K2 results for "young" α-rich stars in the Galaxy

Grisoni, Valeria

doi:10.5281/zenodo.17249127

K2 results for "young" α-rich stars in the Galaxy

Grisoni, Valeria

Publisher: Zenodo
DOI: 10.5281/zenodo.17249127
Repository: zenodo.org
Language: en

Abstract

The origin of apparently young α-rich stars in the Galaxy is still a matter of debate in Galactic archaeology, whether they are genuinely young or might be products of binary evolution, and mergers or mass accretion.Our aim is to shed light on the nature of young α-rich stars in the Milky Way by studying their distribution in the Galaxy thanks to an unprecedented sample of giant stars that cover different Galactic regions and have precise asteroseismic ages, and chemical and kinematic measurements.We analyzed a new sample of ∼6000 stars with precise ages coming from asteroseismology. Our sample combines the global asteroseismic parameters measured from light curves obtained by the K2 mission with stellar parameters and chemical abundances obtained from APOGEE DR17 and GALAH DR3, then cross-matched with Gaia DR3. We define our sample of young α-rich stars and study their chemical, kinematic, and age properties.We investigated young α-rich stars in different parts of the Galaxy and we find that the fraction of young α-rich stars remains constant with respect to the number of high-α stars at ∼10%. Furthermore, young α-rich stars have kinematic and chemical properties similar to high-α stars, except for [C/N] ratios.Thanks to our new K2 sample, we conclude that young α-rich stars have similar occurrence rates in different parts of the Galaxy, and that they share properties similar to the normal high-α population, except for [C/N] ratios. This suggests that these stars are not genuinely young, but are products of binary evolution, and mergers or mass accretion. Under that assumption, we find the fraction of these stars in the field to be similar to that found recently in clusters. This suggests that ∼10% of the low-α field stars could also have their ages underestimated by asteroseismology. This should be kept in mind when using asteroseismic ages to interpret results in Galactic archaeology.

Full text

K2 results for “young” alpha-rich stars in the Galaxy Valeria Grisoni INAF Trieste valeria.griso[email protected] In collaboration with the ERC Asterochronometry team Based on 2024A&A...683A.111G by Grisoni, Chiappini, Miglio et al. July 2025, ISTA Vienna TASC9/KASC16 workshop 2 Galactic archaeology Galactic Archaeology aims at unveiling the history of evolution of our Galaxy from chemical abundances, kinematics and stellar ages. Then, by means of detailed chemical evolution models it is possible to predict the chemical abundances expected in each Galactic component and thus constrain the history of formation of the Milky Way. Our Galaxy has four main stellar populations: halo, thick and thin discs, bulge. Image credit: Chiappini (2001). Image credit: ESA/ATG medialab; background image: ESO/S. Brunier . 3 Observed and predicted [Mg/Fe] vs. [Fe/H] for the two-infall model (upper panel) and the parallel model (lower panel) (Grisoni et al. 2017) compared to AMBRE data (Mikolaitis et al. 2017). Chemical evolution models Thick disc Thin disc Gap Thick disc Thin disc Two-infall Parallel 4 In general, the high-alpha population is found to be genuinely old (“genuine thick disc”, see Miglio et al. 2021, Queiroz et al. 2023). The high-alpha sequence Miglio et al. (2021) 5 Chiappini et al. (2015) with CoRoGEE and Martig et al. (2015) with Kepler first discovered a class of young alpha-rich stars that cannot be explained by classical chemical evolution models. Two possible explanations proposed: i) related to binary evolution/mass transfer or ii) related to a peculiar star formation events. Young alpha-rich stars Martig et al. (2015) 6 Chiappini et al. (2015) used CoRoGEE data to study the spatial distribution of young alpha-rich stars and found they were more abundant in the inner disc regions, and therefore suggested that they might be related to a peculiar star formation events e.g. in the corotation region. Young alpha-rich stars Chiappini et al. (2015) 7 Results from K2 In Grisoni et al. (2024, A&A), we analyze a sample of nearly 6000 stars that combine the global asteroseismic parameters measured from light curves obtained by the K2 mission with stellar parameters and chemical abundances obtained from APOGEE DR17 and GALAH DR3 and then cross-matched with Gaia DR3 (based on the catalogue of Willett et al., submitted). We investigate young-alpha rich stars in this sample; the full K2-APOGEE is in gray, and the young alpha-rich stars are in red. 8 The forbidden region is defined by the multi-zone chemical evolution models of Grisoni et al. (2017) and the data are from K2-APOGEE. The fraction of YAR stars with respect to the high-alpha sequence is around 10% and it is constant in different bins of Rg and height. Results of Grisoni et al. (2024) R<5 kpc 5<R<7 kpc 7<R<8 kpc 8<R<10 kpc R>10 kpc 9 The forbidden region is defined by the multi-zone chemical evolution models of Grisoni et al. (2017) and the data are from K2-APOGEE. The fraction of YAR stars with respect to the high-alpha sequence is around 10% and it is constant in different bins of Rg and height. Grisoni et al. (2024) R<5 kpc 5<R<7 kpc |Z|<0.5 kpc |Z|>1 kpc 0.5<|Z|<1 kpc