K2 results for “young” α-rich stars in the Galaxy

Abstract

Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.