Innermost Ejecta Research Articles

Nucleosynthesis in the Innermost Ejecta of Magnetorotational Supernova Explosions in Three Dimensions

Core-collapse supernova (CCSN) explosions powered by rotation and magnetic fields present an interesting astrophysical site for nucleosynthesis that potentially contributes to the production of r-process elements. Here we present yields of the innermost ejecta in 3D magnetorotational CCSN models simulated using the CoCoNuT-FMT code. Strong magnetic fields tap the rotational energy of the proto−neutron star and lead to earlier and more energetic (∼3 × 1051 erg) explosions than typical neutrino-driven CCSNe. Compared to a reference nonmagnetic model, the ejecta in the magnetorotational models have much more neutron-rich components with Y e down to ∼0.25. Our post-processing calculations with the reaction network SkyNet show significant production of weak r-process elements up to mass number ∼130. We find negligible differences in the synthesis of heavy elements between two magnetorotational models with different initial field strengths of 1010 and 1012 G, in accord with their similar explosion dynamics. The magnetorotational models produce about ∼0.19 and 0.14 M ☉ of radioactive 56Ni, on the low end of inferred hypernova nickel masses. The yields are publicly available at Zenodo (doi: 10.5281/zenodo.10578981) for comparison with stellar abundance patterns, inclusion in modeling galactic chemical evolution, and comparison with other yield calculations. Our results add to the yet-restricted corpus of nucleosynthesis yields from 3D magnetorotational supernova simulations and will help quantify yield uncertainties.

Clumps and Rings of Ejecta in SNR 0540–69.3 as Seen in 3D

Abstract The distribution of ejecta in young supernova remnants offers a powerful observational probe of their explosions and progenitors. Here we present a 3D reconstruction of the ejecta in SNR 0540-69.3, which is an O-rich remnant with a pulsar wind nebula located in the LMC. We use observations from the Very Large Telescope (VLT)/MUSE to study Hβ, [O iii] λ λ4959, 5007, Hα, [S ii] λ λ6717, 6731, [Ar iii] λ7136, and [S iii] λ9069. This is complemented by 2D spectra from VLT/X-shooter, which also cover [O ii] λ λ3726, 3729, and [Fe ii] λ12567. We identify three main emission components: (i) clumpy rings in the inner nebula (≲1000 km s−1) with similar morphologies in all lines; (ii) faint extended [O iii] emission dominated by an irregular ring-like structure with radius ∼1600 km s−1 and inclination ∼40°, but with maximal velocities reaching ∼3000 km s−1; and (iii) a blob of Hα and Hβ located southeast of the pulsar at velocities ∼1500–3500 km s−1. We analyze the geometry using a clump-finding algorithm and use the clumps in the [O iii] ring to estimate an age of 1146 ± 116 yr. The observations favor an interpretation of the [O iii] ring as ejecta, while the origin of the H-blob is more uncertain. An alternative explanation is that it is the blown-off envelope of a binary companion. From the detection of Balmer lines in the innermost ejecta we confirm that SNR 0540 was a Type II supernova and that hydrogen was mixed down to low velocities in the explosion.

Three-dimensional Supernova Models Provide New Insights into the Origins of Stardust

Abstract We present the isotope yields of two post-explosion, three-dimensional 15 core-collapse supernova models, 15S and 15A, and compare them to the carbon, nitrogen, silicon, aluminum, sulfur, calcium, titanium, iron, and nickel isotopic compositions of SiC stardust. We find that these core-collapse supernova models predict similar carbon and nitrogen compositions to SiC X grains and grains with 12C/13C < 20 and 14N/15N < 60, which we will hereafter refer to as SiC ‘D’ grains. Material from the interior of a 15 explosion reaches high enough temperatures shortly after core collapse to produce the large enrichments of 13C and 15N necessary to replicate the compositions of SiC D grains. The innermost ejecta in a core-collapse supernova is operating in the neutrino-driven regime and undergoes fast proton capture after being heated by the supernova shockwave. Both 3D models predict 0.3 Al/27Al < 1.5, comparable to the ratios seen in SiC X, C, and D grains. Models 15S and 15A, in general, predict very large anomalies in calcium isotopes but do compare qualitatively with the SiC X grain measurements that show 44Ca and 43Ca excesses. The titanium isotopic compositions of SiC X grains are well reproduced. The models predict 57Fe excesses and depletions that are observed in SiC X grains, and in addition predict accurately the 60Ni/58Ni, 61Ni/58Ni, and 62Ni/58Ni ratios in SiC X grains, as a result of fast neutron captures initiated by the propagation of the supernova shockwave. Finally, symmetry has a noticeable effect on the production of silicon, sulfur, and iron isotopes in the SN ejecta.

Observations and spectral modelling of the narrow-lined Type Ic SN 2017ein

ABSTRACT SN 2017ein is a narrow-lined Type Ic SN that was found to share a location with a point-like source in the face on spiral galaxy NGC 3938 in pre-supernova images, making SN 2017ein the first credible detection of a Type Ic progenitor. Results in the literature suggest that this point-like source is likely a massive progenitor of 60–80 M⊙, depending on if the source is a binary, a single star, or a compact cluster. Using new photometric and spectral data collected for 200 d, including several nebular spectra, we generate a consistent model covering the photospheric and nebular phase using a Monte Carlo radiation transport code. Photospheric phase modelling finds an ejected mass 1.2–2.0 M⊙ with an Ek of ∼(0.9 ± 0.2) × 1051 erg, with approximately 1 M⊙ of material below 5000 km s−1 found from the nebular spectra. Both photospheric and nebular phase modelling suggests a 56Ni mass of 0.08–0.1 M⊙. Modelling the [O i] emission feature in the nebular spectra suggests that the innermost ejecta are asymmetric. The modelling results favour a low-mass progenitor of 16–20 M⊙, which is in disagreement with the pre-supernova derived high-mass progenitor. This contradiction is likely due to the pre-supernova source not representing the actual progenitor.

The fully developed remnant of a neutrino-driven supernova

Context. The remnants of core-collapse supernovae (SNe) are probes of the physical processes associated with their parent SNe. Aims. Here we aim to explore to which extent the remnant keeps memory of the asymmetries that develop stochastically in the neutrino-heating layer due to hydrodynamic instabilities (e.g., convective overturn and the standing accretion shock instability; SASI) during the first second after core bounce. Methods. We coupled a three-dimensional (3D) hydrodynamic model of a neutrino-driven SN explosion, which has the potential to reproduce the observed morphology of the Cassiopeia A (Cas A) remnant, with 3D (magneto)-hydrodynamic simulations of the remnant formation. The simulations cover ≈2000 yr of expansion and include all physical processes relevant to describe the complexities in the SN evolution and the subsequent interaction of the stellar debris with the wind of the progenitor star. Results. The interaction of large-scale asymmetries left from the earliest phases of the explosion with the reverse shock produces, at the age of ≈350 yr, an ejecta structure and a remnant morphology which are remarkably similar to those observed in Cas A. Small-scale structures in the large-scale Fe-rich plumes that were created during the initial stages of the SN, combined with hydrodynamic instabilities that develop after the passage of the reverse shock, naturally produce a pattern of ring- and crown-like structures of shocked ejecta. The consequence is a spatial inversion of the ejecta layers with Si-rich ejecta being physically interior to Fe-rich ejecta. The full-fledged remnant shows voids and cavities in the innermost unshocked ejecta, which are physically connected with ring-like features of shocked ejecta in the main shell in most cases, resulting from the expansion of Fe-rich plumes and their inflation due to the decay of radioactive species. The asymmetric distributions of 44Ti and 56Fe, which are mostly concentrated in the northern hemisphere, and pointing opposite to the kick velocity of the neutron star, as well as their abundance ratio are both compatible with those inferred from high-energy observations of Chandra and NuSTAR. Finally, the simulations show that the fingerprints of the SN can still be visible ≈2000 yr after the explosion. Conclusions. The main asymmetries and features observed in the ejecta distribution of Cas A can be explained by the interaction of the reverse shock with the initial large-scale asymmetries that developed from stochastic processes (e.g., convective overturn and SASI activity) that originate during the first seconds of the SN blast.

Titanium and Iron in the Cassiopeia A Supernova Remnant

Abstract Mixing above the proto-neutron star is believed to play an important role in the supernova engine, and this mixing results in a supernova explosion with asymmetries. Elements produced in the innermost ejecta, e.g., 56Ni and 44Ti, provide a clean probe of this engine. The production of 44Ti is particularly sensitive to the exact production pathway and, by understanding the available pathways, we can use 44Ti to probe the supernova engine. Using thermodynamic trajectories from a three-dimensional supernova explosion model, we review the production of these elements and the structures expected to form under the “convective-engine” paradigm behind supernovae. We compare our results to recent X-ray and γ-ray observations of the Cassiopeia A supernova remnant.

PUSHing Core-collapse Supernovae to Explosions in Spherical Symmetry. III. Nucleosynthesis Yields

Abstract In a previously presented proof-of-principle study, we established a parameterized spherically symmetric explosion method (PUSH) that can reproduce many features of core-collapse supernovae (CCSNe) for a wide range of pre-explosion models. The method is based on the neutrino-driven mechanism and follows collapse, bounce, and explosion. There are two crucial aspects of our model for nucleosynthesis predictions. First, the mass cut and explosion energy emerge simultaneously from the simulation (determining, for each stellar model, the amount of Fe-group ejecta). Second, the interactions between neutrinos and matter are included consistently (setting the electron fraction of the innermost ejecta). In the present paper, we use the successful explosion models from Ebinger et al. that include two sets of pre-explosion models at solar metallicity, with combined masses between 10.8 and 120 M ⊙. We perform systematic nucleosynthesis studies and predict detailed isotopic yields. The resulting 56Ni ejecta are in overall agreement with observationally derived values from normal CCSNe. The Fe-group yields are also in agreement with derived abundances for metal-poor star HD 84937. We also present a comparison of our results with observational trends in alpha element to iron ratios.

Nucleosynthesis in the Innermost Ejecta of Neutrino-driven Supernova Explosions in Two Dimensions

Abstract We examine nucleosynthesis in the innermost neutrino-processed ejecta (a few ) of self-consistent two-dimensional explosion models of core-collapse supernovae (CCSNe) for six progenitor stars with different initial masses. Three models have initial masses near the low-mass end of the SN range of (e8.8; electron-capture SN), (z9.6), and (u8.1), with initial metallicities of 1, 0, and 10−4 times the solar metallicity, respectively. The other three are solar-metallicity models with initial masses of (s11), (s15), and (s27). The low-mass models e8.8, z9.6, and u8.1 exhibit high production factors (nucleosynthetic abundances relative to the solar abundances) of 100–200 for light trans-Fe elements from Zn to Zr. This is associated with an appreciable ejection of neutron-rich matter in these models. Remarkably, the nucleosynthetic outcomes for the progenitors e8.8 and z9.6 are almost identical, including interesting productions of 48Ca and 60Fe, irrespective of their quite different (O–Ne–Mg and Fe) cores prior to collapse. In the more massive models s11, s15, and s27, several proton-rich isotopes of light trans-Fe elements including the p-isotope 92Mo (for s27) are made, up to production factors of ∼30. Both electron-capture SNe and CCSNe near the low-mass end can therefore be dominant contributors to the Galactic inventory of light trans-Fe elements from Zn to Zr and probably 48Ca and live 60Fe. The innermost ejecta of more massive SNe may have only subdominant contributions to the chemical enrichment of the Galaxy except for 92Mo.

The detonation of a sub-Chandrasekhar-mass white dwarf at the origin of the low-luminosity Type Ia supernova 1999by

While Chandrasekhar-mass ($M_{\rm Ch}$) models with a low $^{56}\rm{Ni}$ yield can match the peak luminosities of fast-declining, 91bg-like Type Ia supernovae (SNe Ia), they systematically fail to reproduce their faster light-curve evolution. Here we illustrate the impact of a low ejecta mass on the radiative display of low-luminosity SNe Ia, by comparing a sub-$M_{\rm Ch}$ model resulting from the pure central detonation of a C-O White Dwarf (WD) to a $M_{\rm Ch}$ delayed-detonation model with the same $^{56}\rm{Ni}$ yield of 0.12 M$_{\odot}$. Our sub-$M_{\rm Ch}$ model from a 0.90 M$_{\odot}$ WD progenitor has a $\sim$5 day shorter rise time in the integrated UV-optical-IR (uvoir) luminosity, as well as in the $B$-band, and a $\sim$20 per cent higher peak uvoir luminosity ($\sim$1 mag brighter peak $M_B$). This sub-$M_{\rm Ch}$ model also displays bluer maximum-light colours due to the larger specific heating rate, and larger post-maximum uvoir and $B$-band decline rates. The luminosity decline at nebular times is also more pronounced, reflecting the enhanced escape of gamma rays resulting from the lower density of the progenitor WD. The deficit of stable nickel in the innermost ejecta leads to a notable absence of forbidden lines of [Ni II] in the nebular spectra. In contrast, the $M_{\rm Ch}$ model displays a strong line due to [Ni II] 1.939 $\mu\rm{m}$, which could in principle serve to distinguish between different progenitor scenarios. Our sub-$M_{\rm Ch}$ model offers an unprecedented agreement with optical and near-infrared observations of the 91bg-like SN 1999by, making a strong case for a WD progenitor significantly below the Chandrasekhar-mass limit for this event and other low-luminosity SNe Ia.

The ν process in the innermost supernova ejecta

The neutrino-induced nucleosynthesis (ν process) in supernova explosions of massive stars of solar metallicity with initial main sequence masses between 13 and 30 M⊙ has been studied with an analytic explosion model using a new extensive set of neutrino-nucleus cross-sections and spectral properties that agree with modern supernova simulations. The production factors for the nuclei 7 Li, 11 B, 19 F, 138 La and 180 Ta, are still significantly enhanced but do not reproduce the full solar abundances. We study the possible contribution of the innermost supernova eject to the production of the light elements 7 Li and 11 B with tracer particles based on a 2D supernova simulation of a 12 M⊙ progenitor and conclude, that a contribution exists but is negligible for the total yield for this explosion model.

Nebular spectra and abundance tomography of the Type Ia supernova SN 2011fe: a normal SN Ia with a stable Fe core

A series of optical and one near-infrared nebular spectra covering the first year of the Type Ia supernova SN 2011fe are presented and modelled. The density profile that proved best for the early optical/ultraviolet spectra, "rho-11fe", was extended to lower velocities to include the regions that emit at nebular epochs. Model rho-11fe is intermediate between the fast deflagration model W7 and a low-energy delayed-detonation. Good fits to the nebular spectra are obtained if the innermost ejecta are dominated by neutron-rich, stable Fe-group species, which contribute to cooling but not to heating. The correct thermal balance can thus be reached for the strongest [FeII] and [FeIII] lines to be reproduced with the observed ratio. The 56Ni mass thus obtained is 0.47 +/- 0.05 Mo. The bulk of 56Ni has an outermost velocity of ~8500 km/s. The mass of stable iron is 0.23 +/- 0.03 Mo. Stable Ni has low abundance, ~10^{-2} Mo. This is sufficient to reproduce an observed emission line near 7400 A. A sub-Chandrasekhar explosion model with mass 1.02 Mo and no central stable Fe does not reproduce the observed line ratios. A mock model where neutron-rich Fe-group species are located above 56Ni following recent suggestions is also shown to yield spectra that are less compatible with the observations. The densities and abundances in the inner layers obtained from the nebular analysis, combined with those of the outer layers previously obtained, are used to compute a synthetic bolometric light curve, which compares favourably with the light curve of SN 2011fe.

RADIATION-HYDRODYNAMICAL MODELING OF CORE-COLLAPSE SUPERNOVAE: LIGHT CURVES AND THE EVOLUTION OF PHOTOSPHERIC VELOCITY AND TEMPERATURE

We have developed a relativistic, radiation-hydrodynamics Lagrangian code, specifically tailored to simulate the evolution of the main observables (light curve and the evolution of photospheric velocity and temperature) in core-collapse supernova (CC-SN) events. The distinctive features of the code are an accurate treatment of radiative transfer coupled to relativistic hydrodynamics, a self-consistent treatment of the evolution of the innermost ejecta taking into account the gravitational effects of the central compact remnant, and a fully implicit Lagrangian approach to the solution of the coupled nonlinear finite difference system of equations. Our aim is to use it as a numerical tool to perform calculations of a grid of models to be compared with observations of CC-SNe. In this paper, we present some testcase simulations and a comparison with observations of SN 1987A, as well as with the results obtained with other numerical codes. We also briefly discuss the influence of the main physical parameters (ejected mass, progenitor radius, explosion energy, amount of 56Ni) on the evolution of the ejecta, and the implications of our results in connection with the possibility to "standardize" hydrogen-rich CC-SNe for using them as candles to measure cosmological distances.

What are the astrophysical sites for the [formula omitted]-process and the production of heavy elements?

This article addresses three of the four nucleosynthesis processes involved in producing heavy nuclei beyond Fe (with a main focus on the r -process). Opposite to the fourth process (the s -process), which operates in stellar evolution during He- and C-burning, they are all related to explosive burning phases, (presumably) linked to core collapse supernova events of massive stars. The (classical) p -process is identified with explosive Ne/O-burning in outer zones of the progenitor star. It is initiated by the passage of the supernova shock wave and acts via photodisintegration reactions like a spallation process which produces neighboring (proton-rich) isotopes from pre-existing heavy nuclei. The reproduction of some of the so-called lighter p -isotopes with A < 100 faces problems in this environment. The only recently discovered ν p -process is related to the innermost ejecta, the neutrino wind expelled from the hot proto-neutron star after core collapse and the supernova explosion. This neutrino wind is proton-rich in its early phase, producing nuclei up to 64Ge. Reactions with neutrinos permit to overcome decay/reaction bottlenecks for the flow beyond 64Ge, thus producing light p -isotopes, which face problems in the classical p -process scenario. The understanding of the r -process, being identified for a long time with rapid neutron captures and passing through nuclei far from stability, is still experiencing major problems. These are on the one hand related to nuclear uncertainties far from stability (masses, half-lives, fission barriers), affecting the process speed and abundance peaks. On the other hand the site is still not definitely located, yet. (i) Later, possibly neutron-rich, high entropy phases of the neutrino wind (if they materialize!) could permit its operation. (ii) Other options include the ejection of very neutron-rich neutron star-like matter, occurring possibly in neutron star mergers or core collapse supernova events with jets, related to prior stellar evolution with high rotation rates and magnetic fields. Two different environments are required for a weak and a main/strong r -process, witnessed by observations of low metallicity stars and meteoritic inclusions, which could possibly be identified with the two options listed above, i.e. the weak r -process could be related to the neutrino wind when changing from p -rich to n -rich conditions.

The r-, p-, and νp-Process

The processes discussed in this review are three of the four nucleosynthesis processes involved in producing heavy nuclei beyond Fe (not counting the rp-process in X-ray bursts). Opposite to the fourth process (the s-process), which operates in stellar evolution during He- and C-burning, they are all related to explosive burning phases, (presumably) linked to core collapse supernova events of massive stars. The (classical) p-process is identified with explosive Ne/O-burning in outer zones of the progenitor star. It is intitiated by the passage of the supernova shock wave and acts via photodisintegration reactions like a spallation process which produces neighboring (proton-rich) isotopes from pre-existing heavy nuclei. The reproduction of some of the so-called lighter p-isotopes with A < 100 faces problems in this environment. The only recently discovered νp-process is related to the innermost ejecta, the neutrino wind expelled from the hot proto-neutron star after core collapse in the supernova explosion. This neutrino wind is proton-rich in its early phase and reactions with neutrinos permit to overcome decay/reaction bottlenecks for the flow beyond the Fe-group, thus permitting the production of those p-isotopes, which face problems in the classical p-process scenario. The understanding of the r-process, being identified for a long time with rapid neutron captures – and passing through nuclei far from stability – is still experiencing major problems. These are on the one hand related to nuclear uncertainties far from stability (masses and half-lives), affecting the process speed and abundance peaks, on the other hand the site is still not definitely located, yet. Later neutron-rich, high entropy phases of the neutrino wind could permit its operation, other options include the ejection of very neutron-rich neutron star matter. Two different environments are required for a weak and a main/strong r-process, witnessed by observations of low metallicity stars.

NEBULAR SPECTRA AND EXPLOSION ASYMMETRY OF TYPE Ia SUPERNOVAE

The spectral signatures of asymmetry in Type Ia Supernova (SN Ia) explosions are investigated, using a sample of late-time nebular spectra. First, a kinematical model is constructed for SN Ia 2003hv, which can account for the main features in its optical, Near-Infrared (NIR), and Mid-Infrared (Mid-IR) late-time spectra. It is found that an asymmetric off-center model can explain the observed characteristics of SN 2003hv. This model includes a relatively high density, Fe-rich region which displays a large velocity off-set, and a relatively low density, extended 56Ni-rich region which is more spherically distributed. The high density region consists of the inner stable Fe-Ni region and outer 56Ni-rich region. Such a distribution may be the result of a delayed-detonation explosion, in which the first deflagration produces the global asymmetry in the innermost ejecta, while the subsequent detonation can lead to the bulk spherical symmetry. This configuration, if viewed from the direction of the off-set, can consistently explain the blueshift in some of the emission lines and virtually no observed shift in other lines in SN 2003hv. For this model, we then explore the effects of different viewing angles and the implications for SNe Ia in general. The model predicts that a variation of the central wavelength, depending on the viewing angle, should be seen in some lines (e.g., [Ni II]7378), while the strongest lines (e.g., [Fe III] blend at 4700A) will not show this effect. By examining optical nebular spectra of 12 SNe Ia, we have found that such a variation indeed exists. We suggest that the global asymmetry in the innermost ejecta, as likely imprint of the deflagration flame propagation, is a generic feature of SNe Ia (abridged).

Detailed Nucleosynthesis Yields from the Explosion of Massive Stars

AbstractDespite the complexity and uncertainties of core collapse supernova simulations there is a need to provide correct nucleosynthesis abundances for the progressing field of galactic evolution and observations of low metallicity stars. Especially the innermost ejecta are directly affected by the explosion mechanism, i.e. most strongly the yields of Fe-group nuclei for which an induced piston or thermal bomb treatment will not provide the correct yields because the effect of neutrino interactions is not included.Recent observations of metal-poor halo stars support the suggested existence of a lighter element primary process (LEPP) which operates very early in the galaxy and is independent of the r-process. We present a candidate for the LEPP, the so-called νp-process.

Signatures of Delayed Detonation, Asymmetry, and Electron Capture in the Mid‐Infrared Spectra of Supernovae 2003hv and 2005df

We present the first mid-infrared (5.2-15.2 micron) spectra of Type Ia supernovae (SNe 2003hv and 2005df). The detection of Ni emission in SN 2005df 135 days after the explosion provides direct observational evidence of high-density nuclear burning forming a significant amount of stable Ni in a Type Ia supernova. The observed emission line profiles in the SN 2005df spectrum indicate a chemically stratified ejecta structure. The SN 2005df Ar lines also exhibit a two-pronged emission profile implying that the Ar emission deviates significantly from spherical symmetry. The spectrum of SN 2003hv also shows signs of asymmetry, exhibiting blueshifted [Co III] which matches the blueshift of [Fe II] lines in nearly coeval NIR spectra. Finally, local thermodynamic equilibrium abundance estimates for the yield of radioactive 56Ni give ~0.5 Msun, for SN 2003hv, but only ~0.13-0.22 Msun for the apparently subluminous SN 2005df, supporting the notion that the luminosity of SNe Ia is primarily a function of the radioactive 56Ni yield. The chemically stratified ejecta structure observed in SN 2005df matches the predictions of delayed-detonation (DD) models, but is entirely incompatible with current three-dimensional deflagration models. Furthermore the degree that this layering persists to the innermost regions of the supernova is difficult to explain even in a DD scenario, where the innermost ejecta are still the product of deflagration burning. Thus, while these results are roughly consistent with a delayed detonation, it is clear that a key piece of physics is still missing from our understanding of the earliest phases of SN Ia explosions.

Composition of the Innermost Core‐Collapse Supernova Ejecta

With presently known input physics and computer simulations in 1D, a self-consistent treatment of core collapse supernovae does not yet lead to successful explosions, while 2D models show some promise. Thus, there are strong indications that the delayed neutrino mechanism works combined with a multi-D convection treatment for unstable layers. On the other hand there is a need to provide correct nucleosynthesis abundances for the progressing field of galactic evolution and observations of low metallicity stars. The innermost ejecta is directly affected by the explosion mechanism, i.e. most strongly the yields of Fe-group nuclei for which an induced piston or thermal bomb treatment will not provide the correct yields because the effect of neutrino interactions is not included. We apply parameterized variations to the neutrino scattering cross sections and alternatively, parameterized variations are applied to the neutrino absorption cross sections on nucleons in the ``gain region''. We find that both measures lead to similar results, causing explosions and a Ye>0.5 in the innermost ejected layers, due to the combined effect of a short weak interaction time scale and a negligible electron degeneracy, unveiling the proton-neutron mass difference. We include all weak interactions (electron and positron capture, beta-decay, neutrino and antineutrino capture on nuclei, and neutrino and antineutrino capture on nucleons) and present first nucleosynthesis results for these innermost ejected layers to discuss how they improve predictions for Fe-group nuclei. The proton-rich environment results in enhanced abundances of 45Sc, 49Ti, and 64Zn as requested by chemical evolution studies and observations of low metallicity stars as well as appreciable production of nuclei in the mass range up to A=80.

Enrichment of Very Metal Poor Stars with Bothr‐Process ands‐Process Elements from 8–10M⊙Stars

Recent spectroscopic studies have revealed the presence of numerous carbon-enhanced, metal-poor stars with [Fe/H] < -2.0 that exhibit strong enhancements of s-process elements. These stars are believed to be the result of a binary mass transfer episode from a former asymptotic giant branch (AGB) companion that underwent s-process nucleosynthesis. However, several such stars exhibit significantly lower Ba/Eu ratios than solar s-process values. This might be explained if there were an additional contribution from the r-process, thereby diluting the Ba/Eu ratio by extra production of Eu. We propose a model in which the double enhancements of r-process and s-process elements originate from a former 8-10 M☉ companion in a wide binary system, which may undergo s-processing during an AGB phase, followed by r-processing during its subsequent supernova explosion. The mass of Eu (as representative of r-process elements) captured by the secondary through the wind from the supernova is estimated and is assumed to be proportional to the geometric fraction of the secondary (low-mass, main-sequence) star with respect to the primary (exploding) star. We find that the estimated mass is in good agreement with a constraint on the Eu yield per supernova event obtained from a Galactic chemical evolution study, when the initial orbital separation is taken to be ~1 yr. If one assumes an orbital period on the order of 5 yr, the efficiency of wind pollution from the supernova must be enhanced by a factor of ~10. This may, in fact, be realized if the expansion velocity of the supernova's innermost ejecta, in which the r-process has taken place, is significantly slow, resulting in an enhancement of accretion efficiency by gravitational focusing.

The Innermost Ejecta of Core Collapse Supernovae

We ensure successful explosions (of otherwise non-explosive models) by enhancing the neutrino luminosity via reducing the neutrino scattering cross sections or by increasing the heating efficiency via enhancing the neutrino absorption cross sections in the heating region. Our investigations show that the resulting electron fraction Y e in the innermost ejecta is close to 0.5, in some areas even exceeding 0.5. We present the effects of the resulting values for Y e on the nucleosynthesis yields of the innermost zones of core collapse Supernovae.