Identification of strontium in the merger of two neutron stars

TL;DR

This paper reports the first robust identification of the neutron-capture element strontium in the spectra of a kilonova from a neutron star merger, confirming neutron star mergers as sites of heavy element creation.

Contribution

It provides the first definitive spectral detection of an $r$-process element in a neutron star merger, confirming these events as sources of heavy elements in the universe.

Findings

Strontium was identified in the spectra of the kilonova AT2017gfo.

The detection confirms neutron star mergers as sites of $r$-process nucleosynthesis.

Neutron star matter is confirmed to be neutron-rich.

Abstract

Half of all the elements in the universe heavier than iron were created by rapid neutron capture. The theory for this astrophysical `$r$-process' was worked out six decades ago and requires an enormous neutron flux to make the bulk of these elements. Where this happens is still debated. A key piece of missing evidence is the identification of freshly-synthesised $r$-process elements in an astrophysical site. Current models and circumstantial evidence point to neutron star mergers as a probable $r$-process site, with the optical/infrared `kilonova' emerging in the days after the merger a likely place to detect the spectral signatures of newly-created neutron-capture elements. The kilonova, AT2017gfo, emerging from the gravitational-wave--discovered neutron star merger, GW170817, was the first kilonova where detailed spectra were recorded. When these spectra were first reported it was argued that they were broadly consonant with an outflow of radioactive heavy elements, however, there was no robust identification of any element. Here we report the identification of the neutron-capture element strontium in a re-analysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of $r$-process elements in neutron star mergers, and demonstrates that neutron stars contain neutron-rich matter.