Astrophysics with Radioactive Isotopes

Accounting for Short-Lived Radionuclides in the Early Solar System in the Context of a Triggered Star Formation Origin of the Solar System

Not scheduled

20m

Oral Presentation

Speaker

Vikram Dwarkadas

Description

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 lower than the Galactic value of 3 $\times 10^{-7}$. 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 $^{26}$Al into the shell. Some portions of this shell subsequently collapses to form the dense cores that give rise to solar-type systems. The 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 $^{60}$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. Besides $^{26}$Al and $^{60}$Fe, many other short-lived radionuclides (SLRs) were present in the ESS, including $^{10}$Be, $^{36}$Cl, $^{41}$Ca, $^{53}$Mn, $^{107}$Pd, $^{129}$I, and $^{182}$Hf. We further investigate whether the triggered star formation model can account for the abundance of these other SLRs, and show that it can adequately explain the abundances of most short-lived radionuclides in the early solar system.