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ABSTRACT We examine the galactic chemical evolution (GCE) of $^4$He in one-zone and multizone models, with particular attention to theoretical predictions of and empirical constraints on initial mass fraction (IMF)-averaged yields. Published models of massive star winds and core collapse supernovae span a factor of 2–3 in the IMF-averaged $^4$He yield, $$y\mathrm{_{He}^{CC}}$$. Published models of intermediate mass, asymptotic giant branch (AGB) stars show better agreement on the IMF-averaged yield, $$y\mathrm{_{He}^{AGB}}$$, and they predict that more than half of this yield comes from stars with $$M=4{\!-\!}8\, \mathrm{ M}_\odot$$, making AGB $^4$He enrichment rapid compared to Fe enrichment from Type Ia supernovae. Although our GCE models include many potentially complicating effects, the short enrichment time delay and mild metallicity dependence of the predicted yields makes the results quite simple: across a wide range of metallicity and age, the non-primordial $^4$He mass fraction $$\Delta Y = Y-Y_{\mathrm{P}}$$ is proportional to the abundance of promptly produced $$\alpha$$-elements such as oxygen, with $$\Delta Y/Z_{\mathrm{O}}\approx (y\mathrm{_{He}^{CC}}+y\mathrm{_{He}^{AGB}})/y\mathrm{_{O}^{CC}}$$. Reproducing solar abundances with our fiducial choice of the oxygen yield $$y\mathrm{_{O}^{CC}}=0.0071$$ implies $$y\mathrm{_{He}^{CC}}+y\mathrm{_{He}^{AGB}}\approx 0.022$$, i.e. $$0.022\,\mathrm{ M}_\odot$$ of net $^4$He production per solar mass of star formation. Our GCE models with this yield normalization are consistent with most available observations, though the implied $$y\mathrm{_{He}^{CC}}$$ is low compared to most of the published massive star yield models. More precise measurements of $$\Delta Y$$ in stars and gas across a wide range of metallicity and [$$\alpha$$/Fe] ratio could test our models more stringently, either confirming the simple picture suggested by our calculations or revealing surprises in the evolution of the second most abundant element. 

Abstract The scale ofα-element yields is difficult to predict from theory because of uncertainties in massive star evolution, supernova physics, and black hole formation, and it is difficult to constrain empirically because the impact of higher yields can be compensated by greater metal loss in galactic winds. We use a recent measurement of the mean iron yield of core collapse supernovae (CCSN) by Rodriguez et al., ${\overline{y}}_{\mathrm{Fe}}^{\mathrm{cc}}=0.058\pm 0.007M_{\odot }$, to infer the scale ofα-element yields by assuming that the plateau of [α/Fe] abundance ratios observed in low-metallicity stars represents the yield ratio of CCSN. For a plateau at [α/Fe]_cc= 0.45, we find that the population-averaged yields of O and Mg are about equal to the solar abundance of these elements, $\log y_{\mathrm{O}}^{\mathrm{cc}}/Z_{\mathrm{O},\odot }=\log y_{\mathrm{Mg}}^{\mathrm{cc}}/Z_{\mathrm{Mg},\odot }=-0.01\pm 0.1$, where $y_{\mathrm{X}}^{\mathrm{cc}}$is the mass of element X produced by massive stars per unit mass of star formation. The inferred O and Fe yields agree with predictions of the Sukhbold et al. CCSN models assuming their Z9.6+N20 neutrino-driven engine, a scenario in which many progenitors withM< 40M_⊙implode to black holes rather than exploding. The yields are lower than assumed in many models of the galaxy mass–metallicity relation, reducing the level of outflows needed to match observed abundances. Our one-zone chemical evolution models with $\eta ={\dot{M}}_{\mathrm{out}}/{\dot{M}}_{*}\approx 0.6$evolve to solar metallicity at late times. By further requiring that models reach [α/Fe] ≈ 0 at late times, we infer a Hubble-time integrated Type Ia supernova rate of $1.1\times {10}^{-3}{M}_{\odot }^{-1}$, compatible with estimates from supernova surveys. 

Abstract Stellar abundance measurements are subject to systematic errors that induce extra scatter and artificial correlations in elemental abundance patterns. We derive empirical calibration offsets to remove systematic trends with surface gravity $\log \left(g\right)$in 17 elemental abundances of 288,789 evolved stars from the SDSS APOGEE survey. We fit these corrected abundances as the sum of a prompt process tracing core-collapse supernovae and a delayed process tracing Type Ia supernovae, thus recasting each star’s measurements into the amplitudesA_ccandA_Iaand the element-by-element residuals from this two-parameter fit. As a first application of this catalog, which is 8× larger than that of previous analyses that used a restricted $\log \left(g\right)$range, we examine the median residual abundances of 14 open clusters, nine globular clusters, and four dwarf satellite galaxies. Relative to field Milky Way disk stars, the open clusters younger than 2 Gyr show ≈0.1−0.2 dex enhancements of the neutron-capture element Ce, and the two clusters younger than 0.5 Gyr also show elevated levels of C+N, Na, S, and Cu. Globular clusters show elevated median abundances of C+N, Na, Al, and Ce, and correlated abundance residuals that follow previously known trends. The four dwarf satellites show similar residual abundance patterns despite their different star formation histories, with ≈0.2–0.3 dex depletions in C+N, Na, and Al and ≈0.1 dex depletions in Ni, V, Mn, and Co. We provide our catalog of corrected APOGEE abundances, two-process amplitudes, and residual abundances, which will be valuable for future studies of abundance patterns in different stellar populations and of additional enrichment processes that affect galactic chemical evolution. 

Abstract Over the next 5 yr, the Dark Energy Spectroscopic Instrument (DESI) will use 10 spectrographs with 5000 fibers on the 4 m Mayall Telescope at Kitt Peak National Observatory to conduct the first Stage IV dark energy galaxy survey. Atz< 0.6, the DESI Bright Galaxy Survey (BGS) will produce the most detailed map of the universe during the dark-energy-dominated epoch with redshifts of >10 million galaxies spanning 14,000 deg². In this work, we present and validate the final BGS target selection and survey design. From the Legacy Surveys, BGS will target anr< 19.5 mag limited sample (BGS Bright), a fainter 19.5 <r< 20.175 color-selected sample (BGS Faint), and a smaller low-zquasar sample. BGS will observe these targets using exposure times scaled to achieve homogeneous completeness and cover the footprint three times. We use observations from the Survey Validation programs conducted prior to the main survey along with simulations to show that BGS can complete its strategy and make optimal use of “bright” time. BGS targets have stellar contamination <1%, and their densities do not depend strongly on imaging properties. BGS Bright will achieve >80% fiber assignment efficiency. Finally, BGS Bright and BGS Faint will achieve >95% redshift success over any observing condition. BGS meets the requirements for an extensive range of scientific applications. BGS will yield the most precise baryon acoustic oscillation and redshift-space distortion measurements atz< 0.4. It presents opportunities for new methods that require highly complete and dense samples (e.g.,N-point statistics, multitracers). BGS further provides a powerful tool to study galaxy populations and the relations between galaxies and dark matter.