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 characterising 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, SNR>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, 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

“…W21 estimate that the intrinsic dispersion of twoprocess 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 (2021) and Ness et al (2021) 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.…”

Stellar Abundance Maps of the Milky Way Disk

“…The observed smooth gradients across all chemical abundances are maybe unsurprising, considering that all elemental abundances are closely linked with each other (e.g. Ness et al 2019Ness et al , 2021, possibly even hewing closely to a two-or threedimensional surface. However, each element comprises some additional unique infor- mation (e.g.…”

“…This implies that likely similar nucleosynthetic processes enriched both bar and disk stars despite the different star formation histories and physical conditions of these regions (e.g. Griffith et al 2021;Ness et al 2021).…”

“…The pursuit of linking stars to their birth places using their current day abundances (Freeman & Bland-Hawthorn 2002;Ness et al 2021;Ratcliffe et al 2021) in practice asks about birth abundances. Because our sample included only the top of the red-giant branch (above the red clump), we calibrated the abundances to what might be thought of as a "reference" evolutionary state at log g = 2.2.…”

Migration and Mixing in the Galactic Disc from Encounters between Sagittarius and the Milky Way

“…Both metallicity and ages of stars are observationally derived quantities, that can inform us on the original birth radii of stars and the overall timescale of dynamical evolution that has taken place since the present-day (e.g. Minchev et al 2013b;Mackereth et al 2017;Ness et al 2019Ness et al , 2021Sharma et al 2021).…”