A Stellar Whodunit: Search for a Near-Earth Supernova

Title: Origin of 60Fe nuclei in cosmic rays: the contribution of local OB associations

Authors: Nicolas de Séréville, Vincent Tatischeff, Pierre Cristofari, Stefano Gabici, Roland Diehl

First Author’s Institution: Université Paris-Saclay, France

Status: arXiv only

Most of the time, when astronomers study supernovae, they’re looking for electromagnetic light, neutrinos or perhaps even gravitational waves. However, another way we can improve our understanding of stellar death is to search directly for supernova isotopes, or more simply, specific elements that are produced only or mainly in supernovae. Using this exact approach, the authors of today’s paper have attempted to find the explosive culprit behind recently detected supernova material. 

Artist’s impression of a supernova. (Wikimedia Commons)

The smoking gun behind this cosmic mystery is Iron-60, or 60Fe, a supernova isotope that is predominantly synthesised during the deaths of massive stars, known as core-collapse supernovae. When a core-collapse supernova occurs, the Iron-60 is ejected, allowing it to be detected by astronomers using a variety of different methods in a slew of exotic locations, including in deep sea sediments, in Antarctic snow, or even in lunar material that was brought back to Earth by the Apollo missions!

However, the authors of today’s paper have instead relied on data from the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA’s Advanced Composition Explorer (ACE) in their Poirot-esque investigation. The mission detected 15 Iron-60 nuclei during its 17 years of operation, hinting at a nearby supernova source.

The Smoking (Radioactive) Gun

What makes Iron-60 particularly special is that it is radioactive. More simply, Iron-60 decays by releasing excess energy in the form of radioactive particles. Therefore, if we’re detecting Iron-60, it must have been produced fairly recently (in cosmic terms), otherwise it would have decayed to a different element. 

What do the authors mean by recently? Well, Iron-60 has a half-life of 3.8 million years, meaning every 3.8 million years, half of the material will decay away. This implies that if we’re seeing Iron-60, it must have been produced not so long ago, in the last 15 million years or so. A mere blip in the 13.7 billion-year history of the universe!

Therefore, the detection of Iron-60 with CRIS indicates that a supernova is likely to have occurred somewhat recently and relatively close to Earth. Since it takes time for the supernova material to reach the detector, if the supernova was too far away, it would all have decayed away before it reached us. 

The Stellar Suspects

Once it was clear that a supernova occurred, the real whodunit began. The authors first assembled the list of suspects. Among them, 25 OB associations, or groups of young, massive stars, located within 1 kpc of the solar system, which are shown in Figure 1. Next, the authors constructed a model to describe the amount of Iron-60 that would arrive at Earth given a set of parameters. 

Figure 1. The OB associations considered in the study. Figure 3 in the paper. The size of each OB association indicates the number of stars that could explode via the core-collapse mechanism, while the colour indicates the age of the association. The location of the Sun is shown by the blue star, and the red arrow points towards the centre of the Galaxy.

Key parameters included the ejecta or material yields from the stars and supernovae in the OB associations, the acceleration of supernova isotopes from their production site, and finally the transport of supernova material across the Milky Way to the Solar System. In addition, the authors had to take into account the explodibility criterion, which indicates how likely it is that a massive star will end its life as a supernova.

To get to the bottom of things, the authors applied their model 4000 times, accounting for each of the 25 OB associations in their sample. This allowed them to determine the most likely culprit behind the source of the Iron-60 detected with CRIS.

Figure 2. How frequently each OB association contributed the most to the Iron-60 quantity in the solar system, with the blue and the red histograms using slightly different Iron-60 metrics. The inset shows the percentage by which certain OB associations dominated for the 4000 realisations of the model. Figure 7 in the paper.

Their results are shown in Figure 2, which shows how often each OB association was the highest contributor to the number of Iron-60 isotopes detected in the Solar System. The blue and red histograms use different Iron-60 quantities based on the region in which the Iron-60 density was calculated, however, for either case, it is clear that the Scorpius-Centaurus, or Sco-Cen, association is a key player. 

Evidently, Sco-Cen is the prime suspect, but the smoking gun is shown in the inset of Figure 2, which gives the fraction of the Iron-60 that is attributed to a subsample of the most contributing associations. Noting that the distributions from the other associations are multiplied by 5, it is clear that when Sco-Cen is the most contributing source, it dominates over the other associations by a large amount, more than 80% in two-thirds of the realisations of their model.

And thus the culprit has been revealed! Alibis have been tested and evidence has been gathered, and at the centre of this stellar mystery appears to be the Sco-Cen association. All that’s left is for a guilty verdict to be issued as more evidence comes in. 

Astrobite edited by Cole Meldorf

Featured image credit: ESO

About Sonja Panjkov

I'm a second-year PhD student at the University of Melbourne. My research focuses on the high-energy emission from the supernova remnants in the Magellanic Clouds. In my spare time, I enjoy hanging out with my cats and going to see live music.

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