Variations in α-element Ratios Trace the Chemical Evolution of the Disk

Abstract

It is well established that the chemical structure of the Milky Way exhibits a bimodality with respect to the α-enhancement of stars at a given [Fe/H]. This has been studied largely based on a bulk α abundance, computed as a summary of several individual α-elements. Inspired by the expected subtle differences in their nucleosynthetic origins, here we probe the higher level of granularity encoded in the inter-family [Mg/Si] abundance ratio. Using a large sample of stars with APOGEE abundance measurements, we first demonstrate that there is additional information in this ratio beyond what is already apparent in [α/Fe] and [Fe/H] alone. We then consider Gaia astrometry and stellar age estimates to empirically characterize the relationships between [Mg/Si] and various stellar properties. We find small but significant trends between this ratio and α-enhancement, age, [Fe/H], location in the Galaxy, and orbital actions. To connect these observed [Mg/Si] variations to a physical origin, we attempt to predict the Mg and Si abundances of stars with the galactic chemical evolution model Chempy. We find that we are unable to reproduce abundances for the stars that we fit, which highlights tensions between the yield tables, the chemical evolution model, and the data. We conclude that a more data-driven approach to nucleosynthetic yield tables and chemical evolution modeling is necessary to maximize insights from large spectroscopic surveys.

Neutron-capture elements record the ordered chemical evolution of the disc over time

“…The parameters we chose to exclude from the optimisation routine are the time delay of SNIa enrichment (log 10 (𝜏I a )), the fraction of stellar yields which outflow to the surrounding gas reservoir (𝜒 out ), and the initial gas reservoir (log 10 ( 𝑓 corona )). As discussed in Blancato et al (2019), the three non-fitted parameters (namely, log 10 (𝜏I a ), 𝜒 out , and log 10 ( 𝑓 corona )) have been shown to be relatively uninformative. Additionally, as described in Rybizki et al (2017), observational constraints on these three parameters are less certain.…”

“…The ability to determine precise chemical abundances from spectra for > 10 5 − 10 6 stars in the Galaxy is providing a wealth of data to be explored. Chemical abundance measurements, when coupled with precise phase space information from the Gaia mission (Gaia Collaboration et al 2020), and stellar ages (e.g., Ness et al 2016;Sharma et al 2020;Hayden et al 2020), permit a full chemo-dynamical-age exploration of stellar populations, especially in the most massive Galactic stellar component: the Milky Way's disc (e.g., Gratton et al 1996;Fuhrmann 1998Fuhrmann , 2004Adibekyan et al 2011;Bovy et al 2012a,b;Minchev et al 2013;Bensby et al 2014;Minchev et al 2014;Hayden et al 2015;Martig et al 2016;Ness et al 2016;Mackereth et al 2017;Minchev et al 2017;Antoja et al 2018;Ciucǎ et al 2018;Gaia Collaboration et al 2018;Blancato et al 2019;Bland-Hawthorn et al 2019;Bovy et al 2019;Buder et al 2019;Ness et al 2019;Mackereth et al 2019b;Hayden et al 2020;Ciucă et al 2021;Yuxi et al 2021) 1 .…”

“…There are many uncertainties and even the Solar abundances can not be reproduced under the most flexible of approaches, indicative of inaccuracies in the yields (e.g. Rybizki et al 2017;Blancato et al 2019). Detailed and complex multi-zone frameworks with prescriptions of migration offer perhaps the most theoretically complete approach to reconciling theory with observations (e.g.…”

Exoplanets in the Galactic context: Planet occurrence rates in the thin disk, thick disk and stellar halo of Kepler stars

“…From elemental abundance perspective, following Blancato et al (2019) and Bashi et al (2020), we used a Gaussian Mixture Model (GMM) classifier and fixed the number of clusters to two in order to capture the bimodality in the [Fe/H]-[α/Fe] plane. Consequently, each point is affiliated with one of the two clusters with some probability P α , where values closer to 1 suggest higher probability to belong to the solar-α cluster while values closer to 0 suggest higher probability to belong to the α-enriched cluster.…”

Two sequences in the age-metallicity relation as seen from [C/N] abundances in APOGEE

“…This element is believed to be produced by both, SNII as well as SNIa, hence it does not correlate directly with Mg which is mostly produced by SNII. Furthermore, the production mechanism in SNII is different to Mg which translates to a dependency on the progenitor's mass (Blancato et al 2019).…”