The distinctive products of supernova explosions have been detected passing through the Solar System, indicating the presence of recent supernova in our vicinity.
Iron-60 (60Fe) is produced in supernovae. Having a half-life of 2.6 million years, any predating the Solar System's source have long since decayed. But recently, evidence emerged of two pulses of 60Fe at the bottom of the ocean, one 8 million years ago, and another lasting from 3.2 to 1.7 million years ago.
A paper in Science reports that NASA's Cosmic Ray Isotope Spectrometer (CRIS) has recorded 15 nuclei of 60Fe. Taken over the 17 years CRIS has been in space, and the 300,000 nuclei it has collected originating outside the Solar System, the number is tiny, but important.
"Our detection of radioactive cosmic-ray iron nuclei is a smoking gun indicating that there has been a supernova in the last few million years in our neighborhood of the galaxy," Professor Robert Binns of Washington University, St Louis said in a statement.
This can't just be a case of 60Fe being produced millions of years ago and floating around the galaxy until CRIS encountered it. Cosmic rays, fast traveling particles from outer space, need to be traveling fast to enter the Solar System against the pressure of the solar wind.
The supernova explosion that forms 60Fe, along with other heavy metal isotopes, was once thought to provide the energy for that speed. However, a previous study Binns co-authored calls this into question. 59Ni and 59Co, another two isotopes formed in supernovae have also been detected by CRIS. 59Ni is radioactive, but 59Co is stable, so the longer the delay between their formation and arrival, the more skewed the ratio.
Based on the numbers detected, Binns concluded that there was a gap of at least 100,000 years between the creation of these nuclei, and their acceleration to speeds at which they could enter the Solar System. Binn's co-author Professor Martin Israel described this as evidence that supernovae produce superbubbles of ejected material which then get accelerated by the shockwaves from one or more subsequent supernova explosions.
Combining the evidence from the 60Fe and the 59Ni/59Co Binns and Israel concluded that the nuclei we are detecting formed in one supernova and were then accelerated by the explosion of a nearby star between 100,000 and a few million years thereafter.
Two supernovae so close in distance and time would be surprising if randomly distributed through the galaxy. However, star-forming nebulae often form numerous giant stars in close proximity that explode within a short (by astronomical standards) time of each other.
The more recent burst of 60Fe deposition is thought to be the product of multiple supernovae from just such a cluster.
The paper estimates these explosions must have been within 2,000 light-years of the Earth. More than 20 associations of stars thought to have formed in such clusters before drifting apart are known. So far we lack enough information to identify which association was responsible for the material we are now detecting.