A legacy sample of Red Giant stars for Galactic Archaeology: joining asteroseismology, spectroscopy and Gaia data

• Applicant: Dr. Marica Valentini
• Subject Area: Astrophysics and Astronomy
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 428473034

Galactic Archaeology, the discipline that investigates how the Milky Way has formed and evolved, strongly depends on the quality of fundamental stellar properties as distances, masses, ages, and chemical composition. Red giant stars are key targets for Galactic studies, thanks to their large number and intrinsic brightness, that allow to sample different components of our Galaxy, such as the Halo, the Thin and the Thick disc, and the Bulge. They cover a wide range in mass, age, and chemical composition and they are therefore the main targets of ongoing and future large Galactic spectroscopic surveys.Until very recently, however, the usefulness of red giants as tools for Galactic Archaeology was severely limited for two reasons: i) the difficulty in determining a precise stellar surface gravity, log(g), which affects the element abundances determination, since the two are strictly coupled; ii) the large uncertainty on the age, of about ~80%, when it is determined with the classical method of isochrone fitting. The modern, sophisticated models of the chemo-dynamical evolution of the Galaxy need much more stringent observational constraints. How do we reach the precision that is required to better understand the Milky Way and its formation and evolution?With the advent of asteroseismology, the situation has changed significantly: asteroseismology can be used to constrain stellar mass with a precision of 10%, leading to a precision of 30% in age, and it provides log(g) with a precision of 0.01 dex, ten times better than what can be achieved with the classic spectroscopic analysis. We have successfully developed both a method and the tools to combine asteroseismology and spectroscopy: i) a spectroscopic analysis pipeline that uses iteratively the seismic gravity, and ii) a Bayesian tool for mass and age determination that combines asteroseismic quantities with the precise atmospheric parameters from spectroscopy and Gaia data as input.We propose here a project that applies our approach for constructing a sample of red giant stars with precise distances, masses, ages, and element abundances: the red giant legacy sample. We have already secured all the data for the completion of the project: spectra coming from various surveys plus successful observing proposals (GES, APOGEE, RAVE and ESO facilities) and seismic data coming from different space missions (CoRoT, Kepler, K2, TESS). All these data will be analysed in a homogeneous way using our tools. This unprecedented sample will probe different regions of the Galaxy, in particular we will provide a unique insight into the Halo and the Bulge. As final step, the legacy sample will be used as calibration and learning sample for data-driven analysis by future, large spectroscopic survey such as 4MOST.

• DFG Programme: Research Grants