On the Effects of Subvirial Initial Conditions and the Birth Temperature of R136

TL;DR

This study explores how initial virial temperature influences star cluster evolution, revealing its significant impact on system dynamics and providing insights into the initial conditions of R136 and other young clusters.

Contribution

It introduces a corrected theoretical estimate for the minimum radius at collapse that includes virial temperature dependence and applies this to infer initial conditions of star clusters.

Findings

Virial temperature strongly affects cluster dynamics and mass segregation.

R136 likely formed with a virial temperature around 0.13.

Most young clusters have initial virial temperatures between 0.18 and 0.25.

Abstract

We investigate the effect of different initial virial temperatures, Q, on the dynamics of star clusters. We find that the virial temperature has a strong effect on many aspects of the resulting system, including among others: the fraction of bodies escaping from the system, the depth of the collapse of the system, and the strength of the mass segregation. These differences deem the practice of using "cold" initial conditions no longer a simple choice of convenience. The choice of initial virial temperature must be carefully considered as its impact on the remainder of the simulation can be profound. We discuss the pitfalls and aim to describe the general behavior of the collapse and the resultant system as a function of the virial temperature so that a well reasoned choice of initial virial temperature can be made. We make a correction to the previous theoretical estimate for the minimum radius, $R_{min}$, of the cluster at the deepest moment of collapse to include a Q dependency, $R_{min}\approx Q + N^{(-1/3)}$, where $N$ is the number of particles. We use our numerical results to infer more about the initial conditions of the young cluster R136. Based on our analysis, we find that R136 was likely formed with a rather cool, but not cold, initial virial temperature ($Q\approx 0.13$). Using the same analysis method, we examined 15 other young clusters and found the most common initial virial temperature to be between 0.18 and 0.25.