Merging neutron stars create more gold than collisions involving black holes

Physics

Biggish bang: artist’s impression of a neutron-star merger (Courtesy: NASA)

The amounts of heavy elements such as gold created when black holes merge with neutron stars have been calculated and compared with the amounts expected when pairs of neutron stars merge. The calculations were done by Hsin-Yu Chen and Salvatore Vitale at the Massachusetts Institute of Technology and Francois Foucart at the University of New Hampshire using advanced simulations and gravitational-wave observations made by the LIGO–Virgo collaboration. Their results suggest that merging pairs of neutron stars are likely to be responsible for more heavy elements in the universe than mergers of black holes with neutron stars.

Today, astrophysicists have an incomplete understanding of how elements heavier than iron are made. In this nucleosynthesis process, lighter nuclei must be able to capture neutrons from their surroundings. Astrophysicists believe this can happen in two ways, each producing about half of the heavy elements in the universe. These are the slow process (s-process) that occurs in large stars and the rapid process (r-process), which is believed to occur in extreme conditions such as the explosion of a star in a supernova. However, exactly where the r-process can take place is hotly debated.

One event that could support the r-process is the merger of a pair of neutron stars, which can result in a huge explosion called a kilonova. Indeed, such an event was seen by LIGO–Virgo in 2017, and simultaneous observations using light-based telescopes suggest that heavy elements were created in that event.

Gravitational disruption

Another possibility is that the r-process occurs just after the merger of a neutron star and a black hole. As the neutron star is disrupted by the huge gravitational field of the black hole, vast amounts of neutron-rich material could be blasted into space – providing an environment for the r-process. Astrophysicists believe this can happen when the black hole has a relatively low mass and is spinning at a relatively high rate. If the black hole is too heavy, the neutron star will be swallowed rapidly, and little neutron-rich material will escape.

Today, however, astrophysicists are unsure of the relative contributions of these two merger types to the universe’s overall abundance of heavy elements.

Ultimately, the amounts of heavy elements produced by these events depends on several factors: including the masses and spins of the merging bodies; the rate of occurrence of the merger types throughout the history of the universe; and the neutron star’s “equation of state”. The latter describes the mathematical relationship between the mass and radius of a neutron star. Over the years a variety of models have been developed to define these quantities.

Improved equation of state

In their study, Chen and colleagues have compared the contributions of both merger types for the first time. They began by studying LIGO–Virgo observations of the two different types of merger. Then, they used the latest simulations of ejections from these events – incorporating improved equation of state measurements, to test several models of how the r-process could proceed, which they deemed consistent with LIGO–Virgo’s observations.

In most simulation scenarios, the researchers found that binary neutron star mergers produced 2–100 times more heavy elements over the past 2.5 billion years than mergers between black holes and neutron stars. This outcome only changed when researchers assumed that black holes tend to have lower masses and faster spins than predicted by current theories.

Chen and colleagues now hope to improve their calculations using future observations from the upgraded LIGO and Virgo detectors – and the new KAGRA detector – which will all be back online in 2022. These efforts could ultimately improve astronomers’ estimates of the rates at which heavy elements are produced across the universe. In turn, this could help them to better determine the ages of distant galaxies, by measuring the abundances of heavy elements they contain.

The research is described in The Astrophysical Journal.

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