Evolution of binary seeds in collapsing protostellar gas clouds

TL;DR

This study uses 3D SPH simulations to explore how binary star seeds evolve during gas accretion, revealing a critical initial mass ratio that determines whether the binary becomes equal-mass or unequal-mass.

Contribution

It introduces a more realistic simulation setup including a dynamic gas envelope and identifies a critical mass ratio influencing binary evolution.

Findings

A critical initial mass ratio q_c = 0.25 separates different evolutionary paths.

Binaries with q_0 > q_c tend toward equal masses.

Binary separation increases over time regardless of initial mass ratio.

Abstract

We perform three dimensional smoothed particle hydrodynamics (SPH) simulations of gas accretion onto the seeds of binary stars to investigate their short-term evolution. Our simulation setup is more realistic compared to the previous works by taking into account of dynamically evolving envelope with non-uniform distribution of gas density and angular momentum of accreting flow. Our initial condition includes a seed binary and a surrounding gas envelope, modelling the phase of core collapse of gas cloud when the fragmentation has already occurred. We assume that the seed binary has no eccentricity and no growth by gas accretion. The envelope is assumed to be an isothermal gas with no self-gravity. We run multiple simulations with different values of initial mass ratio $q_0$ (the ratio of secondary over primary mass) and gas temperature, and find a critical value of $q_{\rm c} = 0.25$ which distinguishes the later evolution of mass ratio $q$ as a function of time. If $q_0$ >~ $q_{\rm c}$, the secondary seed grows faster and $q$ increases monotonically towards unity. If $q_0$ <~ $q_{\rm c}$, on the other hand, the primary seed grows faster and $q$ is lower than $q_0$ at the end of the simulation. Based on our numerical results, we analytically calculate the long-term evolution of the seed binary including the growth of binary by gas accretion. We find that the seed binary with $q_0$ >~ $q_{\rm c}$ evolves towards an equal-mass binary star, and that with $q_0$ <~ $q_{\rm c}$ evolves to a binary with an extreme value of $q$. Binary separation is a monotonically increasing function of time for any $q_0$, suggesting that the binary growth by accretion does not lead to the formation of close binaries.