Neutrino absorption and other physics dependencies in neutrino-cooled black hole accretion discs

Black hole (BH) accretion discs formed in compact-object mergers or collapsars may be major sites of the rapid-neutron-capture (r-)process, but the conditions determining the electron fraction (Y-e) remain uncertain given the complexity of neutrino transfer and angular-momentum transport. After discussing relevant weak-interaction regimes, we study the role of neutrino absorption for shaping Y-e using an extensive set of simulations performed with two-moment neutrino transport and again without neutrino absorption. We vary the torus mass, BH mass and spin, and examine the impact of rest-mass and weak-magnetism corrections in the neutrino rates. We also test the dependence on the angular-momentum transport treatment by comparing axisymmetric models using the standard alpha-viscosity with viscous models assuming constant viscous length-scales (l(t)) and 3D magnetohydrodynamic (MHD) simulations. Finally, we discuss the nucleosynthesis yields and basic kilonova properties. We find that absorption pushes Y-e towards similar to 0.5 outside the torus, while inside increasing the equilibrium value Y-e(eq) by similar to 0.05-0.2. Correspondingly, a substantial ejecta fraction is pushed above Y-e = 0.25, leading to a reduced lanthanide fraction and a brighter, earlier, and bluer kilonova than without absorption. More compact tori with higher neutrino optical depth, tau, tend to have lower Y-e(eq) up to tau similar to 1-10, above which absorption becomes strong enough to reverse this trend. Disc ejecta are less (more) neutron rich when employing an l(t) = const. viscosity (MUD treatment). The solar-like abundance pattern found for our MILD model marginally supports collapsar discs as major r-process sites, although a strong r-process may be limited to phases of high mass-infall rates, M greater than or similar to 2 x 10(-2) M-circle dot s(-1).