Radionuclide Ionization in Protoplanetary Disks: Calculations of Decay Product Radiative Transfer

TL;DR

This paper derives simple analytic solutions for ionization rates from short-lived radionuclide decay in protoplanetary disks, accounting for radiative transfer and disk properties, highlighting the significance of SLRs compared to cosmic rays.

Contribution

It introduces analytic models for SLR-induced ionization in disks, including radiative transfer effects and simple expressions for different disk geometries.

Findings

SLR ionization rates are typically (1-10)×10^{-19} s^{-1}.

Ionization rates depend on disk surface density and evolve over time.

Radiative transfer effects become significant at large disk radii.

Abstract

We present simple analytic solutions for the ionization rate $\zeta_{\rm{SLR}}$ arising from the decay of short-lived radionuclides (SLRs) within protoplanetary disks. We solve the radiative transfer problem for the decay products within the disk, and thereby allow for the loss of radiation at low disk surface densities; energy loss becomes important outside $R\gtrsim30$ for typical disk masses $M_g=0.04$ M$_\odot$. Previous studies of chemistry/physics in these disks have neglected the impact of ionization by SLRs, and often consider only cosmic rays (CRs), because of the high CR-rate present in the ISM. However, recent work suggests that the flux of CRs present in the circumstellar environment could be substantially reduced by relatively modest stellar winds, resulting in severely modulated CR ionization rates, $\zeta_{\rm{CR}}$, equal to or substantially below that of SLRs ($\zeta_{\rm{SLR}}\lesssim10^{-18}$ s$^{-1}$). We compute the net ionizing particle fluxes and corresponding ionization rates as a function of position within the disk for a variety of disk models. The resulting expressions are especially simple for the case of vertically gaussian disks (frequently assumed in the literature). Finally, we provide a power-law fit to the ionization rate in the midplane as a function of gas disk surface density and time. Depending on location in the disk, the ionization rates by SLRs are typically in the range $\zeta_{\rm{SLR}}\sim(1-10)\times10^{-19}$ s$^{-1}$.