Evolution of stellar collision products in open clusters. II. A grid of low-mass collisions

TL;DR

This study models the evolution of stellar collision products in open clusters, creating a grid of detailed models that match observed blue stragglers and providing an analytic recipe for their evolution in simulations.

Contribution

It extends previous work by systematically modeling collision remnants across different masses and ages, and offers an analytic method to incorporate these into N-body simulations.

Findings

Collision models match observed blue stragglers in M67 and NGC 188.

Recent collisions are needed to explain M67 blue stragglers.

An analytic recipe enables easy integration of collision products into simulations.

Abstract

In a companion paper we studied the detailed evolution of stellar collision products that occurred in an $N$-body simulation of the old open cluster M67 and compared our detailed models to simple prescriptions. In this paper we extend this work by studying the evolution of the collision products in open clusters as a function of mass and age of the progenitor stars. We calculated a grid of head-on collisions covering the section of parameter space relevant for collisions in open clusters. We create detailed models of the merger remnants using an entropy-sorting algorithm and follow their subsequent evolution during the initial contraction phase, through the main sequence and up to the giant branch with our detailed stellar evolution code. We compare the location of our models in a colour-magnitude diagram to the observed blue straggler population of the old open clusters M67 and NGC 188 and find that they cover the observed blue straggler region of both clusters. For M67, collisions need to have taken place recently. Differences between the evolution tracks of the collision products and normal main sequence stars can be understood quantitatively using a simple analytic model. We present an analytic recipe that can be used in an $N$-body code to transform a precomputed evolution track for a normal star into an evolution track for a collision product.