Mixing in massive stellar mergers

TL;DR

This paper introduces a simple, computationally efficient algorithm to simulate stellar mergers, capturing shock heating, mixing, and mass loss, and applies it to massive star collisions in dense clusters.

Contribution

The authors develop and calibrate a new approximation method for stellar mergers that includes hydrodynamic effects without intensive simulations, focusing on massive stars.

Findings

Head-on collisions of similar stars lead to significant mixing.

Mergers between different spectral types show less hydrodynamic mixing.

The method prevents unphysical double-valued temperature and composition profiles.

Abstract

The early evolution of dense star clusters is possibly dominated by close interactions between stars, and physical collisions between stars may occur quite frequently. Simulating a stellar collision event can be an intensive numerical task, as detailed calculations of this process require hydrodynamic simulations in three dimensions. We present a computationally inexpensive method in which we approximate the merger process, including shock heating, hydrodynamic mixing and mass loss, with a simple algorithm based on conservation laws and a basic qualitative understanding of the hydrodynamics of stellar mergers. The algorithm relies on Archimedes' principle to dictate the distribution of the fluid in the stable equilibrium situation. We calibrate and apply the method to mergers of massive stars, as these are expected to occur in young and dense star clusters. We find that without the effects of microscopic mixing, the temperature and chemical composition profiles in a collision product can become double-valued functions of enclosed mass. Such an unphysical situation is mended by simulating microscopic mixing as a post-collision effect. In this way we find that head-on collisions between stars of the same spectral type result in substantial mixing, while mergers between stars of different spectral type, such as type B and O stars ($\sim$10 and $\sim$40\msun respectively), are subject to relatively little hydrodynamic mixing.