Did solar energetic particles produce the short-lived nuclides present in the early solar system?

Abstract

Production of the short-lived nuclides ⁴¹Ca, ³⁶Cl, ²⁶Al, and ⁵³Mn by solar energetic particles (SEP) interacting with dust grains of chondritic (=solar) composition is estimated considering a broad range of spectral parameters for the SEP and appropriate nuclear reaction cross sections. The dust grains are assumed to follow a power-law size distribution and to range in size from 10 μm to 1 cm. The possibility that an enhanced flux of SEP from an active early (T Tauri) Sun could have been responsible for the production of these short-lived nuclides in the early solar system is investigated. SEP production of ⁴¹Ca and ³⁶Cl will match their abundances in the early solar system inferred from meteorite data if the SEP irradiation duration was ~ 5 × 10⁵-10⁶ yr and the SEP flux was higher by a factor of more than 5 × 10³ than the contemporary long-term averaged value of Nproton (E 10 MeV) ~100 cm^-2 s^-1. However, corresponding production of ²⁶Al will be much below the level needed to match its inferred abundance in the early solar system. SEP production, therefore, fails to explain the observed correlated presence of ⁴¹Ca and ²⁶Al with canonical initial abundances in early solar system solids. The abundance of ⁵³Mn in the early solar system is not tightly constrained by the meteorite data, and the various estimates differ by a factor of 5. Coproduction of ⁴¹Ca, ³⁶Cl, and ⁵³Mn that will match the meteorite data for the higher initial abundance of ⁵³Mn is possible if the SEP irradiation persisted for about a million years or more with a flux enhancement factor of ~5000-10,000. On the other hand, the lower initial value of ⁵³Mn can also be matched by a flux enhancement factor of ~1000 and an irradiation duration of a few million years; the corresponding production of the other nuclides will be =10% of the level needed to match their abundances in the early solar system. Target abundance consideration rules out the possibility of SEP production of ⁶⁰Fe, another short-lived nuclide present in the early solar system. Thus, SEP production as the primary source of the short-lived nuclides in the early solar system appears to be unlikely. However, the possibility that SEP production could be an important source of ⁵³Mn as well as of the short-lived nuclide ¹⁰Be, whose presence in the early solar system solids has been recently reported, makes it difficult to completely rule out any contribution from this source to the inventory of these nuclides in the early solar system.