Magnetically-Driven Neutron-Rich Ejecta Unleashed: Global 3D Neutrino-GRMHD Simulations of Collapsars Reveal the Conditions for r-process Nucleosynthesis

Abstract: Collapsars - rapidly rotating stellar cores that form black holes (BHs) - can power gamma-ray bursts (GRBs) 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 magnetic fields or the BH, both of which are essential for launching mass outflows. We present $\nu$H-AMR, a novel neutrino-transport general relativistic magnetohydrodynamic ($\nu$GRMHD) code, which we use to perform the first 3D $\nu$GRMHD collapsar simulations. We find a self-consistent formation of a disk with initially weak magnetic flux, resulting in a low accretion speed and leaving sufficient time for the disk to neutronize. However, once substantial magnetic flux accumulates near the BH, it becomes dynamically important, leading to a magnetically arrested disk that unbinds some of the neutron-rich material. The strong flux also accelerates the accretion speed, preventing further disk neutronization. The neutron-rich disk ejecta collides with the infalling stellar gas, generating a shocked cocoon with an electron fraction, $Y_\text{e}\gtrsim0.2$. Continuous mixing between the cocoon and neutron-poor stellar gas incrementally raises the outflow $Y_\text{e}$, but the final $r$-process yield is determined earlier at the point of neutron capture freeze-out. Our models require extreme magnetic fluxes and mass accretion rates to eject neutron-rich material ($Y_\text{e}\lesssim0.3$), implying very high $r$-process ejecta masses $M_\text{ej}\lesssim{}M_\odot$. Future work will explore under what conditions more typical collapsar engines become $r$-process factories.

• The paper introduces νh-amr, a new 3D neutrino-GRMHD code that models collapsar disk formation and magnetically driven ejecta.
• The paper shows that magnetic flux accumulation forms a magnetically arrested disk (MAD), which accelerates neutronization and ejecta production.
• The paper concludes that collapsars with extreme magnetic fields and high accretion rates may significantly contribute to r-process nucleosynthesis, complementing neutron-star mergers.

Insights into Magnetically-Driven Neutron-Rich Ejecta in Collapsars Through Global 3D Neutrino-GRMHD Simulations

Issa et al. present a comprehensive study on magnetically-driven neutron-rich ejecta from collapsars, utilizing the first-ever 3D Neutrino General Relativistic Magnetohydrodynamic (νGRMHD) simulations. These collapsars are rapidly rotating stellar cores that collapse into black holes (BHs) and are essential in powering gamma-ray bursts (GRBs) and potentially significant contributors to the rapid neutron capture process ($r$-process) nucleosynthesis of heavy elements. This research aims to elucidate the conditions under which collapsar disks form, generate, and eject neutron-rich material necessary for $r$-process nucleosynthesis.

Methodology and Numerical Model

The authors introduce $\nu$h-amr, a novel computational code incorporating a two-moment (M1) neutrino transport scheme coupled with general relativistic magnetohydrodynamics. This study uses this code to simulate the collapse of massive stellar cores, highlighting the importance of incorporating magnetic fields and the role of BHs in the collapse process. Unlike earlier simulations which did not include these factors, this work integrates them, facilitating the launch of mass outflows.

The model consists of a uniformly dense rotating stellar core embedded within a numerical grid that employs 3D adaptive mesh refinement and local adaptive time-stepping. The simulation grid is specifically designed to handle the complex physics of rapidly rotating systems that involve significant magnetic field dynamics and high-energy neutrino interactions.

Key Findings

1. Disk and Neutronization Dynamics: The study shows how a disk with initially weak magnetic flux can form around a BH, which subsequently allows for its neutronization, given an appropriate timescale. As magnetic flux accumulates near the BH to dynamically relevant levels, a magnetically arrested disk (MAD) arises, enabling some neutron-rich material to be ejected.
2. Magnetically Arrested Disks (MAD) and Ejecta: The accumulation of significant magnetic flux near the BH leads to increased accretion speed, which decreases the neutronization time available and hence alters the $Y_e$ (electron fraction) distribution in the ejecta. However, the presence of strong magnetic fields promotes conditions suited for potent mass ejection, aligning with observations of neutron-star merger events that produce gravitational waves.
3. Implications for $r$-process Nucleosynthesis: The simulations suggest that under specific conditions, extreme magnetic flux and high mass accretion rates from collapsars can lead to large masses of neutron-rich ejecta with $Y_\text{e}\lesssim0.3$. This points to the potential of collapsars being significant sources of $r$-process elements, complementing neutron-star mergers in accounting for such nucleosynthesis contributions in the universe.
4. Numerical Predictions and Future Directions: This work emphasizes that obtaining sufficient magnetic strength and a high mass accretion rate is critical for ejecta to achieve the conditions necessary for $r$-process nucleosynthesis. The authors propose further explorations into more typical collapsar engines with less extreme conditions to fully understand the potential of collapsar-generated $r$-process element production.

Discussion

Through these 3D simulations, Issa et al. provide a nuanced understanding of how magnetic fields and rapid rotation in stellar collapsars create conditions conducive for $r$-process nucleosynthesis. The insights that extremely high magnetic flux and mass accretion rates are required for significant neutron-rich material ejection highlight the uniqueness of potential $r$-process sites and emphasize the complex interplay of physical processes in collapsars. This serves as a foundation for evolving models of nucleosynthesis and provides a direction for future research investigating more diverse stellar collapse scenarios and their astronomical implications. Additionally, understanding these magnetic-driven dynamics will refine the theoretical frameworks describing GRBs and the closely associated $r$-process element cosmology.

Overall, this research represents a significant progression in the simulation of astrophysical phenomena related to collapsars, offering potential explanations for observations of heavy element formation in the universe. As computing capabilities expand, further detailed simulations will refine the understanding of these mechanisms, potentially leading to new insights into the evolution and distribution of elements across cosmic timescales.