Collapsars Are Not Heavy r-Process Factories
Abstract
Collapsar disks are proposed sources of r-process elements, but neutron-rich disk ejecta require gas to be both neutronized in the disk and ejected before accretion. The magnetic field that enables ejection also thickens the disk and accelerates the inflow, lowering both the density and the time available for weak processing. To compute the resulting neutronization, we evolve the electron fraction of the inflowing gas, adopting at each radius the temperature that maximizes the net neutronization rate for the local density and instantaneous composition. This yields a lower limit on Ye that is independent of the disk's thermal history. We adopt an optimistic unbinding radius of Runb=10 rg for all magnetic fluxes capable of launching outflows. Combining this ejection condition with favorable assumptions of low black hole mass, ejection of midplane gas, and the upper end of the γ-ray burst (GRB) engine-power scale, which limits the accretion rate to ṀBH≲0.4 M⊙ s−1, gives a fiducial lower bound Ye≳0.33, above the characteristic Ye≈ 0.25 threshold for heavy r-process production. Stronger magnetic flux, toward which jet-launching engines evolve, raises the lower bound to Ye≳0.49. We conclude that collapsar disk outflows cannot supply the heavy r-process yields invoked for a dominant enrichment channel. Only engines with unusually high accretion rates, potentially including exceptionally luminous GRBs, such as GRB 221009A, and direct-horizon collapsars, may produce heavy r-process ejecta.
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BibTeX
@misc{gottlieb2026collapsars,
title = {{Collapsars Are Not Heavy $r$-Process Factories}},
author = {Ore Gottlieb and Danat Issa and Jonatan Jacquemin-Ide},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.31758},
eprint = {2610.31758},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.31758},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Collapsars Are Not Heavy r-Process Factories
AU - Ore Gottlieb
AU - Danat Issa
AU - Jonatan Jacquemin-Ide
PY - 2026
DA - 2026/10/22
PB - arXiv
DO - 10.48550/arxiv.2610.31758
UR - https://arxiv.org/abs/2610.31758
AB - Collapsar disks are proposed sources of r-process elements, but neutron-rich disk ejecta require gas to be both neutronized in the disk and ejected before accretion. The magnetic field that enables ejection also thickens the disk and accelerates the inflow, lowering both the density and the time available for weak processing. To compute the resulting neutronization, we evolve the electron fraction of the inflowing gas, adopting at each radius the temperature that maximizes the net neutronization rate for the local density and instantaneous composition. This yields a lower limit on Y_e that is independent of the disk's thermal history. We adopt an optimistic unbinding radius of R_unb=10 r_g for all magnetic fluxes capable of launching outflows. Combining this ejection condition with favorable assumptions of low black hole mass, ejection of midplane gas, and the upper end of the γ-ray burst (GRB) engine-power scale, which limits the accretion rate to Ṁ_BH≲0.4 M_⊙ s^−1, gives a fiducial lower bound Y_e≳0.33, above the characteristic Y_e≈ 0.25 threshold for heavy r-process production. Stronger magnetic flux, toward which jet-launching engines evolve, raises the lower bound to Y_e≳0.49. We conclude that collapsar disk outflows cannot supply the heavy r-process yields invoked for a dominant enrichment channel. Only engines with unusually high accretion rates, potentially including exceptionally luminous GRBs, such as GRB 221009A, and direct-horizon collapsars, may produce heavy r-process ejecta.
ER -