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

TL;DR

The paper investigates the extent of solar energetic particle irradiation in the Sun's protoplanetary disk and aims to bound the flux enhancement factor $Φ$ using both astrophysical observations and meteoritic isotope records. It combines protostellar X-ray luminosities to constrain typical $Φ$ (around $Φ \approx 3\times10^{3}$ at 1 AU) with a detailed transport model for SEPs in the disk that accounts for energy losses and depth-dependent spectra, and it critically evaluates disk-wind exposure. It discusses meteoritic evidence for ${}^{36}{\rm Cl}$, cosmogenic Ne, ${}^{10}{\rm Be}$, and ${}^{7}{\rm Be}$ to test SEP production scenarios, finding Ne cosmogenic consistent with disk irradiation, ${}^{10}{\rm Be}$ largely inherited from the molecular cloud with little in-disk production, and no credible evidence for live ${}^{7}{\rm Be}$. Disk winds are shown to provide negligible SEP exposure for solids, supporting a conclusion that the young Sun's SEP activity was not extraordinary compared with protostars, with $Φ$ remaining below about $10^{4}-10^{5}$ over the first several million years and having significant implications for disk ionization, MRI/outflows, and radionuclide origins.

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 $Φ\approx 3 \times 10^2 - 3 \times 10^3$ times higher than the present-day Sun. Constraining $Φ$ 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 $Φ> 10^5$, even as large as $Φ\approx 6 \times 10^6$, which would make the young Sun extraordinarily active, even for a protostar. We constrain $Φ$ 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 $Φ\approx 3 \times 10^3$ 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.