Young [α/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?

Abstract

We report the discovery of a group of apparently young CoRoT red-giant stars exhibiting enhanced [α/Fe] abundance ratios (as determined from APOGEE spectra) with respect to solar values. Their existence is not explained by standard chemical evolution models of the Milky Way, and shows that the chemical-enrichment history of the Galactic disc is more complex. We find similar stars in previously published samples for which isochrone-ages could be reliably obtained, although in smaller relative numbers. This might explain why these stars have not previously received attention. The young [α/Fe]-rich stars are much more numerous in the CoRoT-APOGEE (CoRoGEE) inner-field sample than in any other high-resolution sample available at present because only CoRoGEE can explore the inner-disc regions and provide ages for its field stars. The kinematic properties of the young [α/Fe]-rich stars are not clearly thick-disc like, despite their rather large distances from the Galactic mid-plane. Our tentative interpretation of these and previous intriguing observations in the Milky Way is that these stars were formed close to the end of the Galactic bar, near corotation -a region where gas can be kept inert for longer times than in other regions that are more frequently shocked by the passage of spiral arms. Moreover, this is where the mass return from older inner-disc stellar generations is expected to be highest (according to an inside-out disc-formation scenario), which additionally dilutes the in-situ gas. Other possibilities to explain these observations (e.g., a recent gas-accretion event) are also discussed.

Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe

“…Mapping of the thick and thin disk at all Galactic longitudes, including the inner disk regions, and at the full range of Galactic radii, with substantial samples at least 6 kpc from the Sun and with a significant subsample having reliably determined ages. These maps expand upon APOGEE-1 results (Anders et al 2014;Nidever et al 2014;Hayden et al 2015), and further test scenarios of inside-out growth, radial migration, and the origin of the α-enriched population (Chiappini et al 2015;Martig et al 2015;Bovy et al 2016b). 2.…”

“…2. Accurate stellar ages and masses from the combination of APOGEE data with asteroseismology (e.g., Epstein et al 2014;Chiappini et al 2015;Martig et al 2015), establishing critical benchmarks in the analysis of Galactic chemistry and dynamics in numerous directions sampled by Kepler and its subsequent K2 mission. 3.…”

TheGaia-ESO Survey: characterisation of the [α/Fe] sequences in the Milky Way discs

“…Finally, we note that Martig et al (2015) and Chiappini et al (2015) have found by combining APOGEE abundances and astero-seismic ages, high-α stars with ages younger than 6 Gyr, including at the super-solar metallicity regime. Such young ages would imply that these stars were formed within the Galaxy, reflecting the complex star formation history and ISM mixing of the Milky Way in the inner most regions.…”

Constraining the Milky Way assembly history with Galactic Archaeology

“…This has recently become evident with the unexpected results of Chiappini et al (2015) and Martig et al (2015), who used CoRoT (Baglin et al 2006) and Kepler (Gilliland et al 2010) asteroseismic ages, respectively, combined with APOGEE chemical information, to show the existence of significantly young high-[α/Fe] stars (but see also Yong et al 2016). This is a largely unexpected result for chemical evolution modeling, because [α/Fe] has been thought to always be a good proxy for age.…”