The Homogeneity of the Star-forming Environment of the Milky Way Disk over Time

Abstract

Stellar abundances and ages afford the means to link chemical enrichment to galactic formation. In the Milky Way, individual element abundances show tight correlations with age, which vary in slope across ([Fe/H]–[α/Fe]). Here, we step from characterizing abundances as measures of age, to understanding how abundances trace properties of stellar birth environment in the disk over time. Using measurements from ∼27,000 APOGEE stars (R = 22,500, signal-to-noise ratio > 200), we build simple local linear models to predict a sample of elements (X = Si, O, Ca, Ti, Ni, Al, Mn, Cr) using (Fe, Mg) abundances alone, as fiducial tracers of supernovae production channels. Given [Fe/H] and [Mg/H], we predict these elements, [X/H], to about double the uncertainty of their measurements. The intrinsic dispersion, after subtracting measurement errors in quadrature is ≈0.015–0.04 dex. The residuals of the prediction (measurement − model) for each element demonstrate that each element has an individual link to birth properties at fixed (Fe, Mg). Residuals from primarily massive-star supernovae (i.e., Si, O, Al) partially correlate with guiding radius. Residuals from primarily supernovae Ia (i.e., Mn, Ni) partially correlate with age. A fraction of the intrinsic scatter that persists at fixed (Fe, Mg), however, after accounting for correlations, does not appear to further discriminate between birth properties that can be traced with present-day measurements. Presumably, this is because the residuals are also, in part, a measure of the typical (in)-homogeneity of the disk’s stellar birth environments, previously inferred only using open cluster systems. Our study implies at fixed birth radius and time that there is a median scatter of ≈0.01–0.015 dex in elements generated in supernovae sources.

Residual Abundances in GALAH DR3: Implications for Nucleosynthesis and Identification of Unique Stellar Populations

“…W22 estimate that the intrinsic dispersion of two-process residuals is ∼0.02-0.03 dex for most of the well-measured APOGEE elements, rising to ∼0.06-0.08 dex for Na, V, and Ce. Ting & Weinberg (2022) and Ness et al (2022) find similar values for the intrinsic dispersion of stellar abundances after conditioning on Mg and Fe, which is similar in practice to fitting the two-process model and computing rms residuals. As emphasized in these papers, the correlations of residuals can provide robust evidence of underlying structure in the element distribution, even when the residuals for any individual star are comparable to the measurement uncertainties.…”

“…If a cluster is enhanced in some element, we expect that all stars in that cluster will show similar enhancements. While processes such as atomic diffusion, planet formation, and planet engulfment can cause surface abundance variations between co-natal stars (Casamiquela et al 2021, and references therein), many works have measured the level of homogeneity among cluster members to be within 0.02-0.03 dex (De Silva et al 2006;Bovy 2016;Liu et al 2016;Casamiquela et al 2020;Ness et al 2022). In this section, we study the residual abundances of known open clusters in GALAH membership and cluster age taken from Spina et al (2021).…”

Chemical Cartography with APOGEE: Mapping Disk Populations with a 2-process Model and Residual Abundances

“…To compare distinct populations such as bulge and disk or disk and satellites, one must create comparison samples that are matched in g log (e.g., Griffith et al 2021a;Hasselquist et al 2021) and/or condition on g log and T eff as variables in addition to abundances (TW22). Such comparisons would become more straightforward if the g T log eff systematics in APOGEE abundances were removed either by empirical calibration (Eilers et al 2022;Ness et al 2022) or, preferably, by identifying and correcting the effects that give rise to them.…”

The chemical enrichment of the Milky Way disk evaluated using conditional abundances

“…The two channels of ([Fe/H], [Mg/Fe]) alone are able to well-predict [X/Fe] information (e.g. Ness et al 2022). However, seven abundances are needed in order to remove residual correlations, and each element may provide individual information linking to birth properties (Ting & Weinberg 2022).…”