Who Pulled the Trigger: A Supernova or an Asymptotic Giant Branch Star?

Abstract

The short-lived radioisotope (SLRI) 60 Fe requires production in a core collapse supernova or asymptotic giant branch (AGB) star immediately before its incorporation into the earliest solar system solids. Shock waves from a somewhat distant supernova, or a relatively nearby AGB star, have the right speeds to simultaneously trigger the collapse of a dense molecular cloud core and to inject shock wave material into the resulting protostar. A new set of FLASH2.5 adaptive mesh refinement hydrodynamic models shows that the injection efficiency depends sensitively on the assumed shock thickness and density. Supernova shock waves appear to be thin enough to inject the amount of shock wave material necessary to match the SLRI abundances measured for primitive meteorites. Planetary nebula shock waves from AGB stars, however, appear to be too thick to achieve the required injection efficiencies. These models imply that a supernova pulled the trigger that led to the formation of our solar system.

The abundance of26Al-rich planetary systems in the Galaxy

“…Until now, the dominant model accounting for the presence of 26 Al in the nascent solar system has been that of the nearby supernova (Cameron & Truran 1977). This model stipulates that having reached the end of its evolution and exploding as a supernova, a massive star delivered 26 Al directly into the SPD (Ouellette et al 2010) or alternatively into the presolar dense core (Boss & Keiser 2010). All SN models consider that supernova progenitors had masses varying between 20 and 60 M (Boss & Keiser 2010;Ouellette et al 2010).…”

“…This model stipulates that having reached the end of its evolution and exploding as a supernova, a massive star delivered 26 Al directly into the SPD (Ouellette et al 2010) or alternatively into the presolar dense core (Boss & Keiser 2010). All SN models consider that supernova progenitors had masses varying between 20 and 60 M (Boss & Keiser 2010;Ouellette et al 2010). The rationale for excluding low-mass supernovae is that stars with masses lower than 20 M have lifetimes that are longer than (or comparable to) those of molecular clouds, so they explode when the starforming region where they were born has disappeared.…”

“…At such a distance, a protoplanetary disk of size r = 100 AU would receive a mass of 26 −12 M /M , three orders of magnitude lower than the solar system value. Because the mixing efficiency is by definition lower than 1 (and possibly as low as a few %, Boss & Keiser 2010), and because 26 Al decays relatively fast (compared to evolutionary timescales of star formation), the discrepancy between the SN-injected 26 Al concentration in a nearby disk with the solar system initial abundance is even larger.…”

Cosmochemical evidence for astrophysical processes during the formation of our solar system

“…Note that there is a critical difference between the newer "triggered star formation model" of Hester and Desch (12) versus the one proposed by Cameron and Truran (17) and later investigated by Boss and Keiser (18). In the Hester and Desch scenario (12), it is not the supernova shockwave itself that triggers the ongoing star formation.…”

“…3 shows a modern numerical simulation of this process; ref. 18). This proposal was countered by an alternative idea (19) that local proton irradiation near our own infant Sun produced the 26 Al, removing the need for any external source.…”