Ryosuke Hirai's Homepage

Research Interests

Supernovae in binaries

My main research interest is on binary stars, especially for high mass stars which explode as core-collapse supernova at the end of their lives. I work intensively on "ejecta-companion interaction", which is the effect of supernova ejecta colliding with a companion star. I have also studied the evolution of binary systems which can reproduce the observed features of some specific supernovae such as iPTF13bvn, Cassiopeia A, SN2006jc and SN2019yvr.

Hydrodynamical simulations of ejecta-companion interaction (Hirai et al. 2020, Hirai, Podsiadlowski & Yamada 2018, Hirai, Sawai & Yamada 2014).
movie movie
Binary evolution simulations for the progenitor of iPTF13bvn (Hirai & Yamada 2015, Hirai 2017a,Hirai 2017b).

I am also investigating what happens when a new-born neutron star gets kicked into a companion star. In particular, I focus on the situations where the neutron star can penetrate the companion's envelope, where it can create interesting objects such as hypervelocity stars, pulsar planets and bumpy superluminous supernova light curves.

3D hydrodynamical simulations of the collision between a new-born neutron star and its binary companion (Hirai & Podsiadlowski 2022).

New-born neutron stars can sometimes be further slowly accelerated through so-called rocket mechanisms. When this happens, we found that it can create binaries with orbits that are impossible to achieve with just traditional kicks.

Orbital evolution of binaries containing a neutron star slowly accelerated through the rocket mechanism. (Hirai et al. 2024).

Stellar mergers

Stars in binary systems can sometimes lead to catastrophic stellar mergers. I work on modelling the outcome of stellar mergers, specifically for the case of the extraordinary star "Eta Carinae", which was most likely created through a stellar merger in a triple system.

Hydrodynamical simulations of the outflow from stellar merger-induced eruptions and its subsequent super-Eddington wind phase (Hirai et al. 2021).
Combination of triple dynamics calculations and N-body simulations to model the Outer Ejecta of Eta Carinae (Hirai et al. 2021).

X-ray binaries

High-mass X-ray binaries are binary systems where a compact object such as a neutron star or black hole accretes part of the stellar wind emitted from a close-by donor star. I work on modelling the effect of tidal forces in binaries on the acceleration of winds and how it can influence the accretion.

Wind stream lines
Comparison of wind streamlines with (right) and without (left) tidal effects (Hirai & Mandel 2021).

Numerical Techniques

Numerical computation is almost inevitable when studying quantitative behaviours of astrophysical phenomena. But we usually have limited computer power, which prevents us from carrying out simulations of the desired resolution. I have investigated how we can improve the computational efficiency of self-gravitational hydrodynamical simulations so that we can speed up simulations.

Test calculation of colliding stars using our "Hyperbolic Self-Gravity Solver" (Hirai, Nagakura, Okawa & Fujisawa 2016).