibration of this quantity. This calibration depends on the prescription
chosen for the extra-mixing, but we find that it should be valid also for the
code MESA, provided the same prescription is used. This study constitutes a
first step towards the calibration of the extension of convective cores in
low-mass stars, which will help reduce the uncertainties on the ages of these
stars.Comment: 27 pages, 15 figures, accepted in A&
Investigating magnetic activity of F stars with the it Kepler mission
The dynamo process is believed to drive the magnetic activity of stars like
the Sun that have an outer convection zone. Large spectroscopic surveys showed
that there is a relation between the rotation periods and the cycle periods:
the longer the rotation period is, the longer the magnetic activity cycle
period will be. We present the analysis of F stars observed by Kepler for which
individual p modes have been measure and with surface rotation periods shorter
than 12 days. We defined magnetic indicators and proxies based on photometric
observations to help characterise the activity levels of the stars. With the
Kepler data, we investigate the existence of stars with cycles (regular or
not), stars with a modulation that could be related to magnetic activity, and
stars that seem to show a flat behaviour.Comment: 2 pages, 1 figure, proceedings of IAU Symposium 302 'Magnetic fields
through stellar evolution', 25-30 August 2013, Biarritz, Franc
Heartbeat stars and the ringing of tidal pulsations
With the advent of high precision photometry from satellites such as Kepler and CoRoT, a whole new layer of interesting and astounding astronomical objects has been revealed: heartbeat stars are an example of such objects. Heartbeat stars are eccen- tric ellipsoidal variables that undergo strong tidal interactions when the stars are almost in contact at the time of closest approach. These interactions deform of the stars and cause a notable light curve variation in the form of a tidal pulse. A subset of these objects (∼20%) show prominent tidally induced pulsations: pulsations forced by the binary orbit. We now have a fully functional code that models binary star features (using phoebe) and stellar pulsations simultaneously, enabling a complete and accurate heartbeat star model to be determined. In this paper we show the results of our new code, which uses emcee, a variant of mcmc, to generate a full set of stellar parameters. We further highlight the interesting features of KIC 8164262, including its tidally induced pulsations and resonantly locked pulsations
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