Progenitor-Explosion Connection and Remnant Birth Masses for Neutrino-Driven Supernovae of Iron-Core Progenitors

Abstract: We perform hydrodynamic supernova simulations in spherical symmetry for over 100 single stars of solar metallicity to explore the progenitor-explosion and progenitor-remnant connections established by the neutrino-driven mechanism. We use an approximative treatment of neutrino transport and replace the high-density interior of the neutron star (NS) by an inner boundary condition based on an analytic proto-NS core-cooling model, whose free parameters are chosen such that explosion energy, nickel production, and energy release by the compact remnant of progenitors around 20 solar masses are compatible with Supernova 1987A. Thus we are able to simulate the accretion phase, initiation of the explosion, subsequent neutrino-driven wind phase for 15-20 s, and the further evolution of the blast wave for hours to days until fallback is completed. Our results challenge long-standing paradigms. We find that remnant mass, launch time, and properties of the explosion depend strongly on the stellar structure and exhibit large variability even in narrow intervals of the progenitors' zero-age-main-sequence mass. While all progenitors with masses below about 15 solar masses yield NSs, black hole (BH) as well as NS formation is possible for more massive stars, where partial loss of the hydrogen envelope leads to weak reverse shocks and weak fallback. Our NS baryonic masses of ~1.2-2.0 solar masses and BH masses >6 solar masses are compatible with a possible lack of low-mass BHs in the empirical distribution. Neutrino heating accounts for SN energies between some 10^{50} erg and about 2*10^{51} erg, but can hardly explain more energetic explosions and nickel masses higher than 0.1-0.2 solar masses. These seem to require an alternative SN mechanism.

• The paper demonstrates that neutrino-driven simulations calibrated with SN 1987A data successfully capture how iron-core progenitors explode and form diverse remnant masses.
• The methodology employs a proto-NS core-cooling model that yields explosion energies up to 2 x 10^51 erg and predicts distinct neutron star (1.2–2.0 solar masses) and black hole (>6 solar masses) mass ranges.
• The findings imply that subtle stellar structure and fallback phenomena necessitate more detailed, multi-dimensional models to fully explain the observed remnant mass distributions.

Exploring Neutrino-Driven Supernovae and Remnant Formation

The paper by Ugliano et al. presents a comprehensive study of the relationship between progenitor stars, supernova explosions, and the formation of compact remnants such as neutron stars (NS) and black holes (BH). The authors carry out one-dimensional (1D) hydrodynamic simulations for more than 100 solar-metallicity progenitors with iron cores. They utilize an approximate neutrino transport method to explore how these progenitors explode and the characteristics of the remnants left behind. The core focus is on understanding the neutrino-driven mechanism in supernova (SN) explosions and its influence on the mass of the remnants birthed from these celestial events.

Methodology and Calibration

The authors adopt a pragmatic approach by replacing the dense core of a forming NS with an analytic proto-NS core-cooling model. The model parameters are finely tuned using observational data from Supernova 1987A to ensure consistent explosion energies, nickel yields, and energy release profiles. This calibration underpins the reliability of the simulations, enabling the study of the accretion phase, explosion initiation, and the evolution of the remnant over days post-explosion.

Key Findings and Numerical Results

• Explosion Dependence on Stellar Structure: The results highlight the significant influence of the progenitor's internal structure on the explosion dynamics and remnant properties. Interestingly, even a narrow mass range (e.g., zero-age-main-sequence (ZAMS) masses) shows large variability in explosion results. This underscores the sensitivity of the explosion mechanism to the finer details of the progenitor's structure, such as the density profiles and the layered composition of the star.
• Remnant Mass Distribution: The study reveals a complex distribution of NS and BH masses, emphasizing that NS are typically formed from progenitors below approximately 15 solar masses, while both NS and BH formation are possible in more massive stars. The baryonic mass of NSs produced ranges from about 1.2 to 2.0 solar masses, whereas BHs are formed with masses greater than 6 solar masses, aligning with the lack of low-mass BH in observations.
• Fallback Phenomena: Interestingly, the amount of fallback - material not ejected and instead added to the remnant - is significant, especially in less massive progenitors pre-explosion. This results in a gap in remnant mass distribution often observed between NS and BH.
• Explosion Energies and Nickel Production: The energies of neutrino-driven SNe are outlined as reaching up to 2 x 10^51 erg. However, these energies and associated nickel production constrain more energetic or nickel-rich supernovae from being explained solely by neutrino-heating processes. Alternate mechanisms may need to be considered for exceptionally energetic explosions.

Implications and Speculations

The research offers insightful implications for our understanding of stellar evolution and compact remnant formation. The variability and nonmonotonic trends in explosion properties stress the necessity for detailed, nuanced modeling of stellar interiors in predicting SN outcomes. The challenge laid by Ugliano et al. is for the astrophysics community to consider multi-dimensional simulations, as 3D models could potentially alter the landscape of SN explosion predictions by mitigating some of the instabilities present in 1D approaches.

The findings have theoretical and observational consequences, particularly in explaining the observed remnant mass distributions. The work underscores the intricate relationship between stellar properties at collapse and the resulting post-explosion phenomena, cautioning against overly simplified models of stellar death.

As a next step, extending these simulations into multi-dimensional analyses will provide a more comprehensive understanding, potentially illuminating the range of explosion energies and elucidating the interplay of rotation and convection in the progenitor phases. Future studies might focus on progenitors of varied metallicity and incorporate binary interactions, which have critical roles in shaping the evolution and final fate of massive stars.