Heavy elements created by crashing neutron stars
These mighty stellar collisions do not appear to be the only source of these elements, though
W e are stardust. Most of the atoms in our bodies come from previous generations of stars, without whose presence none of us could exist, which is not to say that we understand the details. Astronomers are currently especially busy arguing over a site for the production of what are known as ‘r-process’ elements. These are elements such as barium and cerium, heavier than iron, and whose creation requires the rapid addition of neutrons – as many as 100 per second – to atomic nuclei. The conditions needed to create such a process require huge amounts of energy that only occur during some of the Galaxy’s most dramatic events.
On 17 August 2017 the LIGO gravitational wave detector found a merger between two neutron stars, an event spotted and studied by more than 70 different telescopes, and we hoped it might give us the information needed to track down the source.
By observing such events, astronomers hope to work out what elements are produced where. Most of the light we see in a supernova, for example, is not from the explosion directly, but rather produced by the decay of unstable, heavy nuclei produced by the process. Combining observation with a bit of theory, the goal is to try to match the mix of heavy elements we see in the Universe around us.
The research paper I’m spotlighting this month is an attempt to take stock of where we are, especially now we’ve seen a neutron star merger. That single event – a kilonova – produced maybe as much as four per cent of the Sun’s mass just in heavy, r-process elements. Its occurrence soon after the gravitational wave detector switched on also means that such events are most likely pretty common, though we need to observe for a while longer before we can be sure of that.
Even assuming that these things happen often, the paper’s authors come to a surprising conclusion. Kilonovae just don’t work as the only source of heavy elements. In particular, the authors study the presence of the otherwise obscure metal europium as a subtle test of what’s going on; throughout the Milky Way disc, where we see more iron, we measure a lower ratio of europium to iron, though a simple model using only kilonovae to produce r-process elements suggests that this ratio should be constant.
In other words, something is missing from the model. It’s possible we don’t properly understand kilonovae – after all, we’ve only seen one – and new observations will certainly help. But that one event certainly seemed to behave as expected. The alternative is to spice up the recipe for the early Universe with a little extra europium from some previously neglected source. The authors suggest that unusual supernovae involving stars with extreme magnetic fields might be capable of filling the gap. These are very rare now, but might, perhaps, have been common in the early Universe.
Is that the correct answer? I don’t know. Getting the right answer here is a test of everything we think we know about the Universe’s evolution, as well as the physics of some of the most extreme events ever to have taken place. That’s why this work is so important and the recent discoveries so exciting.
CHRIS LINTOTT was reading… Neutron Star Mergers Might not be the Only Source of r-Process Elements in the Milky Way by Benoit Côté, et al. Read it online at: arxiv.org/abs/1809.03525
“0n 17 August 2017 the LIGO gravitational wave detector found a merger between two neutron stars – an event they’ve called a kilonova”