When Stars Attack! AMS Reveals Near-Earth Supernova Explosions
1University of Illinois Urbana-Champaign, IL, USA
Supernova explosions mark the deaths of the most massive stars, and are central actors in galaxy evolution, engines of particle acceleration, and cosmic forges for many of the elements essential for planets and life. Yet these awesome events take a sinister shade when they occur close to home, because an explosion very nearby would pose a grave threat to Earthlings. We will review how AMS measurements of radioisotopes produced by exploding stars can reveal nearby events in the geologic past, and we will highlight isotopes of interest. In particular, geological evidence for live 60Fe has recently been confirmed globally in multiple sites of deep-ocean material, in cosmic rays, and in lunar samples. We will review astrophysical 60Fe production sites and show that the data demand that at least one core-collapse supernova exploded near the Earth over the past few Myr, and explain how debris from the explosion was transported to the Earth as a “radioactive rain.” Deep-ocean and lunar 60Fe measurements thus represent a laboratory for supernova astrophysics, but also with implications for geology, astrobiology, and possibly terrestrial evolutionary biology–possibly including biological extinction events at the end of the Devonian period 360 Myr ago. Finally, we will discuss very recent detections of 244Pu contemporaneous to the 60Fe. The production site of this heaviest of actinides must be rare, thus these surprising data demand either unusual supernovae, or a near-Earth neutron-star merger of the kind recently seen in both light and gravitational radiation as a “kilonova” explosion. We will show that it is possible to distinguish these possibilities through future high-sensitivity AMS measurements of so-called “r-process” radioisotopes including 129I, 93Zr, 107Pd, 135Cs, 236U, 237Np, and 247Cm. These challenging measurements can open a unique new window on the production of the heavy elements, complementary to meteoritic, astronomical, and gravitational-wave measurements.
Brian Fields is a professor of Astronomy and affiliate faculty in Physics at the University of Illinois, and a Fellow of the American Physical Society. He studies the “inner space/outer space” connections that link the science at the smallest and largest scales. He is particularly interested in the highest-energy sites in nature–the big bang, exploding stars (supernovae), and high-energy particles in space (cosmic rays, neutrinos,and gamma rays)–where all fundamental forces play essential roles. Understanding these processes and their interplay allows us to trace the history of matter over cosmic time, ranging from recent near-Earth supernovae to the evolution of elements in our Galaxy to the primordial nucleosynthesis in the early universe. Turning the problem around, he uses these high-energy phenomena as particle accelerators to probe fundamental physics in ways inaccessible to terrestrial experiment.