The identification of strontium in the merger of two neutron stars in a binary system represents one of the major ground-breaking discoveries of the last decades

The origins of the elements heavier than iron remains one of the greatest challenges of (astro)physics. Two major nucleosynthetic processes are responsible for their production, each contributing to about 50% the total trans-iron abundances we observe in the Solar System: the “slow” neutron capture process (s-process) and the “rapid” neutron capture process (r-process). The astrophysical sites of the s-process are well constrained both theoretically and observationally. Stellar models properly reproduce both the massive stars contribution and the observed trans-iron surface abundances of AGB stars. Moreover, unstable trans-iron elements like technetium are actually observed on AGB stars surfaces. Due to its instability, technetium cannot be part of the initial composition of stars. It is hence an evidence that the s-process is actually happening in AGB stars.

Colour composite image of AT 2017gfo from GROND on 18 August 2017, 1.44 days after the discovery of GW170817. Credits: Smartt et al. 2017.

Where does the r-process take place?

On the other hand, the astrophysical sites hosting the r-process remained a mystery for a much longer time. The reasons are many, above all the lack of direct observations of r-process elements production and the very uncertain nuclear-physics inputs, given the involvement of very unstable nuclei produced by the extremely high neutron densities required by the r-process (around 10^21 neutron cm^-3 ). But in 2019, Watson and collaborators presented a ground-breaking discovery, since for the first time a robust spectroscopic identification of a freshly produced r-process element (strontium) was reported during a re-analysis of the AT 2017gfo kilonova event, the first ever transient event to be associated with a gravitational-wave source, GW170817, which arose from a binary neutron-star merger in the galaxy NGC 4993. This study established the origin of r-process elements, and hence of potentially up to 50% the Solar System content in elements heavier than iron, in neutron-star mergers. Additionally, the identification of an element that could only have been synthesized so quickly (fraction of second) under an extreme neutron density provides the first direct spectroscopic evidence that neutron stars are made of neutron-rich matter.


Watson, D., Hansen, C.J., Selsing, al.; “Identification of strontium in the merger of two neutron stars”. Nature 574, 497–500 (2019)

Smartt, S., Chen, TW., Jerkstrand, A. et al. A kilonova as the electromagnetic counterpart to a gravitational-wave source. Nature 551, 75–79 (2017).

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