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1. Fragmentation in Gravitationally Unstable Collapsar Disks and Subsolar Neutron Star Mergers

   https://doi.org/10.3847/2041-8213/ad6990  

   Metzger, Brian D; Hui, Lam; Cantiello, Matteo (August 2024, The Astrophysical Journal Letters)

   Abstract Although stable neutron stars (NSs) can in principle exist down to massesM_ns≈ 0.1M_⊙, standard models of stellar core-collapse predict a robust lower limitM_ns≳ 1.2M_⊙, roughly commensurate with the Chandrasekhar massM_Chof the progenitor’s iron core (electron fractionY_e≈ 0.5). However, this limit may be circumvented in sufficiently dense neutron-rich environments (Y_e< 0.5) for which $M_{\mathrm{Ch}}\propto Y_e^2$is reduced to ≲1M_⊙. Such physical conditions could arise in the black hole accretion disks formed from the collapse of rapidly rotating stars (“collapsars”), as a result of gravitational instabilities and cooling-induced fragmentation, similar to models for planet formation in protostellar disks. We confirm that the conditions to form subsolar-mass NS (ssNS) may be marginally satisfied in the outer regions of massive neutrino-cooled collapsar disks. If the disk fragments into multiple ssNSs, their subsequent coalescence offers a channel for precipitating subsolar mass LIGO/Virgo gravitational-wave mergers that does not implicate primordial black holes. The model makes several additional predictions: (1) ∼Hz frequency Doppler modulation of the ssNS-merger gravitational-wave signals due to the binary’s orbital motion in the disk; (2) at least one additional gravitational-wave event (coincident within ≲hours), from the coalescence of the ssNS-merger remnant(s) with the central black hole; (3) an associated gamma-ray burst and supernova counterpart, the latter boosted in energy and enriched withr-process elements from the NS merger(s) embedded within the exploding stellar envelope (“kilonovae inside a supernova”). 

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2. Magnetically Driven Neutron-rich Ejecta Unleashed: Global 3D Neutrino–General Relativistic Magnetohydrodynamic Simulations of Collapsars Probe the Conditions for r -process Nucleosynthesis

   https://doi.org/10.3847/2041-8213/adc694  

   Issa, Danat; Gottlieb, Ore; Metzger, Brian D; Jacquemin-Ide, Jonatan; Liska, Matthew; Foucart, Francois; Halevi, Goni; Tchekhovskoy, Alexander (May 2025, The Astrophysical Journal Letters)

   Abstract Collapsars—rapidly rotating stellar cores that form black holes—can power gamma-ray bursts and are proposed to be key contributors to the production of heavy elements in the Universe via the rapid neutron capture process (r-process). Previous neutrino-transport collapsar simulations have been unable to unbind neutron-rich material from the disk. However, these simulations have not included sufficiently strong magnetic fields and the black hole (BH), both of which are essential for launching mass outflows. We presentνh-amr, a novel neutrino-transport general relativistic magnetohydrodynamic (νGRMHD) code, which we use to perform the first 3D globalνGRMHD collapsar simulations. We find a self-consistent formation of a weakly magnetized dense accretion disk, which has sufficient time to neutronize. Eventually, substantial magnetic flux accumulates near the BH, becomes dynamically important, leads to a magnetically arrested disk (MAD), and unbinds some of the neutron-rich material. However, the strong flux also hinders accretion, lowers density, and increases neutrino-cooling timescale, which prevents further disk neutronization. Typical collapsar progenitors with mass accretion rates, $\dot{M}\sim 0.1-1M_{\odot }{\mathrm{s}}^{-1}$, do not produce significant neutron-rich (Y_e < 0.25) ejecta. However, we find that MADs at higher mass accretion rates, $\dot{M}\gtrsim \text{few}{M}_{\odot }{\mathrm{s}}^{-1}$(e.g., for more centrally concentrated progenitors), can unbindM_ej ≲ M_⊙of neutron-rich ejecta. The outflows inflate a shocked cocoon that mixes with the infalling neutron-poor stellar gas and raises the final outflowY_e; however, the finalr-process yield may be determined earlier at the point of neutron capture freeze-out. Future work will explore under what conditions more typical collapsar engines becomer-process factories. 

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3. The Scale of Stellar Yields: Implications of the Measured Mean Iron Yield of Core Collapse Supernovae

   https://doi.org/10.3847/1538-4357/ad6313  

   Weinberg, David H; Griffith, Emily J; Johnson, James W; Thompson, Todd A (September 2024, The Astrophysical Journal)

   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. 

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4. Dataset of theoretical multinary perovskite oxides

   https://doi.org/10.1038/s41597-023-02127-w  

   Bare, Zachary J.; Morelock, Ryan J.; Musgrave, Charles B. (December 2023, Scientific Data)

   Abstract Perovskite oxides (ternary chemical formula ABO₃) are a diverse class of materials with applications including heterogeneous catalysis, solid-oxide fuel cells, thermochemical conversion, and oxygen transport membranes. However, their multicomponent (chemical formula$${A}_{x}{A}_{1-x}^{\text{'}}{B}_{y}{B}_{1-y}^{\text{'}}{O}_{3}$$ $A_x{A}_{1-x}^{\text{'}}{B}_y{B}_{1-y}^{\text{'}}{O}_3$) chemical space is underexplored due to the immense number of possible compositions. To expand the number of computed$${A}_{x}{A}_{1-x}^{{\prime} }{B}_{y}{B}_{1-y}^{{\prime} }{O}_{3}$$ $A_x{A}_{1-x}^{\prime }{B}_y{B}_{1-y}^{\prime }{O}_3$compounds we report a dataset of 66,516 theoretical multinary oxides, 59,708 of which are perovskites. First, 69,407$${A}_{0.5}{A}_{0.5}^{{\prime} }{B}_{0.5}{B}_{0.5}^{{\prime} }{O}_{3}$$ $A_{0.5}{A}_{0.5}^{\prime }{B}_{0.5}{B}_{0.5}^{\prime }{O}_3$compositions were generated in thea⁻b⁺a⁻Glazer tilting mode using the computationally-inexpensive Structure Prediction and Diagnostic Software (SPuDS) program. Next, we optimized these structures with density functional theory (DFT) using parameters compatible with the Materials Project (MP) database. Our dataset contains these optimized structures and their formation (ΔH_f) and decomposition enthalpies (ΔH_d) computed relative to MP tabulated elemental references and competing phases, respectively. This dataset can be mined, used to train machine learning models, and rapidly and systematically expanded by optimizing more SPuDS-generated$${A}_{0.5}{A}_{0.5}^{{\prime} }{B}_{0.5}{B}_{0.5}^{{\prime} }{O}_{3}$$ $A_{0.5}{A}_{0.5}^{\prime }{B}_{0.5}{B}_{0.5}^{\prime }{O}_3$perovskite structures using MP-compatible DFT calculations. 

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5. Detailed Chemical Abundances of Stars in the Outskirts of the Tucana II Ultrafaint Dwarf Galaxy*

   https://doi.org/10.3847/1538-3881/aca416  

   Chiti, Anirudh; Frebel, Anna; Ji, Alexander P; Mardini, Mohammad K; Ou, Xiaowei; Simon, Joshua D; Jerjen, Helmut; Kim, Dongwon; Norris, John E (January 2023, The Astronomical Journal)

   Abstract We present chemical abundances and velocities of five stars between 0.3 and 1.1 kpc from the center of the Tucana II ultrafaint dwarf galaxy (UFD) from high-resolution Magellan/MIKE spectroscopy. We find that every star is deficient in metals (−3.6 < [Fe/H] < −1.9) and in neutron-capture elements as is characteristic of UFD stars, unambiguously confirming their association with Tucana II. Other chemical abundances (e.g., C, iron peak) largely follow UFD trends and suggest that faint core-collapse supernovae (SNe) dominated the early evolution of Tucana II. We see a downturn in [α/Fe] at [Fe/H] ≈ −2.8, indicating the onset of Type Ia SN enrichment and somewhat extended chemical evolution. The most metal-rich star has strikingly low [Sc/Fe] = −1.29 ± 0.48 and [Mn/Fe] = −1.33 ± 0.33, implying significant enrichment by a sub-Chandrasekhar mass Type Ia SN. We do not detect a radial velocity gradient in Tucana II ( ${\mathit{dv}}_{\mathrm{helio}}/d{\theta }_1=-{2.6}_{-2.9}^{+3.0}$km s⁻¹kpc⁻¹), reflecting a lack of evidence for tidal disruption, and derive a dynamical mass of $M_{1/2}\left(r_h\right)={1.6}_{-0.7}^{+1.1}\times {10}^6$M_⊙. We revisit formation scenarios of the extended component of Tucana II in light of its stellar chemical abundances. We find no evidence that Tucana II had abnormally energetic SNe, suggesting that if SNe drove in situ stellar halo formation, then other UFDs should show similar such features. Although not a unique explanation, the decline in [α/Fe] is consistent with an early galactic merger triggering later star formation. Future observations may disentangle such formation channels of UFD outskirts. 

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