78 research outputs found
Global Helioseismology
Helioseismology is one of the most successful fields of astrophysics. The
observation and characterization of solar oscillation has allowed solar
seismologists to study the internal structure and dynamics of the Sun with
unprecedented thoroughness. Ground-based networks and dedicated space missions
have delivered data of exquisite quality, enabling the development of
sophisticated inference techniques. The achievements of the fields count,
amongst other, the determination of solar photospheric helium abundance,
unacessible to spectroscopic constraints, the precise positioning of the base
of the convective zone and the demonstration of the importance of microscopic
diffusion in stellar radiative regions. Helioseismology played also a key role
in validating the framework used to compute solar and stellar models and played
an important role in the so-called solar neutrino problem. In the current era
of astrophysics, with the increasing importance of asteroseismology to
precisely characterize stars, the Sun still plays a crucial calibration role,
acting as a benchmark for stellar models. With the revision of the solar
abundances and the current discussions related to radiative opacity
computations, the role of the Sun as a laboratory of fundamental physics is
undisputable. In this brief review, I will discuss some of the inference
techniques developed in the field of helioseismology, dedicated to the
exploitation of the solar global oscillation modes.Comment: To appear in the proceedings of the meeting "How Much do we Trust
Stellar Models?" held in Li\`ege in Sept 201
Constraints on the structure of 16 Cyg A and 16 Cyg B using inversion techniques
Constraining mixing processes and chemical composition is a central problem
in stellar physics as their impact on stellar age determinations leads to
biases in our studies of stellar evolution, galactic history and exoplanetary
systems. In two previous papers, we showed how seismic inversion techniques
could offer strong constraints on such processes by pointing out weaknesses in
theoretical models. We now apply our technique to the solar analogues 16CygA
and 16CygB, being amongst the best targets in the Kepler field to test the
diagnostic potential of seismic inversions. The combination of various seismic
indicators helps to provide more constrained and accurate fundamendal
parameters for these stars. We use the latest seismic, spectroscopic and
interferometric observational constraints in the litterature for this system to
determine reference models independently for both stars. We carry out seismic
inversions of the acoustic radius, the mean density and a core conditions
indicator. We note that a degeneracy exists for the reference models. Namely,
changing the diffusion coefficient or the chemical composition within the
observational values leads to 5% changes in mass, 3% changes in radius and up
to 8% changes in age. We use acoustic radius and mean density inversions to
improve our reference models then carry out inversions for a core conditions
indicator. Thanks to its sensitivity to microscopic diffusion and chemical
composition mismatches, we are able to reduce the mass dispersion to 2%, namely
[0.96, 1.0] M_sun, the radius dispersion to 1%, namely [1.188, 1.200] R_sun and
the age dispersion to 3%, namely [7.0, 7.4] Gy, for 16CygA. For 16CygB, we can
check the consistency of the models but not reduce independently the age
dispersion. Nonetheless, assuming consistency with the age of 16CygA helps to
further constrain its mass and radius.Comment: Submitted to Astronomy and Astrophysic
Using seismic inversions to obtain an internal mixing processes indicator for main-sequence solar-like stars
Determining accurate and precise stellar ages is a major problem in
astrophysics. These determinations are either obtained through empirical
relations or model-dependent approaches. Currently, seismic modelling is one of
the best ways of providing accurate ages. However, current methods are affected
by simplifying assumptions concerning mixing processes. In this context,
providing new structural indicators which are less model-dependent and more
sensitive to such processes is crucial. We build a new indicator for core
conditions on the main sequence, which should be more sensitive to structural
differences and applicable to older stars than the indicator t presented in a
previous paper. We also wish to analyse the importance of the number and type
of modes for the inversion, as well as the impact of various constraints and
levels of accuracy in the forward modelling process that is used to obtain
reference models for the inversion. First, we present a method to obtain new
structural kernels and use them to build an indicator of central conditions in
stars and test it for various effects including atomic diffusion, various
initial helium abundances and metallicities, following the seismic inversion
method presented in our previous paper. We then study its accuracy for 7
different pulsation spectra including those of 16CygA and 16CygB and analyse
its dependence on the reference model by using different constraints and levels
of accuracy for its selection We observe that the inversion of the new
indicator using the SOLA method provides a good diagnostic for additional
mixing processes in central regions of stars. Its sensitivity allows us to test
for diffusive processes and chemical composition mismatch. We also observe that
octupole modes can improve the accuracy of the results, as well as modes of low
radial order.Comment: Accepted for publication in Astronomy and Astrophysic
Probing stellar cores from inversions of frequency separation ratios
With the rapid development of asteroseismology thanks to space-based
photometry missions such as CoRoT, Kepler, TESS, and in the future, PLATO, and
the use of inversion techniques, quasi-model-independent constraints on the
stellar properties can be extracted from a given stellar oscillation spectrum.
In this context, inversions based on frequency separation ratios, that are less
sensitive to surface effects, appear as a promising technique to constrain the
properties of stellar convective cores. We developed an inversion based on
frequency separation ratios with the goal of damping the surface effects of the
oscillation frequencies. Using this new inversion, we defined a new indicator
to constrain the boundary mixing properties of convective cores in solar-like
oscillators. We verified our inversion technique by conducting tests in a
controlled environment, where the stellar mass and radius are known exactly,
and conducted an extensive hare and hounds exercise. The inversion is not
affected by surface effects. With the construction of an extensive set of
models, favoured and forbidden regions can be highlighted in the parameter
space. If the ratios are well fitted, the inversion is unsurprisingly not
providing additional information. The indicator coupled with the inversion
based on frequency separation ratios seems promising at probing the properties
of convective cores, especially for F-type stars exhibiting solar-like
oscillations.Comment: Accepted for publication in Astronomy and Astrophysic
Amplitude of solar gravity modes generated by penetrative plumes
The detection of gravity modes is expected to give us unprecedented insights
into the inner dynamics of the Sun. Within this framework, predicting their
amplitudes is essential to guide future observational strategies and seismic
studies. In this work, we predict the amplitude of low-frequency asymptotic
gravity modes generated by penetrative convection at the top of the radiative
zone. The result is found to depend critically on the time evolution of the
plumes inside the generation region. Using a solar model, we compute the GOLF
apparent surface radial velocity of low-degree gravity modes in the frequency
range . In case of a Gaussian plume time
evolution, gravity modes turn out to be undetectable because of too small
surface amplitudes. This holds true despite a wide range of values considered
for the parameters of the model. In the other limiting case of an exponential
time evolution, plumes are expected to drive gravity modes in a much more
efficient way because of a much higher temporal coupling between the plumes and
the modes than in the Gaussian case. Using reasonable values for the plume
parameters based on semi-analytical models, the apparent surface velocities in
this case turn out to be one order of magnitude smaller than the 22-years GOLF
detection threshold and than the previous estimates considering turbulent
pressure as the driving mechanism, with a maximum value of cm s
for and . When accounting for uncertainties
on the plume parameters, the apparent surface velocities in the most favorable
plausible case become comparable to those predicted with turbulent pressure,
and the GOLF observation time required for a detection at and is reduced to about 50 yrs.Comment: 18 pages, Accepted for publication in A&
The evolution and impact of 3000 M stars in the early Universe
We present evolutionary models of massive, accreting population III stars
with constant and variable accretion rates until the end of silicon burning,
with final masses of 1000 - 3000 Msol. In all our models, after the
core-hydrogen-burning phase, the star expands towards the red side of the
Hertzsprung-Russell diagram is where it spends the rest of its evolution.
During core helium burning, the models exhibit an outer convective envelope as
well as many large intermediate convective zones.These intermediate zones allow
for strong internal mixing to occur which enriches the surface in helium. The
effect of increasing metallicity at a constant accretion rate of 10^{-3}
Msol/yr shows an increase in the lifetime, final mass and distribution of
helium in the envelope. Our fiducial model with mass of 3000 Msol has a final
surface helium abundance of 0.74 and 9% of its total mass or 50% of the core
mass, has a value of Gamma1 < 4/3 at the end of core silicon burning. If the
collapse of the core is accompanied by the ejection of the envelope above the
carbon-oxygen core, this could have a significant impact on the chemical
evolution of the surroundings and subsequent stellar generations. The model has
a final log(N/O) ~ 0.45, above the lower limit in the recently detected
high-redshift galaxy GN-z11. We discuss the impact of a single 3000 Msol star
on chemical, mechanical and radiative feedback, and present directions for
future work.Comment: Accepted for publication in A&
Revisiting Kepler-444. II. Rotational, orbital and high-energy fluxes evolution of the system
Context. Kepler-444 is one of the oldest planetary systems known thus far.
Its peculiar configuration consisting of five sub-Earth-sized planets orbiting
the companion to a binary stellar system makes its early history puzzling.
Moreover, observations of HI-Ly- variations raise many questions
about the potential presence of escaping atmospheres today. Aims. We aim to
study the orbital evolution of Kepler-444-d and Kepler-444-e and the impact of
atmospheric evaporation on Kepler-444-e. Methods. Rotating stellar models of
Kepler-444-A were computed with the Geneva stellar evolution code and coupled
to an orbital evolution code, accounting for the effects of dynamical,
equilibrium tides and atmospheric evaporation. The impacts of multiple stellar
rotational histories and extreme ultraviolet (XUV) luminosity evolutionary
tracks are explored. Results. Using detailed rotating stellar models able to
reproduce the rotation rate of Kepler-444-A, we find that its observed rotation
rate is perfectly in line with what is expected for this old K0-type star,
indicating that there is no reason for it to be exceptionally active as would
be required to explain the observed HI-Ly- variations from a
stellar origin. We show that given the low planetary mass ( 0.03 M) and relatively large orbital distance ( 0.06 AU) of
Kepler-444-d and e, dynamical tides negligibly affect their orbits, regardless
of the stellar rotational history considered. We point out instead how
remarkable the impact is of the stellar rotational history on the estimation of
the lifetime mass loss for Kepler-444-e. We show that, even in the case of an
extremely slow rotating star, it seems unlikely that such a planet could retain
a fraction of the initial water-ice content if we assume that it formed with a
Ganymede-like composition
Constraints on planetary tidal dissipation from a detailed study of Kepler 91b
Context. With the detection of thousands of exoplanets, characterising their
dynamical evolution in detail represents a key step in the understanding of
their formation. Studying the dissipation of tides occurring both in the host
star and in the planets is of great relevance in order to investigate the
distribution of the angular momentum occurring among the objects populating the
system and to studying the evolution of the orbital parameters. From a
theoretical point of view, the dissipation of tides throughout a body may be
studied by relying on the so-called phase or time-lag equilibrium tides model
in which the reduced tidal quality factor Q'p, or equivalently the product
between the love number and the time lag (k2DeltaT), describe how efficiently
tides are dissipated within the perturbed body. Constraining these factors by
looking at the current configuration of the exoplanetary system is extremely
challenging, and simulations accounting for the evolution of the system as a
whole might help to shed some light on the mechanisms governing this process.
Aims. We aim to constrain the tidal dissipation factors of hot-Jupiter-like
planets by studying the orbital evolution of Kepler-91b. Methods. We firstly
carried out a detailed asteroseismc characterisation of Kepler-91 and computed
a dedicated stellar model using both classical and astereoseismic constraints.
We then coupled the evolution of the star to the one of the planets by means of
our orbital evolution code and studied the evolution of the system by
accounting for tides dissipated both in the planet and in the host star.
Results. We found that the maximum value for k2DeltaT (or equivalently the
minimum value for Q'p) determining the efficiency of equilibrium tides
dissipation occurring within Kepler-91b is 0.4 pm 0.25 s (4.5+5.8 * 10^5).Comment: accepted for publication in Astronomy & Astrophysic
A Rapid Estimation of Nitrogen Bound to Neutral Detergent Fibre in Forages by Near Infrared Reflectance Spectroscopy
Near infrared reflectance spectroscopy (NIRS) is widely used as a rapid method for the evaluation of the chemical composition or the nutritive value of foodstuffs (Givens et al., 1997). The determination of the neutral detergent fibre (NDF) bound N (NDF-N), which is highly variable in forages (Shayo & Udén, 1999), is expensive. The purpose of this study was to test the use of NIRS in the prediction of NDF-N in various forages
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