Triggered Star Formation Inside the Shell of a Wolf-Rayet Bubble as the origin of the Solar System

TL;DR

This paper proposes a model where the early solar system formed from triggered star formation at the edge of a Wolf-Rayet bubble, explaining the observed radionuclide abundances without requiring a nearby supernova.

Contribution

It introduces a novel scenario involving Wolf-Rayet star winds and dust grain injection to account for radionuclide levels in the early solar system.

Findings

The model explains the high $^{26}$Al/$^{27}$Al ratio observed.

It accounts for the low $^{60}$Fe/$^{56}$Fe ratio in the solar system.

Estimated 1-16% of Sun-like stars could form via this mechanism.

Abstract

A critical constraint on solar system formation is the high $^{26}$Al/$^{27}$Al abundance ratio of 5 $\times 10^{-5}$ at the time of formation, which was about 17 times higher than the average Galactic ratio, while the $^{60}$Fe/$^{56}$Fe value was about $2 \times 10^{-8}$, lower than the Galactic value. This challenges the assumption that a nearby supernova 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 triggered star formation at the edge of a Wolf-Rayet (W-R) bubble. Aluminium-26 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 collapses to form the dense cores that give rise to solar-type systems. The subsequent aspherical supernova does not inject appreciable amounts of $^{60}$Fe into the proto-solar-system, thus accounting for the observed low abundance of $^{60}$Fe. 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 $^{26}$Al and $^{60}$Fe. 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 WR bubble.