150 research outputs found
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
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
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
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
Application of time-dependent convection models to the photometric mode identification in gamma Doradus stars
We apply the Time-Dependent Convection (TDC) treatment of Gabriel \cite{Gabriel1996} and Grigahcène et al. \cite{Grigahcene} to the photometric mode identification in gamma Dor stars. Comparison of our theoretical results with the observed amplitudes and phases of the star gamma Dor is presented. This comparison makes the identification of the degree l of its pulsation modes possible and shows that our TDC models better agree with observations than Frozen Convection (FC) models
Determining the metallicity of the solar envelope using seismic inversion techniques
peer reviewedThe 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
Higher metal abundances do not solve the solar problem
Context. The Sun acts as a cornerstone of stellar physics. Thanks to
spectroscopic, helioseismic and neutrino flux observations, we can use the Sun
as a laboratory of fundamental physics in extreme conditions. The conclusions
we draw are then used to inform and calibrate evolutionary models of all other
stars in the Universe. However, solar models are in tension with helioseismic
constraints. The debate on the ``solar problem'' has hitherto led to numerous
publications discussing potential issues with solar models and abundances.
Aims. Using the recently suggested high-metallicity abundances for the Sun, we
investigate whether standard solar models, as well as models with macroscopic
transport reproducing the solar surface lithium abundances and analyze their
properties in terms of helioseismic and neutrino flux observations. Methods. We
compute solar evolutionary models and combine spectroscopic and helioseismic
constraints as well as neutrino fluxes to investigate the impact of macroscopic
transport on these measurements. Results. When high-metallicity solar models
are calibrated to reproduce the measured solar lithium depletion, tensions
arise with respect to helioseismology and neutrino fluxes. This is yet another
demonstration that the solar problem is also linked to the physical
prescriptions of solar evolutionary models and not to chemical composition
alone. Conclusions. A revision of the physical ingredients of solar models is
needed in order to improve our understanding of stellar structure and
evolution. The solar problem is not limited to the photospheric abundances if
the depletion of light elements is considered. In addition, tighter constraints
on the solar beryllium abundance will play a key role in the improvement of
solar models.Comment: Accepted for publication in Astronomy and Astrophysic
Evidence for a sharp structure variation inside a red-giant star
The availability of precisely determined frequencies of radial and non-radial
oscillation modes in red giants is finally paving the way for detailed studies
of the internal structure of these stars. We look for the seismic signature of
regions of sharp structure variation in the internal structure of the CoRoT
target HR7349. We analyse the frequency dependence of the large frequency
separation and second frequency differences, as well as the behaviour of the
large frequency separation obtained with the envelope auto-correlation
function. We find evidence for a periodic component in the oscillation
frequencies, i.e. the seismic signature of a sharp structure variation in
HR7349. In a comparison with stellar models we interpret this feature as caused
by a local depression of the sound speed that occurs in the helium
second-ionization region. Using solely seismic constraints this allows us to
estimate the mass (M=1.2^{+0.6}_{-0.4} Msun) and radius (R=12.2^{+2.1}_{-1.8}
Rsun) of HR7349, which agrees with the location of the star in an HR diagram.Comment: 4 pages, 5 figures, accepted in A&A Letter
The CoRoT Evolution and Seismic Tools Activity: Goals and Tasks
The forthcoming data expected from space missions such as CoRoT require the
capacity of the available tools to provide accurate models whose numerical
precision is well above the expected observational errors. In order to secure
that these tools meet the specifications, a team has been established to test
and, when necessary, to improve the codes available in the community. The CoRoT
evolution and seismic tool activity (ESTA) has been set up with this mission.
Several groups have been involved. The present paper describes the motivation
and the organisation of this activity, providing the context and the basis for
the presentation of the results that have been achieved so far. This is not a
finished task as future even better data will continue to demand more precise
and complete tools for asteroseismology.Comment: 11 pages, 3 figures, accepted for publication in Astrophysics and
Space Science, 'CoRoT ESTA' special volum
Solar-like oscillations in a massive star
Seismology of stars provides insight into the physical mechanisms taking
place in their interior, with modes of oscillation probing different layers.
Low-amplitude acoustic oscillations excited by turbulent convection were
detected four decades ago in the Sun and more recently in low-mass
main-sequence stars. Using data gathered by the Convection Rotation and
Planetary Transits mission, we report here on the detection of solar-like
oscillations in a massive star, V1449 Aql, which is a known large-amplitude (b
Cephei) pulsator.Comment: Published in Sience, 19 June 2009, vol. 324, p. 154
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