Planetary Science Seminar fall-2018

A critical constraint on solar system formation is the high 26Al/27Al abundance ratio of 5 x 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 x 10-8, lower than the Galactic value of 3 x 10-7. 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 have been observed around W-R stars in IR observations). 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. The dust grains will be destroyed by grain evaporation or non-thermal sputtering, releasing the 26Al into the shell. Some portions of this shell subsequently collapse due to triggering by shock and ionization fronts, as is frequently observed in wind blown bubbles. This will form the dense cores that give rise to solar-type systems. The W-R star will either collapse directly to a black hole, as in some models, or give rise to a supernova explosion. Even if the latter, the aspherical supernova 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 using numerical simulations, as well as nucleosynthesis modelling, and analytic and semi-analytic calculations. We conclude that it is a viable model that can explain the initial abundances of 26Al and 60Fe, as well as other short-lived radionuclides. 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.