Heavy el­e­ments cre­ated by crash­ing neutron stars

These mighty stel­lar col­li­sions do not ap­pear to be the only source of these el­e­ments, though

Sky at Night Magazine - - BULLETIN -

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”

Could the col­li­sion of neutron stars be the source of ‘r-process’ el­e­ments, the cre­ation of which may be a vi­tal step to­wards life evolv­ing?

CHRIS LIN­TOTT is an as­tro­physi­cist and co-pre­sen­ter of The Skyat Night on BBC TV. He is also the di­rec­tor of the Zooni­verse pro­ject

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