2026ApJ...998...57A

The collapse of rotating massive (∼10M_⊙) stars resulting in hyperaccreting black holes (BHs; “collapsars”) is a leading model for the central engines of long-duration gamma-ray bursts (GRBs) and a promising source of rapid neutron-capture (r-process) elements. R-process nucleosynthesis in disk outflows requires the accretion flow to neutronize. This occurs because of Pauli blocking at finite electron degeneracy, associated with a critical accretion rate $\dot{M}\gt {\dot{M}}_{{\rm{ign}}}$. We analytically examine the assumptions underlying this “ignition threshold” and its possible breakdown with increasing BH mass M_•. Employing three-dimensional general-relativistic magnetohydrodynamic simulations with weak interactions, we explore the physical conditions of collapsar accretion disks with M_• ∼ 80-3000M_⊙ over more than a viscous timescale as they transition through the threshold. There is remarkable agreement between our simulations and the analytic result ${\dot{M}}_{{\rm{ign}}}\propto {\alpha }^{5/3}{M}_{\bullet }^{4/3}$ for M_• ∼ 3-3000M_⊙. Simulations and analytic analyses consistently show that the largest BHs leading to r-process nucleosynthesis at $\gtrsim {\dot{M}}_{{\rm{ign}}}$ are ≈3000M_⊙, beyond which neutronization ceases, since the disk temperature $T\propto {M}_{\bullet }^{-1/6}$ decreases below the neutron-proton mass difference (≃MeV), suppressing the conversion of protons into neutrons. We show that stellar models of ∼250-10⁵M_⊙ can give rise to BHs of M_• ∼ 30-1000M_⊙ accreting at $\dot{M}\gtrsim {\dot{M}}_{{\rm{ign}}}$, yielding potentially up to ∼10-100M_⊙ of light and heavy r-process elements per event. These rare but prolific r-process sources in low-metallicity environments are associated with super-kilonovae and likely extremely energetic GRBs. Such signatures may be used to probe Population III stars.