Using combined asteroseismic and spectroscopic observations of 418 red-giant stars close to the Galactic disc plane (6 kpc < R Gal 13 kpc, |Z Gal | < 0.3 kpc), we measure the age dependence of the radial metallicity distribution in the Milky Way's thin disc over cosmic time. The slope of the radial iron gradient of the young red-giant population (−0.058 ± 0.008 [stat.] ±0.003 [syst.] dex/kpc) is consistent with recent Cepheid measurements. For stellar populations with ages of 1 − 4 Gyr the gradient is slightly steeper, at a value of −0.066 ± 0.007 ± 0.002 dex/kpc, and then flattens again to reach a value of ∼ −0.03 dex/kpc for stars with ages between 6 and 10 Gyr. Our results are in good agreement with a state-of-the-art chemo-dynamical Milky-Way model in which the evolution of the abundance gradient and its scatter can be entirely explained by a non-varying negative metallicity gradient in the interstellar medium, together with stellar radial heating and migration. We also offer an explanation for why intermediate-age open clusters in the Solar Neighbourhood can be more metal-rich, and why their radial metallicity gradient seems to be much steeper than that of the youngest clusters. Already within 2 Gyr, radial mixing can bring metal-rich clusters from the innermost regions of the disc to Galactocentric radii of 5 to 8 kpc. We suggest that these outward-migrating clusters may be less prone to tidal disruption and therefore steepen the local intermediate-age cluster metallicity gradient. Our scenario also explains why the strong steepening of the local iron gradient with age is not seen in field stars. In the near future, asteroseismic data from the K2 mission will allow for improved statistics and a better coverage of the inner-disc regions, thereby providing tighter constraints on the evolution of the central parts of the Milky Way.

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.

The solar metallicity issue is a long-lasting problem of astrophysics, impacting multiple fields and still subject to debate and uncertainties. While spectroscopy has mostly been used to determine the solar heavy elements abundance, helioseismologists attempted 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 providing 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.

The Liège International Astrophysical Colloquia (LIAC) started in 1949 at the initiative of Pol Swings, and they were continued with the efforts of Paul Ledoux and Marcel Migeotte. They were organized each year except for those when a General Assembly of the IAU took place. About sixty years after the first edition we are now at the 40th edition of the series. This is not the place to list all the astrophysical discoveries that were presented before publication at a Liège colloquium. Just an example: the first interpretation by Allan Sandage of the HR diagram of a globular cluster in terms of stellar evolution was presented in 1953, but not published until 1957. At that time, even for a major advance in astrophysics such as this, the rush to publish was much, much smaller than it is nowadays.For the 40th edition of the LIAC we have decided to concentrate on ageing stars. Specifically, we have focused on low-mass red giants, either ascending the red giant branch or in the red clump heliumburning phase, sdB stars and white dwarfs. This choice was justified by the increasing quality of groundbased asteroseismic data and by the enormous amount of splendid asteroseismic observations obtained over the last few years by the space missions CoRoT and Kepler.These stars have very complex internal structures. They are extremely interesting because they bear multiple signatures of their past history. For example, their time as young main sequence stars, still with a growing convective core, left unique observational signatures still visible today. These signatures mainly take the form of chemical discontinuities and anomalies in surface abundances. They are the result of physical processes that are still not fully understood such as rotation, convection, overshooting and diffusion, which are not fully properly implemented in stellar evolution codes.Fortunately we have a powerful tool: asteroseismology, a sort of scanner. When added to other tools such as photometry, spectroscopy and interferometry, asteroseismology allows us to shed some light on the interior of stars. However, in evolved stars with huge density contrasts, fast rotating cores and chemical discontinuities leading to sharp features in the Brunt-Väisälä frequency, the message sent to us through the frequency spectra is extremely difficult to understand and interpret. It is a real challenge to decipher what such stars are really telling us.A main goal of this colloquium was first to present and discuss what we know about the structure of low-mass stars at advanced phases of their evolution, and especially to point out the numerous open problems in their structure and their evolution. Asteroseismology then gives a glimpse into the deep interior of these stars. Moreover, we can determine masses, radii and ages with an unprecedented accuracy. Not forgetting all our other tools (in particular spectroscopy for obtaining stellar metallicities), a better understanding of the evolution of stars and of the structure and the chemical evolution of our Galaxy is now within ...