Gravitational Instability and Fragmentation in Collapsar Disks Supports the Formation of Subsolar Neutron Stars

Abstract 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 and optically thin neutrino cooling and assume an electron fraction set by the balance of e ± 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 τ cool ≡ t cool Ω ≲ 10, where Ω is the orbital frequency; these conditions correspond to accretion rates ≳ M ⊙ s −1 on the upper end of those achieved by collapsar progenitor stars. Fragmentation leads to the formation of neutron-rich clumps (electron fraction Y e ≲ 0.1) spanning a range of masses ∼0.01–1 M ⊙ around the local Jeans value. Most clumps exceed the local Chandrasekhar mass <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Ch</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>Y</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and hence will continue to collapse to nuclear densities, forming neutron stars (NSs) with subsolar masses otherwise challenging to create through ordinary stellar core collapse. Even cool disks dominated by α particles ( Y e ≃ 0.5) can fragment and collapse into neutron-rich clumps capable of forming subsolar NSs. Although our simulations cannot follow this process directly, if the disk-formed NSs subsequently pair into binaries, the GW 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 multimessenger “symphony.”