Triggered Star Formation inside the Shell of a Wolf-Rayet Bubble as the Origin of the Solar System

Abstract

A critical constraint on solar system formation is the high ²⁶Al/²⁷Al abundance ratio of 5 × 10^-5 at the time of formation, which was about 17 times higher than the average Galactic ratio, while the ⁶⁰Fe/⁵⁶Fe value was about 2 × 10^-8, lower than the Galactic value. This challenges the assumption that a nearby supernova (SN) was responsible for the injection of these short-lived radionuclides into the early solar system. We show that this conundrum can be resolved if the solar system was formed by a triggered star formation at the edge of a Wolf-Rayet (W-R) bubble. ²⁶Al is produced during the evolution of the massive star, released in the wind during the W-R phase, and condenses into dust grains that are seen around W-R stars. The dust grains survive passage through the reverse shock and the low-density shocked wind, reach the dense shell swept-up by the bubble, detach from the decelerated wind, and are injected into the shell. Some portions of this shell subsequently collapse to form the dense cores that give rise to solar-type systems. The subsequent aspherical SN does not inject appreciable amounts of ⁶⁰Fe into the proto-solar system, thus accounting for the observed low abundance of ⁶⁰Fe. We discuss the details of various processes within the model and conclude that it is a viable model that can explain the initial abundances of ²⁶Al and ⁶⁰Fe. We estimate that 1%-16% of all Sun-like stars could have formed in such a setting of triggered star formation in the shell of a W-R bubble.