Puzzling Massive Star System Greater Than Any Ever Observed in Galaxy

Image credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet.

A neutron star binary this massive has never been seen in our Galaxy, and scientists have been mystified by how it could have formed—until now. A team of astrophysicists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) think they might have the answer.

Binary neutron stars emit gravitational waves—ripples in space-time— as they orbit each other, and scientists can detect these waves when the neutron stars merge. The gravitational waves contain information about the neutron stars, including their masses.

The gravitational waves from cosmic event GW190425 tell of a neutron star binary more massive than any neutron star binary previously observed, either through radio-wave or gravitational-wave astronomy. A recent study led by OzGrav PhD student Isobel Romero-Shaw from Monash University proposes a formation channel that explains both the high mass of this binary and the fact that similar systems aren’t observed with traditional radio astronomy techniques.

Carl Knox, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)

Romero-Shaw explains: ‘We propose that GW190425 formed through a process called ‘unstable case BB mass transfer’, a procedure that was originally defined in 1981. It starts with a neutron star which has a stellar partner: a helium (He) star with a carbon-oxygen (CO) 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 that form in this way can be significantly more massive than those observed through radio waves. They also merge very fast following the supernova explosion, making them unlikely to be captured in radio astronomy surveys.