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DOE OSTI2022

H α Dots: Direct-method Metal Abundances of Low-luminosity Star-forming Systems

Utilizing low-luminosity star-forming systems discovered in the Hα Dots survey, we present spectroscopic observations undertaken using the Kitt Peak National Observatory 4 m telescope for 26 sources. With determinations of robust, “direct”-method metal abundances, we examine the properties of these dwarf systems, exploring their utility in characterizing starburst galaxies at low luminosities and stellar masses. We find that the Hα Dots survey provides an effective new avenue for identifying star-forming galaxies in these regimes. In addition, we examine abundance characteristics and metallicity scaling relations with these sources, highlighting a flattening of both the luminosity–metallicity (L–Z) and stellar mass–metallicity (M * –Z) relation slopes in these regimes as compared with those utilizing samples covering wider respective dynamic ranges. These local, accessible analogs to the kinds of star-forming dwarfs common at high redshift will help shed light on the building blocks that assembled into the massive galaxies common today.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

CLEAR: The Ionization and Chemical-enrichment Properties of Galaxies at 1.1 < z < 2.3

We use deep spectroscopy from the Hubble Space Telescope Wide-Field-Camera 3 IR grisms combined with broadband photometry to study the stellar populations, gas ionization and chemical abundances in star-forming galaxies at z ~ 1.1–2.3. The data stem from the CANDELS Lyα Emission At Reionization (CLEAR) survey. At these redshifts, the grism spectroscopy measure the [O II] λλ3727, 3729, [O III]λλ4959, 5008, and Hβ strong emission features, which constrain the ionization parameter and oxygen abundance of the nebular gas. We compare the line-flux measurements to predictions from updated photoionization models (MAPPINGS V; Kewley et al.), which include an updated treatment of nebular gas pressure, $\mathrm{log}P/k={n}_{e}{T}_{e}$. Compared to low-redshift samples (z ~ 0.2) at fixed stellar mass, $\mathrm{log}{M}_{* }/{M}_{\odot }\,=$ 9.4–9.8, the CLEAR galaxies at z = 1.35 (1.90) have lower gas-phase metallicity, ${\rm{\Delta }}(\mathrm{log}Z)$ = 0.25 (0.35) dex, and higher ionization parameters, ${\rm{\Delta }}(\mathrm{log}q)$ = 0.25 (0.35) dex, where U ≡ q/c. We provide updated analytic calibrations between the [O III], [O II], and Hβ emission-line ratios, metallicity, and ionization parameter. The CLEAR galaxies show that at fixed stellar mass, the gas ionization parameter is correlated with the galaxy specific star formation rates, where ${\rm{\Delta }}\mathrm{log}q\simeq 0.4\times {\rm{\Delta }}(\mathrm{log}\,\mathrm{sSFR})$, derived from changes in the strength of galaxy Hβ equivalent width. We interpret this as a consequence of higher gas densities, lower gas covering fractions, combined with a higher escape fraction of H-ionizing photons. We discuss both tests to confirm these assertions and implications this has for future observations of galaxies at higher redshifts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Searches for Extremely Metal-poor Galaxies Using Arecibo Legacy Fast ALFA–Selected Dwarf Galaxies

We present a study of nearby dwarf galaxies selected from the Arecibo Legacy Fast ALFA (ALFALFA) blind H I survey. A primary goal of the project was to utilize a nonstandard selection method with the hope of detecting previously unrecognized extremely metal-poor (XMP) galaxies. The study was motivated by the recent discovery of two XMP galaxies, Leo P and Leoncino, which were both originally found via the ALFALFA survey. We have obtained narrowband H α images for 42 dwarf systems, many of which are located in the Local Void in front of the Pisces–Perseus Supercluster. Spectra for 11 of the best candidates resulted in the determination of metal abundances for 10 of the systems. None were found to be extremely metal-poor, although one system (AGC 123350) was found to have an oxygen abundance of log(O/H)+12 = 7.46, or ~6% solar. One of the galaxies in our sample exhibits a high oxygen abundance for its luminosity, suggesting the possibility that it may have a tidal origin.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Semi-analytic spectral fitting: simultaneously modelling the mass accumulation and chemical evolution in MaNGA spiral galaxies

We develop a novel semi-analytic spectral fitting approach to quantify the star formation histories (SFHs) and chemical enrichment histories (ChEHs) of individual galaxies. We construct simple yet general chemical evolution models that account for gas inflow and outflow processes as well as star formation, to investigate the evolution of merger-free star-forming systems. These models are fitted directly to galaxies’ absorption-line spectra, while their emission lines are used to constrain current gas phase metallicity and star formation rate. We apply this method to spiral galaxies selected from the SDSS-IV Mapping Nearby Galaxies at Apache Point Observatory survey. By fitting the co-added absorption-line spectra for each galaxy, and using the emission-line constraints on present-day metallicity and star formation, we reconstruct both the SFHs and the ChEHs for all objects in the sample. We can use these reconstructions to obtain archaeological measures of derived correlations such as the mass–metallicity relation at any redshift, which compare favourably with direct observations. We find that both the SFHs and ChEHs have strong mass dependence: massive galaxies accumulate their stellar masses and become enriched earlier. This mass dependence causes the observed flattening of the mass–metallicity relation at lower redshifts. The model also reproduces the observed gas-to-stellar mass ratio and its mass dependence. Moreover, we are able to determine that more massive galaxies have earlier gas infall times and shorter infall time-scales, and that the early chemical enrichment of low-mass galaxies is suppressed by strong outflows, while outflows are not very significant in massive galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Hydrodynamic Predictions for the Next Outburst of T Coronae Borealis: It Will Be the Brightest Classical or Recurrent Nova Ever Observed in X-Rays*

T Coronae Borealis (TCrB) is a recurrent nova with recorded outbursts in 1866 and 1946 and possible outbursts in 1217 and 1787. It is predicted to explode again in 2025 or 2026 based on multiple observational studies. The system consists of a massive ( M wd ≳ 1.35 M ⊙ ) white dwarf (WD) and a red giant (M3–M4 III). We have performed 1D hydrodynamic simulations with NOVA to predict the behavior of the next outburst. These simulations consist of a range of mass accretion rates onto ∼1.35 M ⊙ WDs, designed to bound the conditions necessary to achieve ignition of an explosion after an ≈80 yr interoutburst period. We have used both carbon–oxygen and oxygen–neon initial compositions, in order to include the possible ejecta abundances to be measured in the observations of the next outburst. As the WD in the TCrB system is observed to be massive, theoretical predictions reported here imply that the WD is growing in mass as a consequence of the thermonuclear runaway. Therefore, the secular evolution of the WD may allow it to approach the Chandrasekhar limit and either explode as a Type Ia supernova or undergo accretion-induced collapse, depending on its underlying composition. We have followed the evolution of just the WD, after removing the ejected matter from the surface layers. Our intent is to illuminate the mystery of the unique, second maximum in the two well-observed outbursts and we have found conditions that bracket the predictions.

Cataclysmic variable stars↗
DOE OSTI2022

Impact of Hypernova νp-process Nucleosynthesis on the Galactic Chemical Evolution of Mo and Ru

We calculate the Galactic Chemical Evolution of Mo and Ru by taking into account the contribution from νp-process nucleosynthesis. We estimate yields of p-nuclei such as 92,94 Mo and 96,98 Ru through the νp-process in various supernova progenitors based upon recent models. In particular, the νp-process in energetic hypernovae produces a large amount of p-nuclei compared to the yield in ordinary core-collapse SNe. Because of this, the abundances of 92,94 Mo and 96,98 Ru in the Galaxy are significantly enhanced at [Fe/H] = 0 by the νp-process. We find that the νp-process in hypernovae is the main contributor to the elemental abundance of 92 Mo at low metallicity [Fe/H] < −2. Our theoretical prediction of the elemental abundances in metal-poor stars becomes more consistent with observational data when the νp-process in hypernovae is taken into account.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

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

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.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Possibility to Identify the Contributions from Collapsars, Supernovae, and Neutron Star Mergers from the Evolution of the r-process Mass Abundance Distribution

We study the evolution of rapid neutron-capture process (r-process) isotopes in the galaxy. We analyze relative contributions from core-collapse supernovae (CCSNe), neutron star mergers, and collapsars under a range of astrophysical conditions and nuclear input data. Here we show that, although the r-process in each of these sites can lead to a similar (universal) elemental distribution, the detailed isotopic abundances can differ from one site to another. These differences may allow for the identification of which sources contributed to the early evolution of r-process material in the galaxy. Our simulations suggest that the early evolution was dominated by CCSNe and collapsar r-process nucleosynthesis. This conclusion may be testable if the next generation of observatories can deduce isotopic r-process abundances.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

The Galactic Distribution of Phosphorus: A Survey of 163 Disk and Halo Stars*

Phosphorus (P) is a critical element for life on Earth, yet the cosmic production sites of P are relatively uncertain. To understand how P has evolved in the solar neighborhood, we measured abundances for 163 FGK stars over a range of –1.09 < [Fe/H] < 0.47 using observations from the Habitable-zone Planet Finder instrument on the Hobby–Eberly Telescope. Atmospheric parameters were calculated by fitting a combination of astrometry, photometry, and Fe I line equivalent widths. Phosphorus abundances were measured by matching synthetic spectra to a P I feature at 10529.52 Å. Our [P/Fe] ratios show that chemical evolution models generally underpredict P over the observed metallicity range. Additionally, we find that the [P/Fe] differs by ~0.1 dex between thin disk and thick disk stars that were identified with kinematics. The P abundances were compared with α-elements, iron-peak, odd-Z, and s-process elements, and we found that the evolution of P in the disk most strongly resembles that of the α-elements. We also find that molar P/C and N/C ratios for our sample match the scatter seen from other abundance studies. Finally, we measure a [P/Fe] = 0.09 ± 0.1 ratio in one low-α halo star and probable Gaia–Sausage–Enceladus member, an abundance ratio ~0.3–0.5 dex lower than the other Milky Way disk and halo stars at similar metallicities. Overall, we find that P is likely most significantly produced by massive stars in core-collapse supernovae, based on the largest P abundance survey to date.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Proton-rich Production of Lanthanides: The vi Process

The astrophysical origin of the lanthanides is an open question in nuclear astrophysics. Besides the widely studied s, i, and r processes in moderately to strongly neutron-rich environments, an intriguing alternative site for lanthanide production could in fact be robustly proton-rich matter outflows from core-collapse supernovae under specific conditions—in particular, high-entropy winds with enhanced neutrino luminosity and fast dynamical timescales. In this environment, excess protons present after charged-particle reactions have ceased can continue to be converted to neutrons by (anti)neutrino interactions, producing a neutron-capture reaction flow up to A ∼ 200. This scenario, christened the νi process in a recent paper, has previously been discussed as a possibility. Here, we examine the prospects for the νi process through the lenses of stellar abundance patterns, bolometric light curves, and galactic chemical evolution models, with a particular focus on hypernovae as candidate sites. We identify specific lanthanide signatures for which the νi process can provide a credible supplement to the r/i processes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Chemical Doppelgangers in GALAH DR3: The Distinguishing Power of Neutron-capture Elements among Milky Way Disk Stars

The observed chemical diversity of Milky Way stars places important constraints on Galactic chemical evolution and the mixing processes that operate within the interstellar medium. Recent works have found that the chemical diversity of disk stars is low. For example, the Apache Point Observatory Galactic Evolution Experiment (APOGEE) "chemical doppelganger rate," or the rate at which random pairs of field stars appear as chemically similar as stars born together, is high, and the chemical distributions of APOGEE stars in some Galactic populations are well-described by two-dimensional models. However, limited attention has been paid to the heavy elements (Z > 30) in this context. In this work, we probe the potential for neutron-capture elements to enhance the chemical diversity of stars by determining their effect on the chemical doppelganger rate. We measure the doppelganger rate in GALactic Archaeology with HERMES DR3, with abundances rederived using The Cannon, and find that considering the neutron-capture elements decreases the doppelganger rate from ~2.2% to 0.4%, nearly a factor of 6, for stars with –0.1 < [Fe/H] < 0.1. While chemical similarity correlates with similarity in age and dynamics, including neutron-capture elements does not appear to select stars that are more similar in these characteristics. Our results highlight that the neutron-capture elements contain information that is distinct from that of the lighter elements and thus add at least one dimension to Milky Way abundance space. This work illustrates the importance of considering the neutron-capture elements when chemically characterizing stars and motivates ongoing work to improve their atomic data and measurements in spectroscopic surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Enrichment by the first stars in a relic dwarf galaxy

Stars that contain trace amounts of elements heavier than helium (that is, ‘metallicity’) preserve the chemical fingerprints of the first stars. In the Milky Way, nearly all of the lowest-metallicity stars show an extreme over-abundance of carbon. The origin of this signature has remained a mystery owing to the lack of observational constraints on the environments in which it originates. Here, in this work, we present observations of a star in the >10-billion-year-old ultrafaint dwarf galaxy Pictor II, showing the lowest iron and calcium abundances outside the Milky Way (<1/43,000th solar and ~1/160,000th solar), with >3,000× relative carbon enhancement. The star’s exceptional paucity in iron and calcium make it clearly preserve enrichment from the first stars in a relic dwarf galaxy; Pictor II is one of the smallest, most chemically primordial systems known. This star supports the hypothesis that extreme carbon enhancement results from low-energy supernovae from the first stars, as the yields of energetic supernovae are harder to retain in small-scale environments. This signature of enrichment by the first stars may trace a regime inaccessible to current high-redshift observations, which can hardly detect the initial enrichment of the smallest galaxies.

chemical evolution↗
DOE OSTI2022

How Many Elements Matter?

Some studies of stars' multielement abundance distributions suggest at least 5–7 significant dimensions, but others show that many elemental abundances can be predicted to high accuracy from [Fe/H] and [Mg/Fe] (or [Fe/H] and age) alone. We show that both propositions can be, and are, simultaneously true. We adopt a machine-learning technique known as normalizing flow to reconstruct the probability distribution of Milky Way disk stars in the space of 15 elemental abundances measured by APOGEE. Conditioning on T eff and $\mathrm{log}\,g$ minimizes the differential systematics. After further conditioning on [Fe/H] and [Mg/Fe], the residual scatter for most abundances is σ [X/H] ≲ 0.02 dex, consistent with APOGEE's reported statistical uncertainties of ~0.01–0.015 dex and intrinsic scatter of 0.01–0.02 dex. Despite the small scatter, residual abundances display clear correlations between elements, which we show are too large to be explained by measurement uncertainties or by the finite sampling noise. We must condition on at least seven elements to reduce the correlations to a level consistent with the observational uncertainties. Our results demonstrate that cross-element correlations are a much more sensitive probe of a hidden structure than dispersion, and they can be measured precisely in a large sample even if the star-by-star measurement noise is comparable to the intrinsic scatter. We conclude that many elements have an independent story to tell, even for the mundane disk stars and elements produced by the core-collapse and Type Ia supernovae. The only way to learn these lessons is to measure the abundances directly, and not merely infer them.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

The Role of Inner H ι Mass in Regulating the Scatter of the Mass–Metallicity Relation

We use 789 disk-like, star-forming galaxies (with 596 H ι detections) from H ι follow-up observations for the SDSS-IV MaNGA survey to study the possible role of inner H ι gas in causing secondary dependences in the mass–gas-phase metallicity relation. We use the gas-phase metallicity derived at the effective radii of the galaxies. We derive the inner H ι mass within the optical radius, but also use the total H ι mass and star formation rate (SFR) for a comparison. We confirm the anticorrelation between the total H ι mass and gas-phase metallicity at fixed stellar mass, but the anticorrelation is significantly strengthened when the total H ι mass is replaced by the inner H ι mass. Introducing a secondary relation with the inner H ι mass can produce a small but noticeable decrease (16%) in the scatter of the mass–gas-phase metallicity relation, in contrast to the negligible effect with the SFR. The correlation with the inner H ι mass is robust when using different diagnostics of metallicity, but the correlation with SFR is not. The correlation with the inner H ι mass becomes much weaker when the gas-phase metallicity is derived in the central region instead of at the effective radius. These results support the idea that the scatter in the mass–metallicity relation is regulated by gas accretion, and not directly by the SFR, and stress the importance of deriving the gas mass and the metallicity from roughly the same region. The new relation between inner H ι mass and gas-phase metallicity will provide new constraints for chemical and galaxy evolution models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Type Ia Supernova Nucleosynthesis: Metallicity-dependent Yields

Abstract Type Ia supernova explosions (SN Ia) are fundamental sources of elements for the chemical evolution of galaxies. They efficiently produce intermediate-mass (with Z between 11 and 20) and iron group elements—for example, about 70% of the solar iron is expected to be made by SN Ia. In this work, we calculate complete abundance yields for 39 models of SN Ia explosions, based on three progenitors—a 1.4 M ⊙ deflagration detonation model, a 1.0 M ⊙ double detonation model, and a 0.8 M ⊙ double detonation model—and 13 metallicities, with 22 Ne mass fractions of 0, 1 × 10 −7 , 1 × 10 −6 , 1 × 10 −5 , 1 × 10 −4 , 1 × 10 −3 , 2 × 10 −3 , 5 × 10 −3 , 1 × 10 −2 , 1.4 × 10 −2 , 5 × 10 −2 , and 0.1, respectively. Nucleosynthesis calculations are done using the NuGrid suite of codes, using a consistent nuclear reaction network between the models. Complete tables with yields and production factors are provided online at Zenodo:Yields ( https://doi.org/10.5281/zenodo.8060323 ). We discuss the main properties of our yields in light of the present understanding of SN Ia nucleosynthesis, depending on different progenitor mass and composition. Finally, we compare our results with a number of relevant models from the literature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Chemical evolution of local post-starburst galaxies: implications for the mass–metallicity relation

ABSTRACT We use the stellar fossil record to constrain the stellar metallicity evolution and star-formation histories of the post-starburst (PSB) regions within 45 local PSB galaxies from the MaNGA survey. The direct measurement of the regions’ stellar metallicity evolution is achieved by a new two-step metallicity model that allows for stellar metallicity to change at the peak of the starburst. We also employ a Gaussian process noise model that accounts for correlated errors introduced by the observational data reduction or inaccuracies in the models. We find that a majority of PSB regions (69 per cent at >1σ significance) increased in stellar metallicity during the recent starburst, with an average increase of 0.8 dex and a standard deviation of 0.4 dex. A much smaller fraction of PSBs are found to have remained constant (22 per cent) or declined in metallicity (9 per cent, average decrease 0.4 dex, standard deviation 0.3 dex). The pre-burst metallicities of the PSB galaxies are in good agreement with the mass–metallicity (MZ) relation of local star-forming galaxies. These results are consistent with hydrodynamic simulations, which suggest that mergers between gas-rich galaxies are the primary formation mechanism of local PSBs, and rapid metal recycling during the starburst outweighs the impact of dilution by any gas inflows. The final mass-weighted metallicities of the PSB galaxies are consistent with the MZ relation of local passive galaxies. Our results suggest that rapid quenching following a merger-driven starburst is entirely consistent with the observed gap between the stellar mass–metallicity relations of local star-forming and passive galaxies.

Leung, Ho-Hin (ORCID:0000000304865178)↗
DOE OSTI2022

The Chemical Composition of Extreme-velocity Stars* †

Abstract Little is known about the origin of the fastest stars in the Galaxy. Our understanding of the chemical evolution history of the Milky Way and surrounding dwarf galaxies allows us to use the chemical composition of a star to investigate its origin and to say whether it was formed in situ or was accreted. However, the fastest stars, the hypervelocity stars, are young and massive and their chemical composition has not yet been analyzed. Though it is difficult to analyze the chemical composition of a massive young star, we are well versed in the analysis of late-type stars. We have used high-resolution ARCES/3.5 m Apache Point Observatory, MIKE/Magellan spectra to study the chemical details of 15 late-type hypervelocity star candidates. With Gaia EDR3 astrometry and spectroscopically determined radial velocities we found total velocities with a range of 274–520 km s −1 and mean value of 381 km s −1 . Therefore, our sample stars are not fast enough to be classified as hypervelocity stars, and are what is known as extreme-velocity stars. Our sample has a wide iron abundance range of −2.5 ≤ [Fe/H] ≤ −0.9. Their chemistry indicates that at least 50% of them are accreted extragalactic stars, with iron-peak elements consistent with prior enrichment by sub-Chandrasekhar mass Type Ia supernovae. Without indication of binary companions, their chemical abundances and orbital parameters indicate that they are the accelerated tidal debris of disrupted dwarf galaxies.

Reggiani, Henrique (ORCID:0000000165336179)↗