Do neutron stars form solely through core-collapse supernovae, or is there another way? New research explores theorized processes in which a white dwarf shrinks down to become a neutron star.
Alternate Pathways
Accretion-induced collapse and merger-induced collapse are potential formation pathways for magnetized neutron stars called magnetars, illustrated here. [NASA/Swift/Sonoma State University/A. Simonnet]
So far, researchers have yet to definitively associate any transient signal with the creation of a neutron star from a collapsing white dwarf, though there are several candidates. To advance the search for these signals, researchers must understand the details of the collapse and predict its electromagnetic signature.
Collapsing Further
Eirini Batziou (Max Planck Institute for Astrophysics and Technical University of Munich) and collaborators investigated the burst of nucleosynthesis that might follow a white dwarf’s collapse, potentially powering an electromagnetic transient. The team performed two-dimensional hydrodynamic simulations of six white dwarfs on the cusp of collapsing into neutron stars. Each simulation run varied the mass, central density, rotation rate, and angular momentum profile of the white dwarf. Two of the modeled white dwarfs had masses very close to the Chandrasekhar limit, the mass above which a white dwarf can hypothetically no longer support itself through electron degeneracy pressure. The remaining synthetic stars had higher masses that were bolstered by rapid rotation.
Each of the modeled white dwarfs began to collapse when the crushing pressure of gravity forced the star’s sea of electrons to invade atomic nuclei, removing the support of electron degeneracy pressure. The white dwarfs’ cores shrank and condensed into protoneutron stars, while their outer layers fell inward and rebounded.
Simulation snapshots showing the evolution of ejected material over time. The top row shows a non-rotating white dwarf and the bottom row shows a rapidly rotating white dwarf. The left side of each panel shows the mass density and the right side shows the radial velocity. Note that the times and the values represented by the color bars are different between the two rows. Click to enlarge. [Adapted from Batziou et al. 2025]
Setting the Stage for Element Creation
The simulations show that rotating and non-rotating white dwarfs have different outcomes when they collapse. Non-rotating white dwarfs cast off material in all directions, while rapidly rotating white dwarfs tend to lose material through wide outflows at their poles. The equator of the rapidly rotating white dwarf is ringed by a torus of material that feeds the neutron star as it forms.
These divergent outcomes affect the amount of ejected material and the production of neutrinos. Ultimately, these differences result in opposite behavior: in the non-rotating case, the ejected material is initially rich in neutrons before transitioning to a proton-rich outflow, while the rotating case starts out with proton-rich ejecta that transitions to a neutron-rich outflow. This suggests that elements created through the rapid capture of neutrons, like gold, can form when white dwarfs collapse into neutron stars, potentially powering an observable electromagnetic signal.
These simulations by Batziou and coauthors are the first to extend substantially beyond the moment the outer layers of the collapsing white dwarf bounce off the central protoneutron star, illuminating the ejection of material, production of neutrinos, and nucleosynthesis. Future work will dive deeper into the details of element creation, predict the luminosity of these events, and explore the role of magnetic fields.
Citation
“Nucleosynthesis Conditions in Outflows of White Dwarfs Collapsing to Neutron Stars,” Eirini Batziou et al 2025 ApJ 984 197. doi:10.3847/1538-4357/adc300