Energetic Constraints on In Situ Production of Short-Lived Radionuclei in the Early Solar System

Abstract

We calculate upper limits on the amount of short-lived radionuclei that can be produced by nonthermal nucleosynthesis in the early solar system. Using energetic constraints obtained from X-ray observations of young stellar objects, we show that irradiation of bare solids can produce 10 Be and 41 Ca at levels compatible with a homogeneous distribution over the entire protoplanetary disk up to the comet-forming region. 53 Mn and 36 Cl cannot be produced at canonical levels together with 10 Be and 41 Ca, unless we posit a heterogeneous spatial distribution. The high level of 7 Be suggested recently is barely compatible with a coproduction of 10 Be up to the cometary reservoir and may indicate the irradiation of a gas phase. Finally, we show that the maximum amount of irradiation-induced 26 Al can only account for a homogeneous distribution of this radionuclide over a rocky reservoir of 2-3 M and that the well-defined canonical 26 Al/ 27 Al ratio observed in Ca-Al-rich inclusions is probably not compatible with an in situ production in the embedded phase of the Sun. If extinct 26 Al is detected in the cometary material from the Stardust mission, the nucleosynthetic process that produced this preeminent high-resolution chronometer should be searched for in stellar events contemporary with the birth of the Sun.

26 Al IN THE EARLY SOLAR SYSTEM: NOT SO UNUSUAL AFTER ALL

“…Although there is evidence that there may have been deviations from this "canonical" ratio across the solar protoplanetary disk by as much as a factor of two (Larsen et al 2011;Liu et al 2012), the overall concentration of 26 Al in the solar disk was more than a factor of 10 greater than the current average value in the interstellar medium of 3.0 × 10 −6 (Tang & Dauphas 2012). While some 26 Al may have been produced within the early solar system, most of it was not (Duprat & Tatischeff 2007;Desch et al 2010); there must have been a significant external source of this short-lived nuclide. Commonly, the natal 26 Al is taken as a signature of a nearby supernova that may have triggered the collapse of the molecular cloud from which the Sun formed (Meyer & Clayton 2000;Gritschneder et al 2012).…”

Short-Lived Radionuclides and Early Solar System Chronology

“…Some short-lived nuclides are likely to have formed by spallation reactions with nuclear particles (protons, a-particles) accelerated by interaction with an active young Sun (e.g., Duprat and Tatischeff, 2007;Gounelle et al, 2001;Lee et al, 1998;Leya et al, 2003;Sahijpal and Soni, 2007). Beryllium-7 certainly requires local production by irradiation in the solar system because of its short half-life, but evidence for its presence in CAIs remains tentative (Chaussidon et al, 2006a,b).…”

A refractory inclusion returned by Stardust from comet 81P/Wild 2

“…If they did, the solar system inventory of 26 Al that would have to be accounted for by any mechanism would be much greater than previously considered. This would make it even more difficult to account for the abundance of 26 Al in the early solar system by irradiation only, supporting a nucleosynthetic origin (Duprat and Tatischeff 2007). Furthermore, if Inti had the same initial ratio as CAIs in chondrites, that would imply that either a) Inti formed in a typical CAI formation region in the inner solar nebula; or b) the formation region of the dust in Jupiter family comets had the same initial ratio as the region where CAIs in chondrites formed.…”