When Stars Collide

TL;DR

This paper investigates the evolution of stars formed from collisions in star clusters, highlighting how their increased helium content affects their brightness and evolution, which is crucial for understanding blue stragglers and black hole formation.

Contribution

It provides detailed evolution calculations of merger remnants from main sequence star collisions, emphasizing the importance of helium enrichment in their development.

Findings

Collision remnants are significantly brighter due to helium enrichment.

Simplified models underestimate the lifetime of merger products.

Merger processes can lead to the formation of intermediate mass black holes.

Abstract

When two stars collide and merge they form a new star that can stand out against the background population in a starcluster as a blue straggler. In so called collision runaways many stars can merge and may form a very massive star that eventually forms an intermediate mass blackhole. We have performed detailed evolution calculations of merger remnants from collisions between main sequence stars, both for lower mass stars and higher mass stars. These stars can be significantly brighter than ordinary stars of the same mass due to their increased helium abundance. Simplified treatments ignoring this effect give incorrect predictions for the collision product lifetime and evolution in the Hertzsprung-Russell diagram.