Simulating Smashing Neutron Stars: Interview with Dr Ryosuke Hirai

People tend to think of scientists as either theorists or experimentalists with the former being stereotyped as bespectacled savants huddled over a mess of papers and books and the latter, a class of lab coat-wearing kooks, confined to basement laboratories filled with bubbling beakers and crackling electrical devices that may or may not be powered by lightning.   

In reality, however, the roles or types of scientists are far more varied, with analytical and computational techniques - like numerical modelling and machine learning - becoming standard tools for many researchers.

The astronomy community in Australia recently attended the 2022 Astronomical Society of Australia’s (ASA) Annual Science Meeting where this reality was very prevalent. Students, early career researchers, senior scientists and collaborators, all proudly showed off the excellent work which they and their teams have been pursuing over the last year. 

One great example of this more modern, multidisciplinary research is the work presented by Dr Ryosuke Hirai, which was presented at the ASA meeting.

Dr Hirai has worked extensively on modelling the behaviour of stars that live in pairs, known as binary systems, and how they interact with each other when one of the stars reaches the end of its stellar life as an energetic supernova. These violent events occur when one or both of the pair are high-mass stars, which burn through their nuclear fuel in relatively short timeframes.

More than 80% of the stars we see in the night sky are in fact binary systems, we just need powerful telescopes to differentiate two objects from one distant, twinkling point. Generally speaking, stars in a binary are separated by enough distance to significantly affect their cosmic companion as they orbit a common centre of gravity. But for those stars that stray too close when their partner runs out of nuclear fuel and explodes, the results can produce some exotic stellar objects.   

These supernovae can leave behind a core of dense, neutron-rich material from the progenitor star - in a compact remnant object known as a neutron star - which can be kicked towards the companion by the force of the explosion. Some neutron stars tend to produce radio and high-energy emissions which, when observed from Earth, appear to pulsate as they rapidly rotate and have thus earned the nickname of pulsars. 

Dr Hirai’s new work shows how such neutron stars, depending on the speed and direction of their natal kick, can be thrown into the envelope of their companions, disrupting the secondary star due to their strong gravity. As the neutron star follows a trajectory inside the secondary, the system’s centre of mass changes. This allows the neutron star to follow a different trajectory, depending on how powerful the original natal kick is. 

Sometimes, when moving fast enough, it might pass right through the star, and at other times, when moving slower, it will hang around and start to gravitationally attract enough material to emerge with an additional disc of matter for itself. These interactions allow the neutron star to form planetary disks or even merge with their stellar companion into a strange hybrid star.

After his talk I spoke with Dr Hirai, who is a research fellow at the School of Physics and Astronomy at Monash University, to learn more about his latest work in simulating supernova explosions in binary star systems. 

Can you tell us a bit about yourself and your hobbies?

I'm Japanese, so I did my PhD in Japan but I also spent half of my life in the UK; moving to England when I was two and staying there until I was ten. In terms of hobbies, when I was in Japan I was actually a stunt man. I used to do stunts in movies and live shows. It started as part-time work when I was 15 and I was doing it along with my high school, undergrad, PhD studies.  

Wow, what a fantastic hobby, how did you get into that?

It started because I was a fan of this show called Power Rangers. It was originally made in Japan. So we have this huge thing in Japan about Power Rangers and a superhero called Masked Rider. I enjoyed performing stunts in these live shows of Power Rangers.

So now that you’re a full-time scientist, what do you like most about your work in astrophysics? 

In terms of research, I just enjoy explosions in general - like supernovae - and I've worked on stellar mergers and X-ray bursts. These explosions and shocks and things like that are the things I'm most interested in. But I also enjoy the numerical side, like doing hydrodynamical simulations.

What sort of software do you use to do that?

I use my own code-named Hormone, after my favourite band Maximum the Hormone - which I made during my PhD. This includes lots of new numerical algorithms as well, which we’ve written papers about. So yeah, I kind of enjoy thinking about the physics of explosions and also just everyday numerical coding and thinking about new ways to solve equations. 

Last month at the ASA ASM you spoke about your work on supernovae in binary systems. Could you tell us a bit about what happens in a binary when one of the stars explodes?

More than half of my past research is based on things related to binaries. What happens is that you have a supernova hitting the companion star. People traditionally thought that it doesn't do much because the amount of energy that goes into the star isn't that big. But through some simulations we’ve found that though this energy is small, it's actually also deposited in really small regions, making it quite important and causing the star to swell up quite a bit. Sometimes, depending on the companion star's type, the supernova can strip off some of the mass. But that's only if this companion star is a very fluffy red giant or something. These are the two main effects that I've been working on in the past - the swelling up because of the heat injected and also some stripping.  

More recently, as I talked about at the ASA meeting, I've been thinking about the remaining binary orbit. Usually, in about 90% of the cases, the binary gets disrupted in a supernova event simply because there's lots of mass loss, which reduces the gravity that holds them together. And also, you have this natal kick given to the neutron star [produced from the supernova], which kind of disrupts the binding. But sometimes the kick can be very luckily directed in a way that it will remain in a binary or even go into the companion. That's where we found that it may do some interesting stuff.

What kind of interesting things are we talking about? 

People usually thought that if you just kicked the neutron star into the companion, it would just swallow it and form a Thorne–Żytkow object [an exotic hybrid star with a neutron star core and a puffy outer layer]. But now we found that about half of the time it actually penetrates the outer layers of the companion star - sometimes penetrating multiple times - before it merges.  

How did these Thorne–Żytkow objects form in your simulations? 

For most of my simulations, we saw that the neutron stars penetrate, but they decelerate enough to become [gravitationally] bound. And if it's on a bound orbit., they will penetrate multiple times, but eventually, it has to merge. So if you direct that kick into the star, it will create a Thorne–Żytkow object. Only if the kick is very strong, say above about 700 kilometres per second, then there’s a chance that the neutron star penetrates through the companion on an unbound object; never to meet again.

How did you study the interactions between these newly formed Neutron stars with their nearby companions?

Initially, we're hoping to make hypervelocity stars. But in terms of high-velocity stars, it turned out that it's quite hard to get beyond the classical limit. So now I think it's more about [using these simulations to understand] bumpy light curves and also planet formation. Pulsar planets are rare enough that we will have to explain them with some very rare scenarios. And this envelope penetration is a very rare thing. If it happens, the material carried away from the supernova could create some possible planets. 

How do these scenarios then go on to potentially produce pulsar planets?

My model can basically explain the formation of a protoplanetary disk. My simulations show that we can pull roughly two Jupiter masses of material off the companion through penetration and that it's also taken away in a sort of disk-like form that can eventually go into a planetary disk.

What’s the next big research idea you’re looking into?

I have quite a few different topics I'm working on. One of my main things that some of my students are investigating but I'm also working on, is this common envelope evolution - when a star evolves and starts swallowing the other star, then it will spiral into the envelope and become a compact binary.

This is kind of the largest uncertainty in binary evolution at the moment. Many people have been impacted through simulations and analytical work for the past three decades. But we still keep on saying it's the most uncertain thing because of how little we know. So my students are working on simulations, but also I'm working on the analytical modelling side, which I think might provide a significant step forward and make it less uncertain.

If you had unlimited funding, what project would you design/work on?

Good question. Usually, as a theorist, we don't require too much money. So, it's hard to come up with something with unlimited resources. I'm not sure if I can work on it myself, but I would. I would love to have a replacement for Hubble. My own Hubble space telescope to probe the UV.

Was there anything else about what you presented that you wanted to discuss?

One of the main questions I usually get about this research is the rates. So we're just cautioning everyone that this isn't a common thing that should happen. It's supposed to be a rare thing, but we're trying to explain rare phenomena. The rate of the collision strongly depends on this uncertain level of efficiency, because that determines how close binaries can be before a supernova. Depending on that, I would say at most we can probably have about 1% of stripped-envelope supernovae that could have these sorts of collisions.