299 research outputs found
Constraints on the core mu-gradient of the solar-like star HD 49385 via low-degree mixed modes
The existence of an l=1 avoided crossing in the spectrum of the solar-like
pulsator CoRoT-target HD 49385 was established by Deheuvels & Michel (2009). It
is the first confirmed detection of such a phenomenon. The authors showed in a
preliminary modeling of the star that it was in a post main sequence status.
Being a 1.3 Msun-star, HD 49385 has had a convective core during its main
sequence phase. The mu-gradient left by the withdrawal of this core bears
information about the processes of transport at the boundary of the core. We
here investigate the constraints that the observed avoided crossing brings on
the mu-gradient in the core of the star.Comment: 5 pages, 5 figures, accepted in Astron. Nach. This work was presented
at the HELAS Conference in Lanzarote in February 201
Stellar ages from asteroseismology
Asteroseismology provides powerful means to probe stellar interiors. The
oscillations frequencies are closely related to stellar interior properties via
the density and sound speed profiles. Since these are tightly linked with the
mass and evolutionary state, we can expect to determine the age and mass of a
star from the comparison of its oscillation spectrum with predictions of
stellar models. Such a comparison suffers both from the problems we face when
modeling a particular star (as the uncertainties on global parameters and
chemical composition) and from our misunderstanding of processes at work in
stellar interiors (as the transport processes that may lead to core mixing and
affect the model ages). For stars where observations have provided precise and
numerous oscillation frequencies together with accurate global parameters and
additional information (as the radius or the mass if the star is in a binary
system, the interferometric radius or the mean density if the star is an
exoplanet host), we can also expect to better constrain the physical
description of the stellar structure and to get a more reliable age estimation.
After a survey of stellar pulsations, we present some seismic diagnostics that
can be used to infer the age of a star as well as their limitations. We then
illustrate the ability of asteroseismology to scrutinize stellar interiors on
the basis of a few exemples. In the years to come, extended very precise
asteroseismic observations are expected, in photometry or in spectroscopy, from
ground-based (HARPS, CORALIE, ELODIE, UVES, UCLES, SIAMOIS, SONG) or spatial
devices (MOST, CoRoT, WIRE, Kepler, PLATO). This will considerably enlarge the
sample of stars eligible to asteroseismic age determination and should allow to
estimate the age of individual stars with a 10-20% accuracy.Comment: 10 pages, 15 figures, Proc. of the IAU Symp. 258 "The Ages of Stars",
Baltimore USA 13-17 Oct 2008, eds D. Soderblom et al., CUP in pres
Inference from adiabatic analysis of solar-like oscillations in Red giants
The clear detection with CoRoT and KEPLER of radial and non-radial solar-like
oscillations in many red giants paves the way to seismic inferences on the
structure of such stars. We present an overview of the properties of the
adiabatic frequencies and frequency separations of radial and non-radial
oscillation modes, highlighting how their detection allows a deeper insight
into the properties of the internal structure of red giants. In our study we
consider models of red giants in different evolutionary stages, as well as of
different masses and chemical composition. We describe how the large and small
separations computed with radial modes and with non-radial modes mostly trapped
in the envelope depend on the stellar global parameters and evolutionary state,
and we compare our theoretical predictions and first KEPLER data.Finally, we
find that the properties of dipole modes constitute a promising seismic
diagnostic of the evolutionary state of red-giant stars.Comment: 6 pages, 5 figures. Proceedings of IV Helas International Conference:
"Seismological Challenges for Stellar Structure", Lanzarote (Canary Islands,
Spain), 1-5 February 201
Theoretical study of Doradus pulsations in pre-main sequence stars
The question of the existence of pre-main sequence (PMS) ~Doradus
(~Dor) has been raised by the observations of young clusters such as
NGC~884 hosting ~Dor members. We have explored the properties of
~Dor type pulsations in a grid of PMS models covering the mass range
and we derive the theoretical instability
strip (IS) for the PMS ~Dor pulsators. We explore the possibility of
distinguishing between PMS and MS ~Dor by the behaviour of the period
spacing of their high order -modes (-modes).Comment: 5 pages, 6 figures, Proc. HELAS IV Conference, Lanzarote, February
2010. Eds T. Roca Cort\'es, P. Pall\'e and S. Jim\'enez Reyes. Accepted in
Astron. Nac
A new seismic analysis of Alpha Centauri
Models of alpha Cen A & B have been computed using the masses determined by
Pourbaix et al. (2002) and the data derived from the spectroscopic analysis of
Neuforge and Magain (1997). The seismological data obtained by Bouchy and
Carrier (2001, 2002) do help improve our knowledge of the evolutionary status
of the system. All the constraints are satisfied with a model which gives an
age of about 6 Gyr for the binary.Comment: to be published in Astronomy and Astrophysic
Determining the metallicity of the solar envelope using seismic inversion techniques
The solar metallicity issue is a long-lasting problem of astrophysics,
impacting multi- ple fields and still subject to debate and uncertainties.
While spectroscopy has mostly been used to determine the solar heavy elements
abundance, helioseismologists at- tempted providing a seismic determination of
the metallicity in the solar convective enveloppe. However, the puzzle remains
since two independent groups prodived two radically different values for this
crucial astrophysical parameter. We aim at provid- ing an independent seismic
measurement of the solar metallicity in the convective enveloppe. Our main goal
is to help provide new information to break the current stalemate amongst
seismic determinations of the solar heavy element abundance. We start by
presenting the kernels, the inversion technique and the target function of the
inversion we have developed. We then test our approach in multiple
hare-and-hounds exercises to assess its reliability and accuracy. We then apply
our technique to solar data using calibrated solar models and determine an
interval of seismic measurements for the solar metallicity. We show that our
inversion can indeed be used to estimate the solar metallicity thanks to our
hare-and-hounds exercises. However, we also show that further dependencies in
the physical ingredients of solar models lead to a low accuracy. Nevertheless,
using various physical ingredients for our solar models, we determine
metallicity values between 0.008 and 0.014.Comment: Accepted for publication in MNRA
Analysis of MERCATOR data Part I: variable B stars
We re-classified 31 variable B stars which were observed more than 50 times
in the Geneva photometric system with the P7 photometer attached to the
MERCATOR telescope (La Palma) during its first 3 years of scientific
observations. HD89688 is a possible beta Cephei/slowly pulsating B star hybrid
and the main mode of the COROT target HD180642 shows non-linear effects. The
Maia candidates are re-classified as either ellipsoidal variables or spotted
stars. Although the mode identification is still ongoing, all the
well-identified modes so far have a degree l = 0, 1 or 2.Comment: 4 pages, 3 figures. To appear in: Proceedings of JENAM 2005 'Distant
worlds', Communications in Asteroseismolog
Thorough analysis of input physics in CESAM and CLES codes
This contribution is not about the quality of the agreement between stellar
models computed by CESAM and CLES codes, but more interesting, on what
ESTA-Task~1 run has taught us about these codes and about the input physics
they use. We also quantify the effects of different implementations of the same
physics on the seismic properties of the stellar models, that in fact is the
main aim of ESTA experiments.Comment: 11 pages, 12 fig. Accepted for publication in ApSS CoRoT/ESTA Volu
Seismic modelling of the Cep star HD\,180642 (V1449\,Aql)
We present modelling of the Cep star HD\,180642 based on its
observational properties deduced from CoRoT and ground-based photometry as well
as from time-resolved spectroscopy. We investigate whether present-day
state-of-the-art models are able to explain the full seismic behaviour of this
star, which has extended observational constraints for this type of pulsator.
We constructed a dedicated database of stellar models and their oscillation
modes tuned to fit the dominant radial mode frequency of HD\,180642, by means
of varying the hydrogen content, metallicity, mass, age, and core overshooting
parameter. We compared the seismic properties of these models with those
observed. We find models that are able to explain the numerous observed
oscillation properties of the star, for a narrow range in mass of
11.4--11.8\,M and no or very mild overshooting (with up to 0.05 local
pressure scale heights), except for an excitation problem of the ,
p mode. We deduce a rotation period of about 13\,d, which is fully
compatible with recent magnetic field measurements. The seismic models do not
support the earlier claim of solar-like oscillations in the star. We instead
ascribe the power excess at high frequency to non-linear resonant mode coupling
between the high-amplitude radial fundamental mode and several of the low-order
pressure modes. We report a discrepancy between the seismic and spectroscopic
gravity at the level.Comment: 10 pages, 2 Tables, 6 Figures. Accepted for publication in Astronomy
and Astrophysic
Seismic signature of envelope penetrative convection: the CoRoT star HD 52265
Aims: We aim at characterizing the inward transition from convective to
radiative energy transport at the base of the convective envelope of the
solar-like oscillator HD 52265 recently observed by the CoRoT satellite.
Methods: We investigated the origin of one specific feature found in the HD
52265 frequency spectrum. We modelled the star to derive the internal structure
and the oscillation frequencies that best match the observations and used a
seismic indicator sensitive to the properties of the base of the envelope
convection zone. Results: The seismic indicators clearly reveal that to best
represent the observed properties of HD 52265, models must include penetrative
convection below the outer convective envelope. The penetrative distance is
estimated to be , which corresponds to an extent over a distance
representing 6.0 per cents of the total stellar radius, significantly larger
than what is found for the Sun. The inner boundary of the extra-mixing region
is found at where is the stellar radius.
Conclusions: These results contribute to the tachocline characterization in
stars other than the Sun.Comment: 4 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics Letter
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