Keywords

We report Li–Be–B and Al–Mg isotopic compositions of Ca-Al-rich inclusions (CAIs) in Sayh al Uhaymir 290 (CH) and Isheyevo (CH/CB) metal-rich carbonaceous chondrites. All CAIs studied here do not show resolvable excesses in ²⁶Mg, a decay product of the short-lived radionuclide ²⁶Al, which suggests their formation occurred prior to the injection of ²⁶Al into the solar system from a nearby stellar source. The inferred initial ¹⁰Be/⁹Be ratios obtained for these CAIs range from 0.17 × 10⁻³ to 6.1 × 10⁻³, which tend to be much higher and more variable than those of CAIs in CV3 chondrites. The high ¹⁰Be/⁹Be ratios suggest that ¹⁰Be was most likely synthesized through solar cosmic-ray irradiation. The lithium isotopic compositions of these CAIs are nearly chondritic, independent of their initial ¹⁰Be/⁹Be ratios. This can be explained by the irradiation targets being of chondritic composition; in other words, targets were most likely not solid CAI themselves, but their precursors in solar composition. The larger variations in ¹⁰Be/⁹Be ratios observed in CH and CH/CB CAIs than in CV CAIs may reflect more variable cosmic-ray fluxes from the earlier, more active Sun at an earlier evolutionary stage (class 0-I) for the former, and a later, less active stage of the Sun (class II) for the latter. If this is the case, our new Be–B and Al–Mg data set implies that the earliest formed CAIs tend to be transported into the outer part of the solar protoplanetary disk, where the parent bodies of metal-rich chondrites likely accreted.

We examine the contribution of electron-capture supernovae (ECSNe), low-mass SNe from collapsing Fe cores (FeCCSNe), and rotating massive stars to the chemical composition of the Galaxy. Our model includes contributions to chemical evolution from both thermonuclear ECSNe (tECSNe) and gravitational collapse ECSNe (cECSNe). We show that if ECSNe are predominantly gravitational collapse SNe but about 15% are partial thermonuclear explosions, the model is able to reproduce the solar abundances of several important and problematic isotopes including ${}^{48}\mathrm{Ca}$, ${}^{50}\mathrm{Ti}$, and ⁵⁴Cr together with ⁵⁸Fe, ⁶⁴Ni, ⁸²Se, and ⁸⁶Kr and several of the Zn–Zr isotopes. A model in which no cECSNe occur, only tECSNe with low-mass FeCCSNe or rotating massive stars, proves also very successful at reproducing the solar abundances for these isotopes. Despite the small mass range for the progenitors of ECSNe and low-mass FeCCSNe, the large production factors suffice for the solar inventory of the above isotopes. Our model is compelling because it introduces no new tensions with the solar abundance distribution for a Milky Way model—only tending to improve the model predictions for several isotopes. The proposed astrophysical production model thus provides a natural and elegant way to explain one of the last uncharted territories on the periodic table of astrophysical element production.

Short gamma-ray bursts require a rotating black hole, surrounded by a magnetized relativistic accretion disk, such as the one formed by coalescing binary neutron stars or neutron star–black hole systems. The accretion onto a Kerr black hole is the mechanism of launching a baryon-free relativistic jet. An additional uncollimated outflow, consisting of subrelativistic neutron-rich material, which becomes unbound by thermal, magnetic, and viscous forces, is responsible for blue and red kilonova. We explore the formation, composition, and geometry of the secondary outflow by means of simulating accretion disks with relativistic magnetohydrodynamics and employing a realistic nuclear equation of state. We calculate the nucleosynthetic r-process yields by sampling the outflow with a dense set of tracer particles. Nuclear heating from the residual r-process radioactivities in the freshly synthesized nuclei is expected to power a red kilonova, contributing independently from the dynamical ejecta component, launched at the time of merger, and neutron-poor broad polar outflow, launched from the surface of the hypermassive neutron star by neutrino wind. Our simulations show that both magnetization of the disk and high black hole spin are able to launch fast wind outflows (v/c ∼ 0.11–0.23) with a broad range of electron fraction Y_e ∼ 0.1–0.4, and help explain the multiple components observed in the kilonova light curves. The total mass loss from the post-merger disk via unbound outflows is between 2% and 17% of the initial disk mass.

Millimeter molecular line observations have been conducted toward the young (∼900 yr) bipolar planetary nebula (PN) K4-47, using the 12 m antenna and the Submillimeter Telescope of the Arizona Radio Observatory, and the Institut de Radioastronomie Millimétrique 30 m Telescope. Measurements at 1, 2, and 3 mm of multiple transitions were carried out to ensure the accuracy of all molecular identifications. K4-47 was found to be unusually chemically rich, containing three complex species, CH₃CN, H₂CNH, and CH₃CCH, which have never before been observed in a planetary nebula. In addition, HC₃N, N₂H⁺, H₂CO, c-C₃H₂, and SiO have been identified in this object, as well as a variety of ¹³C-substituted isotopologues (${{{\rm{H}}}_{2}}^{13}$CO, c-¹³CCCH₂, c-CC¹³CH₂, ${{\mathrm{CH}}_{3}}^{13}$CN, ¹³CH₃CN, ${{\mathrm{CH}}_{3}}^{13}$CCH, and ¹³CH₃CCH), including all three doubly¹³C-substituted varieties of HC₃N—the first known object in which all three species have been detected. After CO and H₂, the most abundant molecules in K4-47 are CCH and CN, which have abundances of f  ∼ 8 × 10⁻⁷, relative to molecular hydrogen. Surprisingly, the next most abundant molecule is CH₃CCH, which has f  ∼ 6 × 10⁻⁷, followed by HCN with an abundance of ∼5 × 10⁻⁷. The results suggest that K4-47 is the most chemically complex planetary nebula currently known. The molecular content of K4-47 closely resembles that of the C-star IRC+10216, but with lower abundances, except for HCO⁺, H₂CO, and CH₃CCH. The PN also chemically and morphologically resembles the bipolar protoplanetary nebula CRL 618, with similar enrichments of ¹³C, ¹⁵N, and ¹⁷O, suggestive of an explosive process at the end of the asymptotic giant branch.

Electron capture on ²⁰Ne is critically important for the final stage of evolution of stars with initial masses of 8–10 M_⊙. In the present paper, we evaluate electron-capture rates for a forbidden transition ²⁰Ne (${0}_{g.s.}^{+}$) $\to$ ²⁰F (${2}_{g.s.}^{+}$) in stellar environments by the multipole expansion method with the use of shell-model Hamiltonians. These rates have not been accurately determined in theory as well as in experiments. Our newly evaluated rates are compared with those obtained by a prescription that treats the transition as an allowed Gamow–Teller transition with the strength determined from a recent β-decay experiment for ²⁰F (${2}_{g.s.}^{+}$) $\to$ ²⁰Ne (${0}_{g.s.}^{+}$). We find that different electron energy dependence of the transition strengths between the two methods leads to sizable differences in the weak rates of the two methods. We also find that the Coulomb effects, that is, the effects of screening on ions and electrons are nonnegligible. We apply our electron-capture rates on ²⁰Ne to the calculation of the evolution of high-density O–Ne–Mg cores of 8–10 M_⊙ stars. We find that our new rates affect the abundance distribution and the central density at the final stage of evolution.

There is no consensus on the progenitors of Type Ia supernovae (SNe Ia) despite their importance for cosmology and chemical evolution. We address this question using our previously published catalogs of Mg, Si, Ca, Cr, Fe, Co, and Ni abundances in dwarf galaxy satellites of the Milky Way (MW) to constrain the mass at which the white dwarf (WD) explodes during a typical SN Ia. We fit a simple bi-linear model to the evolution of [X/Fe] with [Fe/H], where X represents each of the elements mentioned above. We use the evolution of [Mg/Fe] coupled with theoretical supernova yields to isolate what fraction of the elements originated in SNe Ia. Then, we infer the [X/Fe] yield of SNe Ia for all of the elements except Mg. We compare these observationally inferred yields to recent theoretical predictions for two classes of Chandrasekhar-mass (M_Ch) SN Ia as well as sub-M_Ch SNe Ia. Most of the inferred SN Ia yields are consistent with all of the theoretical models, but [Ni/Fe] is consistent only with sub-M_Ch models. We conclude that the dominant type of SN Ia in ancient dwarf galaxies is the explosion of a sub-M_Ch WD. The MW and dwarf galaxies with extended star formation histories have higher [Ni/Fe] abundances, which could indicate that the dominant class of SN Ia is different for galaxies where star formation lasted for at least several Gyr.

We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19) and Z (18), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and Z grain data with data for mainstream grains from AGB stars of close-to-solar metallicity demonstrates that the three types of grains have indistinguishable Mo isotopic compositions. We show that the Mo isotope data can be used to constrain the maximum stellar temperatures (T_MAX) during thermal pulses in AGB stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with the grain data for Mo isotopes points to an origin from low-mass (∼1.5–3 M_⊙) rather than intermediate-mass (>3–∼9 M_⊙) AGB stars. Because of the low efficiency of ²²Ne(α, n)²⁵Mg at the low T_MAX values attained in low-mass AGB stars, model calculations cannot explain the large ³⁰Si excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.

The astrophysical production site of the heaviest elements in the universe remains a mystery. Incorporating heavy-element signatures of metal-poor, r-process-enhanced stars into theoretical studies of r-process production can offer crucial constraints on the origin of heavy elements. In this study, we introduce and apply the "actinide-dilution with matching" model to a variety of stellar groups, ranging from actinide-deficient to actinide-enhanced, to empirically characterize r-process ejecta mass as a function of electron fraction. We find that actinide-boost stars do not indicate the need for a unique and separate r-process progenitor. Rather, small variations of neutron richness within the same type of r-process event can account for all observed levels of actinide enhancements. The very low-Y_e, fission-cycling ejecta of an r-process event need only constitute 10%–30% of the total ejecta mass to accommodate most actinide abundances of metal-poor stars. We find that our empirical Y_e distributions of ejecta are similar to those inferred from studies of GW170817 mass ejecta ratios, which is consistent with neutron-star mergers being a source of the heavy elements in metal-poor, r-process-enhanced stars.

A protoneutron star produced in a core-collapse supernova (CCSN) drives a wind by its intense neutrino emission. We implement active–sterile neutrino oscillations in a steady-state model of this neutrino-driven wind to study their effects on the dynamics and nucleosynthesis of the wind in a self-consistent manner. Using vacuum mixing parameters indicated by some experiments for a sterile ν_s of ∼1 eV in mass, we observe interesting features of oscillations due to various feedback. For the higher ν_s mass values, we find that oscillations can reduce the mass-loss rate and the wind velocity by a factor of ∼1.6–2.7 and change the electron fraction critical to nucleosynthesis by a significant to large amount. In the most dramatic cases, oscillations shift nucleosynthesis from dominant production of ⁴⁵Sc to that of ⁸⁶Kr and ⁹⁰Zr during the early epochs of the CCSN evolution.

The discovery of a binary neutron star merger (NSM) through both its gravitational wave and electromagnetic emission has revealed these events to be key sites of r-process nucleosynthesis. Here, we evaluate the prospects of finding the remnants of Galactic NSMs by detecting the gamma-ray decay lines from their radioactive r-process ejecta. We find that ¹²⁶Sn, which has several lines in the energy range 415–695 keV and resides close to the second r-process peak, is the most promising isotope, because of its half-life t_1/2 = 2.30(14) × 10⁵ yr being comparable to the ages of recent NSMs. Using a Monte Carlo procedure, we predict that multiple remnants are detectable as individual sources by next-generation γ-ray telescopes which achieve sub-MeV line sensitivities of ∼10⁻⁸–10⁻⁶ γ cm⁻² s⁻¹. However, given the unknown locations of the remnants, the most promising search strategy is a systematic survey of the Galactic plane and bulge extending to high Galactic latitudes. Individual known supernova remnants which may be misclassified NSM remnants could also be targeted, especially those located outside the Galactic plane. Detection of a moderate sample of Galactic NSM remnants would provide important clues to unresolved issues such as the production of actinides in NSMs, properties of merging NS binaries, and even help distinguish them from rare supernovae as current Galactic r-process sources. We also investigate the diffuse flux from longer-lived nuclei (e.g., ¹⁸²Hf) that could in principle trace the Galactic spatial distribution of NSMs over longer timescales, but find that the detection of the diffuse flux appears challenging even with next-generation telescopes.