Modelling the evolution of double neutron stars

On 17 August 2017, physicists around the world detected a disturbance in the very fabric of spacetime when two neutron stars that had been spiraling into each other finally collided and merged into one. As the stars approached their impending doom, gravitational waves propagated away at the speed of light, and the collision was marked by a spectacular gamma-ray burst seen by orbiting telescopes just seconds later. The merger had occurred about 140 million light-years away, but here on Earth scientists were already questioning how much we really know about how pairs of neutron stars like this form.

Neutron stars are created when giant stars, usually some 10 to 30 times as massive as the Sun, exhaust their fuel and explode in a supernova leaving behind a core of subatomic particles. Gravity crushes the protons and electrons together forming neutrinos and neutrons, and the neutrons prevent further collapse of the star leaving something akin to an atomic nucleus within a 10 km radius. When the progenitor stars exist in binary systems, gas may be transferred from one star to the other, hastening the demise of the unlucky donor. But there is uncertainty as to how initially widely spaced binary stars later evolve into close double neutron stars.

The key lies in understanding a phase of binary star evolution known as the common-envelope episode (CEE). For a while now it has been thought that if one star expanded rapidly its envelope could engulf its companion and the resulting drag would force the two closer together. In fact, most evolutionary pathways leading to close compact binaries are expected to have experienced at least one CEE.

"It's a bit like having the two stars move through molasses", explains Professor Ilya Mandel, senior author, and OzGrav Chief Investigator at Monash University. "They slow down, and any orbiting objects come close together". The end result is either the envelope being ejected, leading to a closely spaced binary, or a merger of the two stars. But the detailed physics remains poorly understood.

A research team led by Dr. Alejandro Vigna-Gómez, alumnus from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and DARK Fellow at the Niels Bohr Institute, has been working to change that. The team, who also have associations with Victoria's Melbourne and Monash Universities and other international organisations, have run detailed simulations on the common-envelope phase of evolution. By looking at progenitors with a wide range of properties, they were able to determine which were the most likely to experience a CEE and become close double neutron stars.

Their results will go a long way towards simplifying future studies of this important phase of double neutron star evolution. Firstly, they confirmed that there are two dominant evolutionary pathways to the formation of double neutron stars. In most cases, the progenitors prior to the CEE were a donor star with a neutron star companion, but they also found that the CEE could occur when two similarly sized massive stars were in a binary.

Next, they found that the brightest of a particular type of stellar explosion known as a luminous red nova would, in about 10% of cases, evolve into double neutron stars. In this case, the luminous red nova itself could be a signature of envelope ejection. When a recently observed luminous red nova named M101 OT2015-1 was analysed, its progenitor star was found to have similar properties to the expected pre-CEE properties of double neutron star-forming systems.

Finally, they managed to constrain the orbital evolution of binary systems before the CEE, finding that it may be different than previously thought. According to Dr. Vigna-Gómez, rather than being circular, "the orbit might remain eccentric until the common-envelope forms". This implies that existing computer simulation code, which had previously assumed orbits were circular before the CEE, needs to be updated to fit the new models. 

Dr. Vigna-Gómez and his team calculated that roughly one double neutron star merger should occur in the Milky Way every million years or so. At that rate, we may be waiting a long time to see one in our own galaxy, but with better constraints on the star systems that produce these phenomena, we'll now be able to run more accurate computer simulations to help improve our understanding. And to really know neutron stars, there is still a lot more we need to understand.