Recent Publications
KPM: A Flexible and Data-Driven K-Process Model for Nucleosynthesis
The element abundance pattern found by APOGEE and GALAH in Milky Way disk stars is close to two-dimensional, dominated by production from one prompt process and one delayed process. This simplicity is remarkable, since the elements are produced by a multitude of nucleosynthesis mechanisms operating in stars with a wide range of progenitor masses. We fit the abundances of 14 elements for 48,659 red-giant stars from APOGEE DR17 using a flexible, data-driven K-process model--dubbed KPM. In our fiducial model, with K=2, each abundance in each star is described as the sum of a prompt and a delayed process contribution. We find that KPM with K=2 is able to explain the abundances well, recover the observed abundance bimodality, and detect the bimodality over a greater range in metallicity than previously has been possible. The model makes assumptions, especially that it fixes some parameters to break degeneracies and improve interpretability; we find that some of the nucleosynthetic implications are dependent upon these detailed parameter choices. The results of KPM have implications for the formation of the Galaxy disk, the relationship between abundances and ages, and the physics of nucleosynthesis.
Untangling the Sources of Abundance Dispersion in Low-metallicity Stars
We measure abundances of 12 elements (Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni) in a sample of 86 metal-poor (-2 ≲ [Fe/H] ≲ -1) subgiant stars in the solar neighborhood. Abundances are derived from high-resolution spectra taken with the Potsdam Echelle Polarimetric and Spectroscopic Instrument on the Large Binocular Telescope, modeled using iSpec and MOOG. By carefully quantifying the impact of photon-noise (<0.05 dex for all elements), we robustly measure the intrinsic scatter of abundance ratios. At fixed [Fe/H], the RMS intrinsic scatter in [X/Fe] ranges from 0.04 (Cr) to 0.16 dex (Na), with a median of 0.08 dex. Scatter in [X/Mg] is similar, and accounting for [α/Fe] only reduces the overall scatter moderately. Stochastic sampling of the CCSN progenitor mass distribution provides a good quantitative explanation of our data if the effective number of CCSN contributing to the enrichment of a typical sample star is N ~ 50. At the median metallicity of our sample, this interpretation implies that the CCSN ejecta are mixed over a gas mass ~6 × 104 M ⊙ before forming stars. The scatter of elemental abundance ratios is a powerful diagnostic test for simulations of star formation, feedback, and gas mixing in the early phases of the Galaxy.
Residual Abundances in GALAH DR3: Implications for Nucleosynthesis and Identification of Unique Stellar Populations
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. 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. 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 and identify a population of stars, preferentially young, that have much higher AGB enrichment than expected from their SNIa enrichment.
The Impact of Black Hole Formation on Population Averaged Supernova Yields
The landscape of black hole (BH) formation -- which massive stars explode as core-collapse supernovae (CCSN) and which implode to BHs -- profoundly affects the IMF-averaged nucleosynthetic yields of a stellar population. Building on the work of Sukhbold et al. (2016), we compute IMF-averaged yields at solar metallicity for a wide range of assumptions, including neutrino-driven engine models with extensive BH formation, models with a simple mass threshold for BH formation, and a model in which all stars from 8−120M⊙ explode. For constraining the overall level of BH formation, ratios of C and N to O or Mg are promising diagnostics. For distinguishing complex, theoretically motivated landscapes from simple mass thresholds, abundance ratios involving Mn or Ni are promising because of their sensitivity to the core structure of the CCSN progenitors. No landscape choice achieves across-the-board agreement with observed abundance ratios; the discrepancies offer empirical clues to aspects of massive star evolution or explosion physics still missing from the models.
The Similarity of Abundance Ratio Trends and Nucleosynthetic Patterns in the Milky Way Disk and Bulge
We compare abundance ratio trends in a sample of ∼11,000 Milky Way bulge stars (R < 3 kpc) from APOGEE to those of stars in the Galactic disk (5 kpc < R < 11 kpc). We divide each sample into low-Ia and high-Iapopulations, and in each population, we examine the median trends of [X/Mg] vs. [Mg/H] for elements X = Fe, O, Na, Al, Si, P, S, K, Ca, V, Cr, Mn, Co, Ni, Cu, and Ce. After removing small systematic trends of APOGEE abundances with stellar log(g), we find nearly identical median trends for low-Ia disk and bulge stars for all elements. High-Ia trends are similar for most elements, with noticeable (0.05-0.1 dex) differences for Mn, Na, and Co. The close agreement of abundance trends (with typical differences ≲0.03 dex) implies that similar nucleosynthetic processes enriched bulge and disk stars despite the different star formation histories and physical conditions of these regions.
Abundance Ratios in GALAH DR2 and Their Implications for Nucleosynthesis
Using a sample of 70,924 stars from the second data release of the GALAH optical spectroscopic survey, we construct median sequences of [X/Mg] versus [Mg/H] for 21 elements, separating the low-Ia and high-Ia stellar populations through cuts in [Mg/Fe]. The separation between the two [X/Mg] sequences indicates the relative importance of prompt and delayed enrichment mechanisms, while the sequences’ slopes indicate metallicity dependence of the yields. GALAH and APOGEE measurements agree for some of their common elements, but differ in sequence separation or metallicity trends for others. We infer that about 75% of solar C comes from core-collapse supernovae and 25% from delayed mechanisms. We find core-collapse fractions of 60%-80% for Sc, Ti, Cu, and Zn, with strong metallicity dependence of the core-collapse Cu yield. We infer large delayed contributions to Y, Ba, and La with non-monotonic metallicity dependence. The separation of the [Eu/Mg] sequences implies that at least ∼30% of Eu enrichment is delayed with respect to star formation. We compare our results to predictions of several supernova and asymptotic giant branch yield models; C, Na, K, Mn, and Ca all show discrepancies with models that could make them useful diagnostics of nucleosynthesis physics.
A Comparison of Stellar and Gas-Phase Chemical Abundances in Dusty Early-Type Galaxies
While we observe a large amount of cold interstellar gas and dust in a subset of the early-type galaxy (ETG) population, the source of this material remains unclear. The two main, competing scenarios are external accretion of lower mass, gas-rich dwarfs, and internal production from stellar mass loss and/or cooling from the hot interstellar medium. We test these hypotheses with measurements of the stellar and nebular metallicities of three ETGs (NGC 2768, NGC 3245, and NGC 4694) from new long-slit, high signal-to-noise ratio spectroscopy from the Multi-Object Double Spectrographs on the Large Binocular Telescope. We model the stellar continuum to derive chemical abundances and measure gas-phase abundances with standard nebular diagnostics. We find that the stellar and gas-phase abundances are very similar, which supports internal production and is very inconsistent with the accretion of smaller, lower metallicity dwarfs. All three of these galaxies are also consistent with an extrapolation of the mass-metallicity relation to higher mass galaxies with lower specific star formation rates.
