Science Summaries

This page features summaries of selected recent publications, written for non-experts. They will highlight some of the range of research done by the group.

Hertzsprung-Russell Diagrams from astrometric+asteroseismic+spectroscopic data

One of the main challenges of Galactic archaeology is to reveal the Galaxy assembly and evolution history via the age, chemical composition, and kinematics of stars in a large fraction of the volume of the Milky Way. In particular, asteroseismology provides us with the crucial chronological information: solar-like oscillating giants are unique evolutionary clocks owing to the availability of seismic constraints on their mass and to the tight age-initial mass relation they adhere to. Even so, a key information for a better understanding of stellar structure within red giants is still missing: their luminosity. This is where Gaia DR2 comes into play, providing distances for over 1.3 billion sources - which includes red-giant stars observed by Kepler. The synergy between Kepler, Gaia DR2, and spectroscopic surveys offers unprecedented possibilities for stress-testing models of stellar evolution unravelling long-standing problems of stellar evolution and, looking at the bigger picture, for helping disentangle the multidimensional problem of Galaxy assembly.

Hertzsprung-Russell Diagrams of red giants observed by Kepler. The K-band absolute magnitude is inferred using Gaia DR2 parallaxes and the parallax zero-point measured in the Kepler field by Khan et al. (2019). Spectroscopic constraints from APOGEE are used to provide information on the effective temperature and chemical composition. Finally, asteroseismology offers precise and accurate ages for these stars (Rodrigues et al. 2017), leading to a very informative Hertzsprung-Russell Diagram and allowing us to create cluster-like sequences of field stars. In the leftmost panel, several red-giant structures are indicated: the red-giant branch bump (RGBb; e.g. see Cassisi 2012), the red clump (RC), the secondary red clump (SRC), and the vertical red clump (VRC; e.g. see Girardi 2016). The other panels first show the selection in age, and then the effect of varying the metallicity.

Mean density inversions for red giants and red clump stars

Accurately weighing red giants is paramount for Galactic archaeology, as these stars act as the standard clocks and rulers to unravel the history of the Milky Way. Currently, the excellent data of the CoRoT and Kepler missions allows asteroseismology to fulfill this role for thousands of stars. More high-quality data can be expected in the coming years thanks to the TESS and PLATO missions. In this study, we adapt and test advanced seismic analysis techniques which had been developed for the main-sequence phase (core hydrogen burning stars) on red-giant stars. We show that these techniques can provide very accurate values of the mean density of red giants, beyond what is achievable by current approaches. We demonstrate the accuracy of our modelling approach by applying it to a sample of eclipsing binaries. The enclosed figure demonstrates that we obtain masses in all cases in good agreement with those determined from eclipses, where other seismic approaches failed to do so. Moreover, the robustness of our approach, which couples seismic inversions to the Bayesian AIMS modelling software, is well-suited for automated pipelines and thus the analyses of large samples of red giants required for Galactic archaeology. Coupled with the recent Gaia data release, our method leads to mass determinations of red giants below 10% in accuracy, reducing significantly the uncertainties on stellar ages and paving the way for in-depth Galactic archaeology.

Masses and radii for a subsample of eclipsing binaries determined from the eclipses and from various seismic methods.