5 mins read 15 Jun 2020

Massive Neutron Star a Problem for Evolutionary Models

The discovery of the heaviest binary neutron star system ever is requiring astronomers to revisit their models of how these bizarre systems form.

Artist impression of merging neutron stars. Credit: University of Warwick/Mark Garlick

Considering that the first detection of a binary neutron star merger from gravitational waves was made back in 2017, it is hard to believe that the second only occurred last year. In the intervening period, around ten black hole merger events were published in scientific journals, and most of the data coming in from the latest gravitational wave observing run also display the signatures of pairs of black holes. But being a rarity is only one reason that last year’s detection, called GW190425, was met with excitement by astronomers from around the world. If initial analyses stand up to scrutiny, it has the greatest total mass of any binary neutron star system known.

And the discrepancy is significant. There are currently about a dozen binary neutron star systems in our galaxy for which we have reliable masses for both stellar components. The average mass of the neutron stars that would result from mergers within all these systems would be a bit more than two and a half times the Sun. But you need to add a whole other Sun to get the mass of GW190425. And that makes it very interesting, even paradigm-shifting, as scientists work to modify their theories of binary neutron star formation to fit this unusual observation.

Unstable 'Case BB' Mass Transfer. A possible evolutionary path for the high mass binary neutron star system GW190425. Credit: Carl Knox

There are thought to be two ways in which a binary neutron star system can form. The first, and thought to be the most common case, is where a binary star system undergoes successive supernovae, each one leaving behind a neutron star. This is known as isolated evolution, and while it is an adequate explanation for the known binary neutron systems in the Milky Way, it is unlikely that GW190425 could have formed in this way. Its supernovae probably would have been energetic enough to completely disrupt the binary system due to their high-mass progenitors, meaning that the eventual merger would never have happened.

The second way in which these systems can form is through stellar interactions inside heavily populated areas of space such as globular clusters. A neutron star with a companion moves towards the centre of the star cluster where it swaps its companion for another neutron star. The dynamics of this arrangement ensures that the new system will merge within a comparatively short timeframe. Such a hypothesis would provide an explanation for the high mass of GW190425, but computer models suggest that we would need to run our detectors for up to 12,500 years before we found a binary neutron star system formed in this way. It just does not seem likely.

But a team of Victorian scientists led by Monash PhD student Isobel Romero-Shaw think that they can explain the evolution of high mass binary neutron star systems like GW190425 another way. Romero-Shaw and her team, who are all linked with the ARC Centre of Excellence for Gravitational Wave Discovery, or OzGrav, hypothesised a model known as unstable ‘case BB’ mass transfer. Romero-Shaw explains. “It starts with a neutron star which has a stellar partner: a helium star with a carbon-oxygen core. If the helium part of the star expands far enough to engulf the neutron star, this helium cloud ends up pushing the binary closer together before it dissipates. The carbon-oxygen core of the star then explodes in a supernova and collapses to a neutron star”.

Binary neutron stars formed in this way can be both more massive and form more rapidly than those that evolve through other formation channels. And a nice feature of this hypothesis is that it can be tested directly by measuring how elliptical the orbits of the stars were before they merged. Rather than being circular, as is the case for stars that are the result of isolated evolution, they should have some measurable orbital eccentricity. When Romero-Shaw and her collaborators measured the eccentricity of GW190425, they found promising signs that it could have been formed through unstable ‘case BB’ mass transfer.

“Understanding the evolution of these systems gives us insight into the hidden workings of binary systems,” says Romero-Shaw. But to really gain these insights, astronomers are going to need to wait for next-generation gravitational wave detectors to come online, like the space-based Laser Interferometer Space Antenna, nicknamed LISA. “LISA will be able to probe much lower gravitational-wave frequencies, so it will be able to observe systems like GW190425 much earlier in their lives”. The study of gravitational waves is still in its relative infancy, but exciting discoveries like GW190425 are forcing us to re-evaluate some of what we thought we already knew and giving industrious researchers, like those at OzGrav, plenty of food for thought.

Read the paper on the MNRAS here