Ignition of Weak Interactions and <i>r</i> -process Outflows in Supercollapsar Accretion Disks

Abstract The collapse of rotating massive (∼10 M ⊙ ) 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 M ̇ > M ̇ 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 • ∼ 80−3000 M ⊙ over more than a viscous timescale as they transition through the threshold. There is remarkable agreement between our simulations and the analytic result M ̇ ign ∝ α 5 / 3 M • 4 / 3 for M • ∼ 3−3000 M ⊙ . Simulations and analytic analyses consistently show that the largest BHs leading to r -process nucleosynthesis at ≳ M ̇ ign are ≈3000 M ⊙ , beyond which neutronization ceases, since the disk temperature T ∝ M • − 1 / 6 decreases below the neutron–proton mass difference (≃MeV), suppressing the conversion of protons into neutrons. We show that stellar models of ∼250−10 5 M ⊙ can give rise to BHs of M • ∼ 30−1000 M ⊙ accreting at M ̇ ≳ M ̇ ign , yielding potentially up to ∼10−100 M ⊙ 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