Survival of ultraheavy nuclei in astrophysical sources: applications to protomagnetar outflows

TL;DR

This paper investigates whether ultraheavy nuclei can survive photodisintegration in protomagnetar outflows, providing new formulas for cross sections and analyzing survival conditions in different outflow models and progenitor environments.

Contribution

It introduces new analytic formulas for heavy nuclei photodisintegration cross sections and applies them to assess nuclei survival in protomagnetar outflows.

Findings

Nuclei may survive in spherical winds for ~100s post-collapse.

Survival depends on outflow Lorentz factor and engine properties.

Progenitor envelope size affects jet breakout time and nuclei survival.

Abstract

Outflows of rapidly rotating protomagnetars have been considered as attractive sites for the synthesis of nuclei heavier than iron, but the question remains whether these nuclei are able to survive against photodisintegration as they make their way out of their formation environments. In this work, we present new analytic fitting formulae for the photodisintegration cross sections applicable to heavy nuclei beyond iron. We confirm that the results from the TALYS simulations are consistent with the theory of the giant dipole resonance, and apply the obtained new formulae to investigate whether ultraheavy nuclei entrained in protomagnetar outflows can be disintegrated by thermal and nonthermal photons before leaving the stellar envelope. We explore two outflow models: a spherical wind model and a jetted outflow model. For nuclei accelerated to the bulk speed of these outflows, their survival depends on the model and engine properties. For spherical winds, nuclei may survive for the first $\sim100\,{\rm s}$ post-core collapse, but as the wind Lorentz factor increases, the photodisintegration optical depth sharply rises and nuclei may no longer survive. For the jetted outflows arising from progenitors surrounded with stellar envelopes, nuclei can only survive before the jet breakout time in cases where the central engine has low spin-down energy, that is, with a low magnetic field strength and longer spin period. In progenitors with more extended envelopes, the jet break out time is much longer, allowing for nonthermal photons to readily photodisintegrate nuclei in high spin-down energy cases. These results also have implications for the capabilities of protomagnetars to source ultra-high energy cosmic ray nuclei.