Telescopes versus Microscopes: the puzzle of Iron-60

TL;DR

The paper investigates the astrophysical origins of iron-60 in the early solar system, evaluating different scenarios and suggesting that large star-forming clumps near supernovae are the most plausible source.

Contribution

It critically assesses various models for iron-60 origin and proposes that massive star-forming regions at tens of parsecs from supernovae best explain the data.

Findings

AGB star winds and supernova ejecta injection models largely fail to match observed abundances.

Large star-forming clumps near supernovae are the most likely environment for Earth's formation.

Galactic background gamma-ray observations support a higher past star formation rate.

Abstract

The discovery that the short-lived radionucleide iron-60 was present in the oldest meteorites suggests that the formation of the Earth closely followed the death of a massive star. I discuss three astrophysical origins: winds from an AGB star, injection of supernova ejecta into circumstellar disks, and induced star formation on the boundaries of HII regions. I show that the first two fail to match the solar system iron-60 abundance in the vast majority of star forming systems. The cores and pillars on the edges of HII regions are spectacular but rare sites of star formation and larger clumps with masses 1e3-1e4 solar masses at tens of parsec from a supernova are a more likely birth environment for our Sun. I also examine gamma-ray observations of iron-60 decay and show that the Galactic background could account for the low end of the range of meteoritic measurements if the massive star formation rate was at least a factor of 2 higher 4.6 Gyr ago.