Investigating the Relationship Between Chemical Abundance Ratios and Ages in Red Giant Stars

Abstract

One of the big questions in Astrophysics is how galaxies form and evolve. To understand that it is necessary to study our own Galaxy, the Milky Way, where we can gather information with the highest amount of detail. However, a crucial parameter in that mission, the measurement of stellar ages for red giant stars, has been a challenge. Red giants are important to study because they span a large range of ages, and, due to their intrinsic brightness, allow us to probe further in the Galaxy. Recent developments have enabled the large-scale usage of asteroseismology for measurement of stellar masses, hence ages, at a high level of precision and accuracy for red giants, thanks to the exquisite photometry from space-based observatories.

In this work, we combine the chronological information made possible by the observation of stellar pulsations by Kepler with chemical abundances from high quality spectra collected with HIRES, at the Keck Telescope, and kinematic information made available by the Gaia mission. Here, we dissect in depth a sample of 23 stars, making a detailed chemical analysis from their observed spectra, a careful asteroseismic grid-based modelling to determine their fundamental parameters, ages included, and their kinematics to complete their big picture. These stars were selected due to being identified as outliers in past surveys, with their previously estimated ages in disagreement with their overall chemistry, from the expectations of Galactic chemical evolution models: old and metal-rich, young and metal-poor, young and alpha-rich. Our aim is to understand why these objects are (or appear to be) outliers. The chemical abundances are derived with a classical spectroscopic method, relying on 1D LTE atmospheric models, measurement of equivalent widths and spectral synthesis, using the code MOOG. The asteroseismic grid-based modelling uses the code BASTA for interpolation of several observables, including the characteristic parameters from solar-like oscillations, Delta_nu and nu_max. The kinematic analysis relies on modelling with GalPy. We also detail the code written to streamline the spectroscopic part of the analysis.

We confirm the need for a model-based correction in the determination of stellar masses in red giants. However, such correction was not enough to make the 'young and metal-poor' objects be old enough to have ages compatible with the expectations for the formation of the Milky Way halo. From the analysis of their chrono-chemo-dynamics, two of these stars were identified as being likely accreted by the Milky Way, while two stars are potential blue stragglers. Some outliers are likely due to bad fitting of Delta_nu and nu_max by automatic pipelines, as a careful visual inspection suggests. Our findings also suggest that the uncertainties of Gaia (E)DR3 parallaxes might be underestimated. Also, the age-inference of the stars in the core He-burning phase is still hampered by uncertainties regarding mass loss in the previous evolutionary stage. Finally, we show that our method yields better precision in the determination of stellar masses of red giants when compared to recent large-scale surveys. Show more