Gravitational Instability and Fragmentation in Collapsar Disks Supports the Formation of Sub-Solar Neutron Stars
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| Format: | Preprint |
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2025
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| author | Chen, Yi-Xian Metzger, Brian D. |
| author_facet | Chen, Yi-Xian Metzger, Brian D. |
| contents | We perform three-dimensional shearing-box hydrodynamical simulations to explore the outcome of gravitational instability in the outer regions of neutrino-cooled disks such as those formed from the collapse of rotating massive stars ("collapsars''). We employ a physical equation of state, optically-thin neutrino cooling, and assume an electron fraction set by the balance of electron/positron pair-capture reactions. Disks in a marginally stable initial state (Toomre parameter Q~ 1) undergo runaway cooling and fragmentation when the dimensionless cooling timescale obeys tau_cool = t_cool*Omega < 10, where Omega is the orbital frequency; these conditions correspond to accretion rates > Msun/s on the upper end of those achieved by collapsar progenitor stars. Fragmentation leads to the formation of neutron-rich clumps (electron fraction Ye ~ 0.1) spanning a range of masses ~0.01-1 Msun around the local Jeans value. Most clumps exceed the local Chandrasekhar mass M_Ch ~ Ye^2 and hence will continue to collapse to nuclear densities, forming neutron stars (NS) with sub-solar masses otherwise challenging to create through ordinary stellar core-collapse. Even cool disks dominated by alpha-particles (Ye ~ 0.5) can fragment and collapse into neutron-rich clumps capable of forming sub-solar NSs. Although our simulations cannot follow this process directly, if the disk-formed NSs subsequently pair into binaries, the gravitational wave chirps from their rapid mergers are potentially detectable by ground based observatories. The temporal coincidence of such a hierarchical NS merger chain with the collapsar gamma-ray burst and supernova would offer a uniquely spectacular multi-messenger "symphony''. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2508_17183 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Gravitational Instability and Fragmentation in Collapsar Disks Supports the Formation of Sub-Solar Neutron Stars Chen, Yi-Xian Metzger, Brian D. High Energy Astrophysical Phenomena We perform three-dimensional shearing-box hydrodynamical simulations to explore the outcome of gravitational instability in the outer regions of neutrino-cooled disks such as those formed from the collapse of rotating massive stars ("collapsars''). We employ a physical equation of state, optically-thin neutrino cooling, and assume an electron fraction set by the balance of electron/positron pair-capture reactions. Disks in a marginally stable initial state (Toomre parameter Q~ 1) undergo runaway cooling and fragmentation when the dimensionless cooling timescale obeys tau_cool = t_cool*Omega < 10, where Omega is the orbital frequency; these conditions correspond to accretion rates > Msun/s on the upper end of those achieved by collapsar progenitor stars. Fragmentation leads to the formation of neutron-rich clumps (electron fraction Ye ~ 0.1) spanning a range of masses ~0.01-1 Msun around the local Jeans value. Most clumps exceed the local Chandrasekhar mass M_Ch ~ Ye^2 and hence will continue to collapse to nuclear densities, forming neutron stars (NS) with sub-solar masses otherwise challenging to create through ordinary stellar core-collapse. Even cool disks dominated by alpha-particles (Ye ~ 0.5) can fragment and collapse into neutron-rich clumps capable of forming sub-solar NSs. Although our simulations cannot follow this process directly, if the disk-formed NSs subsequently pair into binaries, the gravitational wave chirps from their rapid mergers are potentially detectable by ground based observatories. The temporal coincidence of such a hierarchical NS merger chain with the collapsar gamma-ray burst and supernova would offer a uniquely spectacular multi-messenger "symphony''. |
| title | Gravitational Instability and Fragmentation in Collapsar Disks Supports the Formation of Sub-Solar Neutron Stars |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2508.17183 |