The Extent of Solar Energetic Particle Irradiation in the Sun's Protoplanetary Disk

TL;DR

This paper re-evaluates the flux of solar energetic particles in the early solar system, concluding it was much lower than some previous estimates, aligning with typical protostellar activity levels.

Contribution

It provides a comprehensive re-analysis of meteoritic data to constrain the early Sun's SEP flux, challenging prior claims of extraordinarily high activity.

Findings

Cosmogenic Ne was produced in the disk, not in the molecular cloud.

${}^{36}{ m Cl}$ was created after disk dissipation.

${}^{10}{ m Be}$ was inherited, not produced in the disk.

Abstract

Solar flares emit X rays and high-energy (MeV-GeV) ions (Solar Energetic Particles, or SEPs). Astronomical observations show solar mass-protostellar fluxes are a factor $\Phi \approx 3 \times 10^2 - 3 \times 10^3$ times higher than the present-day Sun. Constraining $\Phi$ in the early solar system is important for modeling ionization in the Sun's protoplanetary disk, the extent of magnetorotational instability or magnetocentrifugal outflows, or even production of short-lived radionuclides. Recent interpretations of meteoritic data -- cosmogenic Ne in hibonite grains, initial $({}^{10}{\rm Be}/{}^{9}{\rm Be})_0$ ratios in Ca-rich, Al-rich inclusions (CAIs), or even inferences of live ${}^{7}{\rm Be}$ in CAIs -- have suggested values $\Phi > 10^5$, even as large as $\Phi \approx 6 \times 10^6$, which would make the young Sun extraordinarily active, even for a protostar. We constrain $\Phi$ by re-examining these data. We conclude: cosmogenic Ne was produced in hibonite grains as they resided in the disk; ${}^{36}{\rm Cl}$ was created in Cl-poor grains after the disk dissipated; ${}^{10}{\rm Be}$ was inherited from the molecular cloud, with almost no ($< 1\%$) ${}^{10}{\rm Be}$ created in the disk; and there is no evidence whatsoever for any live ${}^{7}{\rm Be}$ in CAIs. We show these data are consistent with a value $\Phi \approx 3 \times 10^3$ for the first $> 5$ Myr of the solar nebula. The early Sun evidently emitted a flux of X rays and SEPs not atypical for a protostar.