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 .