Telescopes versus Microscopes: the puzzle of iron-60

Abstract

The discovery that the short-lived radionucleide ⁶⁰Fe 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 ⁶⁰Fe 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 10^3-4 M_๏ at tens of parsec from a supernova are a more likely birth environment for our Sun. I also examine γ-ray observations of ⁶⁰Fe 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.