Research Highlights

Nuclear Neural Networks to Explore Supernovae

In the case of supernova explosions, observations are way ahead of theory. One of the main challenges in modeling massive stars to the onset of core-collapse is the computational bottleneck of nucleosynthesis during late burning stages. The large number of isotopes formed makes the simulations computationally intensive and prone to numerical instability.

To overcome this barrier, I designed a nuclear neural network framework to replace the nuclear solver in stellar evolution codes following oxygen core depletion (Grichener et al. 2025). I find that the nuclear neural network successfully emulates the results obtained with large nuclear networks, which are crucial for multidimensional simulations of core collapse supernovae, at a computational cost comparable to that of the small commonly used networks. This approach is promising for facilitating large-scale generation of supernova progenitors with higher physical fidelity, thus advancing our understanding of the explosion mechanism as well as neutron star and black hole formation, supernova kicks and gravitational wave sources.

For documentation on how to use our nuclear neural networks in your science and how to create your own nuclear neural networks, check out this guide.

Mergers of Compact Objects with Cores of Giant Stars

When a star at least few times more massive than the sun and a neutron star coexist in a close binary system, the immense swelling of the star as it evolves into a red supergiant might result in the engulfment of the neutron star that keeps orbiting inside the envelope of its giant companion. During this common envelope evolution phase dynamical friction transfers energy from the orbit of the neutron star to the envelope leading to a substantial decrease in the orbital separation. If the neutron star gets close enough to the core of the red supergiant they could merge, forming a thick accretion disk from the dense core matter. Alternatively, the system could become a double compact object binary that might merge emitting detectable gravitational waves.

The mergers of neutron stars and black holes with cores of giant stars are a fascinating research area due to the variety of astrophysical phenomena they harbor, such as heavy elements formation, high energy neutrinos emission .and luminous transients. For a recent review of these mergers see Grichener 2025

Heavy Elements Formation and Enrichment

One of the main questions I address in my research is the origin of the heaviest elements in the Universe. The formation sites of elements such as gold, mercury and uranium remain an open question in nuclear astrophysics. Light elements like the nitrogen and oxygen that compose our atmosphere are produced through nuclear fusion reactions in stellar interiors and distributed across the Universe by supernova explosions. These reactions release energy in the formation of elements up to iron, but become energetically unfavorable for heavier elements, requiring a different process for their production. The heaviest elements in the Universe are created through a rapid neutron capture process (r-process) that occurs in exotic extremely neutron-rich environments.

I study a scenario in which the heaviest elements are produced in the merger of a neutron star with the core of a giant star. The extreme temperatures and densities in the accretion disk that is formed around the neutron star as it merger with the core of its giant companion can lead protons and electrons to merge into neutrons, which are then captured by iron nuclei producing heavy elements. This occurs on relatively short timescales after star formation, potentially explaining their presence in old stellar populations.

I explored this novel scenario for r-process nucleosynthesis using various numerical methods: I demonstrated its feasibility by performing stellar evolution simulations with the open source code MESA (Grichener & Soker 2019), found it can be a major contributor to europium abundances in the Milky-Way by using a Galactic chemical evolution model (Grichener et al. 2022) and assessed its contribution to observations of heavy elements by carrying out population synthesis simulations using the open-source code COMPAS (Grichener 2023; see publicly avilable datasets here).

High Energy Neutrinos Emission

Over the last decade, IceCube Observatory has been detecting neutrinos with energies at least nine orders of magnitude higher than neutrinos produced in supernova explosions. Such high energy neutrinos are formed in cooling processes of cosmic rays that are accelerated to extremely high energies inside shocks. I showed that relativistic jets launched during common envelope evolution of a giant and a black hole prior to its merger with the core can accelerate cosmic rays to high enough energies to produce these neutrinos through photohadronic interactions (Grichener & Soker 2021). I found the event rate of massive binaries that can emit these neutrinos and concluded they contribute to a substantial fraction of the flux detected by IceCube (Grichener 2023).

The Role of Recombination Energy in Common Envelope Removal

The reason for the ejection of the shared envelope at the end of the common envelop evolution phase is one of the greatest mysteries in binary evolution. One of the main contenders to serve as an explanation for envelope removal is the absorption of hydrogen and helium recombination energy by the envelope gas. As an undergraduate student, I studied the role recombination energy can play in this process and showed that a large part of it is transported outwards by either photon diffusion or convection, and hence it is less likely it could contribute much to the kinetic energy of the outflowing envelope (Grichener et al. 2018; Soker et al. 2018).

Morphologies of Supernova Remnants

The morphologies of supernova remnants might hint at the processes that occurred at the final stages of the supernova explosion and shed light on the explosion mechanism. During my bachelor’s I used analytical methods to explore the morphologies of core collapse supernova remnants and found that many of them exhibit 'ear'-like features that could be produced by jets launched prior and during the explosion (Grichener & Soker 2017; Bear, Grichener & Soker 2017). Recently, I returned to the field of supernova remnants tackling it from another angle by suggesting that the enigmatic supernova remnant W49B originated from a neutron star that tidally disrupted the core of a giant star leading to a thermonuclear outburst, and showed this can explain both the morphology and the distribution of metals in W49B (Grichener & Soker 2023).

Page updated

Google Sites

Report abuse