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1. On the Origin of Abundance Variations in the Milky Way’s High- α Plateau

   https://doi.org/10.3847/1538-4357/ae0b5d  

   Sit, Tawny; Weinberg, David H; Griffith, Emily J (November 2025, The Astrophysical Journal)

   Abstract Using multielement abundances from the Sloan Digital Sky Survey APOGEE survey, we investigate the origin of abundance variations in Milky Way (MW) disk stars on the “high-αplateau,” with −0.5 ≤ [Mg/H] ≤  −0.1 and 0.25 ≤ [Mg/Fe] ≤ 0.35. The elevated [α/Fe] ratios of these stars imply low enrichment contributions from Type Ia supernovae (SN Ia), but it is unclear whether their abundance patterns reflect pure core-collapse supernova (CCSN) enrichment. We find that plateau stars with higher [Fe/Mg] ratios also have higher [X/Mg] ratios for other iron-peak elements, suggesting that the [Fe/Mg] variations in the plateau population do reflect variations in the SN Ia/CCSN ratio. To quantify this finding, we fit the observed abundance patterns with a two-process model, calibrated on the full MW disk, which represents each star’s abundances as the sum of a prompt CCSN process with amplitudeA_ccand a delayed SN Ia process with amplitudeA_Ia. This model is generally successful at explaining the observed trends of [X/Mg] withA_Ia/A_cc, which are steeper for elements with a large SN Ia contribution (e.g., Cr, Ni, Mn) and flatter for elements with low SN Ia contribution (e.g., O, Si, Ca). Our analysis does not determine the value of [Mg/Fe] corresponding to pure CCSN enrichment, but it should be at least as high as the upper edge of the plateau at [Mg/Fe] ≈ 0.35, and could be significantly higher. Compared to the two-process predictions, the observed trends of [X/Mg] withA_Ia/A_ccare steeper for (C+N) but shallower for Ce, providing intriguing but contradictory clues about asymptotic giant branch enrichment in the early disk. 

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2. Predictions for complex distributions of stellar elemental abundances in low-mass galaxies

   https://doi.org/10.1093/mnras/stac834  

   Patel, Preet B; Loebman, Sarah R; Wetzel, Andrew; Faucher-Giguère, Claude-André; El-Badry, Kareem; Bailin, Jeremy (April 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We investigate stellar elemental abundance patterns at $$z$$ = 0 in eight low-mass ($$M_{*}=10^{6}{-}10^{9}\ \text{M}_{\odot }$$) galaxies in the Feedback in Realistic Environments cosmological simulations. Using magnesium (Mg) as a representative α-element, we explore stellar abundance patterns in magnesium-to-iron ([Mg/Fe]) versus iron-to-hydrogen ([Fe/H]), which follow an overall monotonic trend that evolved slowly over time. Additionally, we explore three notable secondary features in enrichment (in three different case-study galaxies) that arise from a galaxy merger or bursty star formation. First, we observe a secondary track with a lower [Mg/Fe] than the main trend. At $$z$$ = 0, stars from this track are predominantly found within 2–6 kpc of the centre; they were accreted in a 1:3 total-mass-ratio merger ∼0.4 Gyr ago. Second, we find a distinct elemental bimodality that forms following a strong burst in star formation in a galaxy at $$t_{\text{lookback}}\, \sim 10$$ Gyr. This burst quenched star formation for ∼0.66 Gyr, allowing Type Ia supernovae to enrich the system with iron (Fe) before star formation resumed. Third, we examine stripes in enrichment that run roughly orthogonal to the dominant [Mg/Fe] versus [Fe/H] trend; these stripes correspond to short bursts of star formation during which core-collapse supernovae enrich the surrounding medium with Mg (and Fe) on short time-scales. If observed, these features would substantiate the utility of elemental abundances in revealing the assembly and star-formation histories of dwarf galaxies. We explore the observability of these features for upcoming spectroscopic studies. Our results show that precise measurements of elemental abundance patterns can reveal critical events in the formation histories of low-mass galaxies. 

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

   https://doi.org/10.3847/1538-4365/ac6028  

   Weinberg, David H.; Holtzman, Jon A.; Johnson, Jennifer A.; Hayes, Christian; Hasselquist, Sten; Shetrone, Matthew; Ting, Yuan-Sen; Beaton, Rachael L.; Beers, Timothy C.; Bird, Jonathan C.; et al (June 2022, The Astrophysical Journal Supplement Series)

   Abstract We apply a novel statistical analysis to measurements of 16 elemental abundances in 34,410 Milky Way disk stars from the final data release (DR17) of APOGEE-2. Building on recent work, we fit median abundance ratio trends [X/Mg] versus [Mg/H] with a 2-process model, which decomposes abundance patterns into a “prompt” component tracing core-collapse supernovae and a “delayed” component tracing Type Ia supernovae. For each sample star, we fit the amplitudes of these two components, then compute the residuals Δ[X/H] from this two-parameter fit. The rms residuals range from ∼0.01–0.03 dex for the most precisely measured APOGEE abundances to ∼0.1 dex for Na, V, and Ce. The correlations of residuals reveal a complex underlying structure, including a correlated element group comprised of Ca, Na, Al, K, Cr, and Ce and a separate group comprised of Ni, V, Mn, and Co. Selecting stars poorly fit by the 2-process model reveals a rich variety of physical outliers and sometimes subtle measurement errors. Residual abundances allow for the comparison of populations controlled for differences in metallicity and [ α /Fe]. Relative to the main disk ( R = 3–13 kpc), we find nearly identical abundance patterns in the outer disk ( R = 15–17 kpc), 0.05–0.2 dex depressions of multiple elements in LMC and Gaia Sausage/Enceladus stars, and wild deviations (0.4–1 dex) of multiple elements in ω Cen. The residual abundance analysis opens new opportunities for discovering chemically distinctive stars and stellar populations, for empirically constraining nucleosynthetic yields, and for testing chemical evolution models that include stochasticity in the production and redistribution of elements. 

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

   https://doi.org/10.3847/1538-4357/ac5826  

   Griffith, Emily J.; Weinberg, David H.; Buder, Sven; Johnson, Jennifer A.; Johnson, James W.; Vincenzo, Fiorenzo (May 2022, The Astrophysical Journal)

   Abstract We investigate the [X/Mg] abundances of 16 elements for 82,910 Galactic disk stars from GALAH+ DR3. We fit the median trends of low-Ia and high-Ia populations with a two-process model, which describes stellar abundances in terms of a prompt core-collapse and delayed Type-Ia supernova component. For each sample star, we fit the amplitudes of these two components and compute the residual Δ[X/H] abundances from this two-parameter fit. We find rms residuals ≲0.07 dex for well-measured elements and correlated residuals among some elements (such as Ba, Y, and Zn) that indicate common enrichment sources. From a detailed investigation of stars with large residuals, we infer that roughly 40% of the large deviations are physical and 60% are caused by problematic data such as unflagged binarity, poor wavelength solutions, and poor telluric subtraction. As one example of a population with distinctive abundance patterns, we identify 15 stars that have 0.3–0.6 dex enhancements of Na but normal abundances of other elements from O to Ni and positive average residuals of Cu, Zn, Y, and Ba. We measure the median elemental residuals of 14 open clusters, finding systematic ∼0.1–0.4 dex enhancements of O, Ca, K, Y, and Ba and ∼0.2 dex depletion of Cu in young clusters. Finally, we present a restricted three-process model where we add an asymptotic giant branch star (AGB) component to better fit Ba and Y. With the addition of the third process, we identify a population of stars, preferentially young, that have much higher AGB enrichment than expected from their SNIa enrichment. 

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5. Neutron-capture elements in dwarf galaxies III: A homogenized analysis of 13 dwarf spheroidal and ultra-faint galaxies

   Reichert, M.; Hansen, C.; Hanke, M.; Skúladóttir, A.; Arcones, A.; Grebel, E.K. (September 2020, ArXivorg)

   We present a large homogeneous set of stellar parameters and abundances across a broad range of metallicities, involving 13 classical dwarf spheroidal (dSph) and ultra-faint dSph (UFD) galaxies. In total this study includes 380 stars in Fornax, Sagittarius, Sculptor, Sextans, Carina, Ursa Minor, Draco, Reticulum II, Bootes I, Ursa Major II, Leo I, Segue I, and Triangulum II. This sample represents the largest, homogeneous, high-resolution study of dSph galaxies to date. With our homogeneously derived catalog, we are able to search for similar and deviating trends across different galaxies. We investigate the mass dependence of the individual systems on the production of α-elements, but also try to shed light on the long-standing puzzle of the dominant production site of r-process elements. We use data from the Keck observatory archive and the ESO reduced archive to reanalyze stars from these 13 dSph galaxies. We automatize the step of obtaining stellar parameters, but run a full spectrum synthesis to derive all abundances except for iron. The homogenized set of abundances yielded the unique possibility to derive a relation between the onset of type Ia supernovae and the stellar mass of the galaxy. Furthermore, we derived a formula to estimate the evolution of α-elements. Placing all abundances consistently on the same scale is crucial to answer questions about the chemical history of galaxies. By homogeneously analysing Ba and Eu in the 13 systems, we have traced the onset of the s-process and found it to increase with metallicity as a function of the galaxy's stellar mass. Moreover, the r-process material correlates with the α-elements indicating some co-production of these, which in turn would point towards rare core-collapse supernovae rather than binary neutron star mergers as host for the r-process at low [Fe/H] in the investigated dSph systems. 

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