Nucleosynthesis of26Al in the early solar system and in cosmic rays

The nucleus²⁶Al(^1/2 = 7.4 × 10⁵ yr) has long been considered as a possible heat source during the formation of the solar system. Recent experimental work has found no evidence for²⁶Al at the time of final solidification of the meteorites. Due to the short half-life these measurements do not rule out the possibility that²⁶Al was a significant heat source a few million years prior to final solidification. For²⁶Al to be an effective heat source in the early solar system it is necessary for the ratio²⁶Al/Si to be 2×10^–7 at the time of a solidification. The nucleosynthetic yields of²⁶Al by silicon burning, carbon burning, and spallation are discussed. It is shown that²⁶Al can be synthesized in carbon and/or silicon-burning supernovae. However, time scales in the early solar system make it more likely that²⁶Al, if present in planets, was synthesized by a proton irradiation in the early solar system. An integrated proton flux >4×10¹⁸ cm^–2 is shown to be necessary in order for²⁶Al to be a significant heat source. No conclusive evidence has been observed for an irradiation of this magnitude. Therefore, unless such evidence is found, it should be assumed that²⁶Alwas not involved in the formation of the solar system. In addition, the production of²⁶Al in cosmic rays is discussed and it is shown that either resolution of the Al isotopes in the cosmic rays or accurate measurements of the variation with energy of the cosmic ray abundance ratios Al/Si and Mg/Si can be used to determine the age of the cosmic rays, _C.R.. Current abundance data are extremely uncertain; however, the trend tends toward values of _C.R. 10⁶ yr.Supported in part by the National Science Foundation GP-19887, GP-28027, GP-27304].