Colliding Neutron Stars as the Source of Heavy Elements

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Where do the heavy elements — the chemical elements beyond iron — in our universe come from? One of the primary candidate sources is the merger of two neutron stars, but recent observations have cast doubt on this model. Can neutron-star mergers really be responsible?

Elements from Collisions?

element origins

Periodic table showing the origin of each chemical element. Those produced by the r-process are shaded orange and attributed to supernovae in this image; though supernovae are one proposed source of r-process elements, an alternative source is the merger of two neutron stars. [Cmglee]

When a binary-neutron-star system inspirals and the two neutron stars smash into each other, a shower of neutrons are released. These neutrons are thought to bombard the surrounding atoms, rapidly producing heavy elements in what is known as r-process nucleosynthesis.

So could these mergers be responsible for producing the majority of the universe’s heavy r-process elements? Proponents of this model argue that it’s supported by observations. The overall amount of heavy r-process material in the Milky Way, for instance, is consistent with the expected ejection amounts from mergers, based both on predicted merger rates for neutron stars in the galaxy, and on the observed rates of soft gamma-ray bursts (which are thought to accompany double-neutron-star mergers).

Challenges from Ultra-Faint Dwarfs

Recently, however, r-process elements have been observed in ultra-faint dwarf satellite galaxies. This discovery raises two major challenges to the merger model for heavy-element production:

  1. When neutron stars are born during a core-collapse supernova, mass is ejected, providing the stars with asymmetric natal kicks. During the second collapse in a double-neutron-star binary, wouldn’t the kick exceed the low escape velocity of an ultra-faint dwarf, ejecting the binary before it could merge and enrich the galaxy?
  2. Ultra-faint dwarfs have very old stellar populations — and the observation of r-process elements in these stars requires mergers to have occurred very early in the galaxy’s history. Can double-neutron-star systems merge quickly enough to account for the observed chemical enrichment?

Small Kicks and Fast Mergers

kick velocities

Fraction of double-neutron-star systems that remain bound, vs. the magnitude of the kick they receive. A typical escape velocity for an ultra-faint dwarf is ~15 km/s; roughly 55-65% of binaries receive smaller kicks than that and wouldn’t be ejected from an ultra-faint dwarf. [Beniamini et al. 2016]

Led by Paz Beniamini, a team of scientists from the Racah Institute of Physics at the Hebrew University of Jerusalem has set out to answer these questions. Using the statistics of our galaxy’s double-neutron-star population, the team performed Monte Carlo simulations to estimate the distributions of mass ejection and kick velocities for the systems.

Beniamini and collaborators find that, for typical initial separations, more than half of neutron star binaries are born with small enough kicks that they remain bound and aren’t ejected — even from small, ultra-faint dwarf galaxies.

The team also used their statistics to calculate the time until merger for the population of binaries, finding that ~90% of the double-neutron-star systems merge within 300 Myr, and around 15% merge within 100 Myr — quick enough to enrich even the old population of stars.

This population of systems that remain confined to the galaxy and merge rapidly can therefore explain the observations of r-process material in ultra-faint dwarf galaxies. Beniamini and collaborators’ work suggests that the merger of neutron stars is indeed a viable model for the production of heavy elements in our universe.

Citation

Paz Beniamini et al 2016 ApJ 829 L13. doi:10.3847/2041-8205/829/1/L13

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