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