Fast Neutrino Conversion in Hydrodynamic Simulations of Neutrino-Cooled Accretion Disks

Abstract

The outflows from neutrino-cooled black-hole (BH) accretion disks formed in neutron-star mergers or cores of collapsing stars are expected to be neutron-rich enough to explain a large fraction of elements created by the rapid neutron-capture (r-) process, but their precise chemical composition remains elusive. Here, we investigate the role of fast neutrino flavor conversion, motivated by the findings of our post-processing analysis that shows evidence of electron-neutrino lepton-number (ELN) crossings deep inside the disk, hence suggesting possibly non-trivial effects due to neutrino flavor mixing. We implement a parametric, dynamically self-consistent treatment of fast conversion in time-dependent simulations and examine the impact on the disk and its outflows. By activating the, otherwise inefficient, emission of heavy-lepton neutrinos, fast conversions enhance the disk cooling rates and reduce the absorption rates of electron-type neutrinos, causing a reduction of the electron fraction in the disk by 0.03 − 0.06 and in the ejected material by 0.01 − 0.03. The r-process yields are enhanced by typically no more than a factor of two, rendering the overall impact of fast conversions modest. The kilonova is prolonged as a net result of increased lanthanide opacities and enhanced radioactive heating rates. We observe only mild sensitivity to the disk mass, the condition for the onset of flavor conversion, and to the considered cases of flavor mixing. Remarkably, parametric models of flavor mixing that conserve the lepton numbers per family result in an overall smaller impact than models invoking three-flavor equipartition, often assumed in previous works.

Comprehensive study of mass ejection and nucleosynthesis in binary neutron star mergers leaving short-lived massive neutron stars

“…A word of caution is appropriate here, because in the present study, we do not take into account the effects of the neutrino oscillation. The latest numerical simulations (e.g., Li &Siegel 2021 andJust et al 2022a) indicate that the value of Y e for the post-merger ejecta may be decreased by up to 0.03. In such a case, the production of the heavy r-process elements are enhanced while that of the lighter one is reduced.…”

Evaluating Approximate Flavor Instability Metrics in Neutron Star Mergers

“…Extrapolating beyond the calculation domain, collisional processes are very weak due to low densities. Trajectories that have equal numbers of neutrinos and antineutrinos will remain so, implying that crossings should remain present at larger distances, in agreement with [15][16][17]. In following subsections, we will try to reproduce these results with a variety of momentbased tests.…”

“…It thus seems unlikely that polynomial crossing tests will be able to detect the full range of crossings in neutron star merger simulations. However, [16,17] already show that the FFI is present in a large fraction of the domain during simulations that dynamically include FFI-inspired flavor mixing. In addition, we note that all of the polynomial tests encompass the region where I + ≈ I − in which significant flavor transformation is possible, so the inability of the polynomial method to detect crossings at large radii does not necessarily make it any less realistic.…”

“…In CCSNe and neutron star mergers (NSMs), neutrinos can be sufficiently dense that neutrino-neutrino interactions significantly modify flavor transformations in a nonlinear manner that has yielded a rich phenomenology, including collective oscillations [7], the neutrino halo effect [8], and the matter-neutrino resonance [9]. More recently, the neutrino fast flavor instability (FFI) [10,11] was shown to occur nearly ubiquitously in CC-SNe ( [12,13] and references therein) and NSMs [14][15][16][17] in a way that can significantly modify the properties of ejected matter that enrich the universe with heavy elements. The FFI can occur in regions inaccessible to * srichers@berkeley.edu other flavor transformation mechanisms, but the short timescales and lengthscales associated with the FFI preclude a direct treatment in global simulations.…”

“…The k 0 and polynomial tests have been dynamically incorporated into global simulations of NSMs [16,17], and similar simulations of CCSNe are likely underway. The ability of some of the tests to accurately predict in-stability has been tested in the context of simulations of one-dimensional (spherically symmetric) CCSNe with Newtonian gravity.…”