Luca Boccioli

We explore the impact of a more efficient ¹² C+ ¹² C reaction on the structure and nucleosynthesis of massive stars. We calculate nonrotating stellar models with initial masses of 15, 16, 18, 20, 22, 25, and 40 M _⊙ and solar metallicity by means of the FRANEC code. Furthermore, we simulate the core-collapse supernova (SN) of these models with the thermal bomb technique, using two different approaches to inject the thermal energy into the pre-SN structure. Our results show that a more efficient ¹² C+ ¹² C rate extends the duration of the central carbon-burning phase, developing more massive convective cores and leading to a different and less compact pre-SN structure with respect to models calculated with a standard ¹² C+ ¹² C rate. These structural differences significantly impact nucleosynthesis. In particular, an increased rate enhances the production of elements heavier than Fe, produced by the s- process nucleosynthesis and driven by the more efficient activation of the ¹³ C( α , n) ¹⁶ O neutron source in the early carbon-burning shells. We find that the differences in the chemical composition of the core-collapse SN ejecta are primarily determined by these pre-SN structural changes, which dominate over the effects of different explosion prescriptions.

Explosive nucleosynthesis is affected by many uncertainties, particularly regarding the assumptions and prescriptions adopted during a star’s evolution. Moreover, simple explosion models are often used, which can introduce large errors in the assumed explosion energy and mass cut. In this paper, our goal is to analyze the explosion properties and nucleosynthesis of a wide range of progenitors from three different stellar evolution codes: FRANEC, KEPLER, and MESA. In particular, we show the differences between the neutrino-driven explosions simulated in this work and the much simpler bomb and piston models typically used in the literature. We then focus on the impact of different explodabilities and different explosion dynamics on the nucleosynthetic yields. We adopted the neutrino-driven core-collapse supernova explosion code GR1D+, i.e., a spherically symmetric model with state-of-the-art microphysics and neutrino transport and a time-dependent mixing-length model for neutrino-driven convection. We carried out explosions up to several seconds after bounce, then calculated the nucleosynthetic yields with the post-processing code SkyNet. We find that our 1D+ simulations yield explosion energies and remnant masses in agreement with observations of type II-P, IIb, and Ib supernovae, as well as with the most recent 3D simulations of the explosion. We provide a complete set of yields for all the stars simulated, including rotating, low-metallicity, and binary progenitors. Finally, we find that piston and bomb models, compared with more realistic neutrino-driven explosions, can artificially increase the production of Fe-peak elements, whereas the different explodabilities cause discrepancies in lighter elements.

The merger between the carbon (C) and oxygen (O) shells hours to days before the collapse of a massive star significantly changes its nucleosynthesis, which is reflected in the elemental ratios observed in supernova remnants (SNRs). We present a nucleosynthesis study of ⁴⁴ Ti production in core-collapse supernovae (CCSNe), highlighting large silicon (Si), sulfur (S), calcium (Ca), and, most importantly, argon (Ar) to neon (Ne) ratios as diagnostics for carbon–oxygen (C–O) shell mergers. We compare yields from eight different sets of CCSNe models to observations of Cassiopeia A (Cas A), and show that C–O shell mergers are consistently the models that best match X-ray and infrared observations. These models produce high Ar/Ne ratios (≳0.1), due to ²⁰ Ne depletion and production of ³⁶ Ar and ³⁸ Ar, while lower ratios are obtained from nonmerger cases. Based on the Ar/Ne diagnostic, we compare the range of expected ⁴⁴ Ti produced by C–O shell mergers, which is up to ∼20%–30% of the overall ⁴⁴ Ti, but expected to be located outside the reverse shock. Based on the sets of models considered, the photon flux expected from the ⁴⁴ Ti synthesized in the C–O shell merger in Cas A is below the NuSTAR and COS I detection limits, compatible with current limits locating most of the ⁴⁴ Ti interior to the reverse shock, but might be detectable from proposed missions like ASCENT. Finally, for the SNR of 1987A, a dominant C–O merger origin of the observed ⁴⁴ Ti is unlikely based on the observed redshift in its ⁴⁴ Ti line.

Massive stars can end their lives with a successful supernova explosion (leaving behind a neutron star or, more rarely, a black hole), or a failed explosion that leaves behind a black hole. The density structure of the pre-collapse progenitor star already encodes much of the information regarding the outcome and properties of the explosion. However, the complexity of the collapse and subsequent shock expansion phases prevents drawing a straightforward connection between the pre-collapse and post-explosion properties. In order to derive such a connection several explodability studies have been performed in recent years. However, different studies can predict different explosion outcomes. In this article, we show how compactness, which is related to the average density of the star’s core, has an important role in determining the efficiency of neutrino heating, and therefore the outcome of the explosion. Commonly, high-compactness progenitors are assumed to yield failed explosions, due to their large mass accretion rates, preventing the shock from expanding. We show by analysing $\sim$150 2D flash and F ornax simulations and 20 3D F ornax simulations that this is not the case. Instead, due to the rapid increase of neutrino heating with compactness, high-compactness progenitors lead to successful shock revival. We also show that 1D+ simulations that include $\nu$-driven convection using a mixing-length theory approach correctly reproduce this trend. Finally, we compare 1D+ models, which we show can reproduce some aspects of multi-D simulations with reasonable accuracy, with other widely used 1D models in the literature.

The explosion mechanism of a core-collapse supernova (SN) is a complex interplay between neutrino heating and cooling (including the effects of neutrino-driven convection), the gravitational potential, and the ram pressure of the infalling material. To analyse the post-bounce phase of an SN, one can use the generalized force explosion condition (FEC+), which succinctly formalizes the interplay among these four phenomena in an analytical condition, consistent with realistic simulations. In this paper, we use the FEC+ to study the post-bounce phase of 341 spherically symmetric simulations, where convection is included through a time-dependent mixing-length approach. We find that the accretion of the Si/O interface through the expanding shock can significantly change the outcome of the SN by driving the FEC+ above the explosion threshold. We systematically explore this by (i) artificially smoothing the pre-SN density profile, and (ii) artificially varying the mixing length. In both cases, we find that large-enough density contrasts at the Si/O interface lead to successful shock revival only if the FEC+ is already close to the explosion threshold. Furthermore, we find that the accretion of the Si/O interface has a substantial effect on the critical condition for SN explosions, contributing between 5 and 15 per cent, depending on how pronounced the density contrast at the interface is. Earlier studies showed that convection affects the critical condition by 25–30 per cent, which demonstrates that the accretion of the Si/O interface through the shock can play a nearly comparable role in influencing shock dynamics.

Most massive stars in our universe are born in binary systems, and yet their fates are quite uncertain, due to a poor understanding of their evolution. It is well known, however, that the density profile of a collapsing core, particularly at the interface between silicon and oxygen shells, is important when determining the explodability of a star. While this has been studied for single stars, it has been rarely studied for binary stars. In this note, we explore the explodability of single and binary stars using self-consistent supernova simulations. We study the differences in the silicon–oxygen interface and compactness of the progenitor stars and how they affect the explodability. We find that binary-stripped stars with initial masses above 37 M _⊙ are not as explodable as their single-star counterparts.

Recent developments in multi-dimensional simulations of core-collapse supernovae have considerably improved our understanding of this complex phenomenon. In addition to that, one-dimensional (1D) studies have been employed to study the explosion mechanism and its causal connection to the pre-collapse structure of the star, as well as to explore the vast parameter space of supernovae. Nonetheless, many uncertainties still affect the late stages of the evolution of massive stars, their collapse, and the subsequent shock propagation. In this review, we will briefly summarize the state-of-the-art of both 1D and 3D simulations and how they can be employed to study the evolution of massive stars, supernova explosions, and shock propagation, focusing on the uncertainties that affect each of these phases. Finally, we will illustrate the typical nucleosynthesis products that emerge from the explosion.

Recent developments in multi-dimensional simulations of core-collapse supernovae have considerably improved our understanding of this complex phenomenon. In addition to that, one-dimensional (1D) studies have been employed to study the explosion mechanism and its causal connection to the pre-collapse structure of the star, as well as to explore the vast parameter space of supernovae. Nonetheless, many uncertainties still affect the late stages of the evolution of massive stars, their collapse, and the subsequent shock propagation. In this review, we will briefly summarize the state-of-the-art of both 1D and 3D simulations and how they can be employed to study the evolution of massive stars, supernova explosions, and shock propagation, focusing on the uncertainties that affect each of these phases. Finally, we will illustrate the typical nucleosynthesis products that emerge from the explosion.