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 $10~\mu H_z\le \nu \le 100~\mu H_z$. 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 $0.05$ cm s${}^{-1}$ for $\ell =1$ and $\nu\approx 100~\mu H_z$. 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 $\nu \approx100~\mu H_z$ and $\ell=1$ is reduced to about 50 yrs.Comment: 18 pages, Accepted for publication in A&

The evolution and impact of 3000 M⊙_\odot 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&

Degradability and Fermentescibility According to Fiber Content in Low Quality Forages

The nylon bag technique and the gas-test method were compared with 6 different roughages representing a wide range of fiber contents in samples collected in the Senegalese Groundnut Basin. The results showed high correlation coefficients and low residual standard errors between the chemical composition of the roughages (mainly the acid detergent fiber fraction) and their degradability or fermentescibility. Reliable relationships between the two methods were also identified

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-$\rm \alpha$ 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-$\rm \alpha$ variations from a stellar origin. We show that given the low planetary mass ($\sim$ 0.03 M$_{\rm \oplus}$) and relatively large orbital distance ($\sim$ 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