Arlette Noels-Grötsch

The underlying physical meaning of the νmax νc relation

Asteroseismology of stars that exhibit solar-like oscillations are enjoying a growing interest with the wealth of observational results obtained with the CoRoT and Kepler missions. In this framework, scaling laws between asteroseismic quantities and stellar parameters are becoming essential tools to study a rich variety of stars. However, the physical underlying mechanisms of those scaling laws are still poorly known. Our objective is to provide a theoretical basis for the scaling between the frequency of the maximum in the power spectrum (νmax) of solar-like oscillations and the cut-off frequency (νc). Using the SoHO GOLF observations together with theoretical considerations, we first confirm that the maximum of the height in oscillation power spectrum is determined by the so-called plateau of the damping rates. The physical origin of the plateau can be traced to the destabilizing effect of the Lagrangian perturbation of entropy in the upper-most layers, which becomes important when the modal period and the local thermal relaxation time-scale are comparable. Based on this analysis, we then find a linear relation between νmax and νc, with a coefficient that depends on the ratio of the Mach number of the exciting turbulence to the third power to the mixing-length parameter.

Red Giants as Probes of the Structure and Evolution of the Milky Way

Part I Asteroseismology of red giants.- Aspects of observational red giant population seismology.- Asteroseismology of red giants as a tool for studying stellar populations: first steps.- Adiabatic solar-like oscillations in red giant stars.- Energetic aspects of non-radial solar-like oscillations in red giants.- Part II Internal structure, atmosphere, and evolution of red giants: current models and their uncertainties.- Evolution and internal structure of red giants.- Uncertainties and systematics in stellar evolution models of Red Giant Stars.- Convection modelling and the morphology of RGBs in stellar clusters.- Helium burning in moderate-mass stars.- Hydrodynamic simulations of shell convection in stellar cores.- Impact of rotational mixing on the global and asteroseismic properties of red giants.- 3D picture of the convective envelope of a rotating RGB star.- Effects of rotation and thermohaline mixing in red giant stars.- 3D Model Atmospheres of Red Giant Stars.- Part III Stellar populations in the Milky Way.- Structure and Evolution of the Milky Way.- Red Giant stars: probing the MilkyWay chemical enrichment.- Chemical abundances of giants in globular clusters.- TRILEGAL, a TRIdimensional modeL of thE GALaxy: status and future.- The Besanc,on model of stellar population synthesis of the Galaxy.

The IACOB project. IV. New predictions for high-degree non-radial mode instability domains in massive stars and their connection with macroturbulent broadening

Context. Asteroseismology is a powerful tool to access the internal structure of stars. Apart from the important impact of theoretical developments, progress in this field has been commonly associated with the analysis of time-resolved observations. Recently, the so-called macroturbulent broadening has been proposed as a complementary and less expensive way – in terms of observational time – to investigate pulsations in massive stars. Aims. We assess to what extent this ubiquitous non-rotational broadening component which shapes the line profiles of O stars and B supergiants is a spectroscopic signature of pulsation modes driven by a heat mechanism. Methods. We compute stellar main-sequence and post-main-sequence models from 3 to 70 M with the ATON stellar evolution code, and determine the instability domains for heat-driven modes for degrees `= 1–20 using the adiabatic and non-adiabatic codes LOSC and MAD. We use the observational material compiled in the framework of the IACOB project to investigate possible correlations between the single snapshot line-broadening properties of a sample of ≈260 O and B-type stars and their location inside or outside the various predicted instability domains. Results. We present an homogeneous prediction for the non-radial instability domains of massive stars for degree ` up to 20. We provide a global picture of what to expect from an observational point of view in terms of the frequency range of excited modes, and we investigate the behavior of the instabilities with respect to stellar evolution and the degree of the mode. Furthermore, our pulsational stability analysis, once compared to the empirical results, indicates that stellar oscillations originated by a heat mechanism cannot explain alone the occurrence of the large non-rotational line-broadening component commonly detected in the O star and B supergiant domain.

Uncertainties in Models of Stellar Structure and Evolution

Numerous physical aspects of stellar physics have been presented in Session 2 and the underlying uncertainties have been tentatively assessed. We try here to highlight some specific points raised after the talks and during the general discussion at the end of the session and eventually at the end of the workshop. A table of model uncertainties is then drawn with the help of the participants in order to give the state of the art in stellar modeling uncertainties as of July 2013.

Inversions of the Ledoux discriminant: a closer look at the tachocline

Modelling the base of the solar convective envelope is a tedious problem. Since the first rotation inversions, solar modellers are confronted with the fact that a region of very limited extent has an enormous physical impact on the Sun. Indeed, it is the transition region from differential to solid body rotation, the tachocline, which furthermore is influenced by turbulence and is also supposed to be the seat of the solar magnetic dynamo. Moreover, solar models show significant disagreement with the sound speed profile in this region. In this paper, we show how helioseismology can provide further constraints on this region by carrying out an inversion of the Ledoux discriminant. We compare these inversions for Standard Solar Models built using various opacity tables and chemical abundances and discuss the origins of the discrepancies between Solar Models and the Sun.

Seismic inversion of the solar entropy. A case for improving the standard solar model

The Sun is the most constrained and well-studied of all stars. As a consequence, the physical ingredients entering solar models are used as a reference to study all other stars observed in the Universe. However, our understanding of the solar structure is still imperfect, as illustrated by the current debate on the heavy element abundances in the Sun. We wish to provide additional information on the solar structure by carrying out structural inversions of a new physical quantity, a proxy of the entropy of the solar plasma which properties are very sensitive to the temperature gradient below the convective zone. We use new structural kernels to carry out direct inversions of an entropy proxy of the solar plasma and compare the solar structure to various standard solar models built using various opacity tables and chemical abundances. We also link our results to classical tests commonly found in the literature. Our analysis allows us to probe more efficiently the uncertain regions of the solar models, just below the convective zone, paving the way for new in-depth analyses of the Sun taking into account additional physical uncertainties of solar models beyond the specific question of chemical abundances.

Effects of extra-mixing processes on the periods of high-order gravity modes in main-sequence stars

In main-sequence stars, the chemical composition gradient that develops at the edge of the convective core is responsible for a non-uniform period spacing of high-order gravity modes. In this work we investigate, in the case of a 1.6 Msun star, the effects on the period-spacing of extra mixing processes in the core (such as diffusion and overshooting).

Nonradial oscillations of solar models with an initial discontinuity in hydrogen abundance

Solar models are calculated with low central hydrogen abundance. The stability of these models is investigated. The eigenspectrum is computed and compared with the SCLERA observations of solar oscillation.

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.

Non-radial, non-adiabatic solar-like oscillations in RGB and HB stars

CoRoT and Kepler observations of red giants reveal rich spectra of non-radial solar-like oscillations allowing to probe their internal structure. We compare the theoretical spectrum of two red giants in the same region of the HR diagram but in different evolutionary phases. We present here our first results on the inertia, lifetimes and amplitudes of the oscillations and discuss the differences between the two stars.