Models of Individual Blue Stragglers

TL;DR

This paper reviews current models of individual blue stragglers, focusing on stellar collisions, binary mergers, and mass transfer, highlighting their outcomes, differences, and the importance of rotation and surface abundances.

Contribution

It provides a comprehensive overview of the physical processes and modeling challenges involved in blue straggler formation, emphasizing the need for improved understanding of rotation and mixing.

Findings

Collision products retain parent star memory and are not fully mixed.

Mass transfer produces a wider range of blue stragglers than collisions.

Rotation and surface abundances are key to distinguishing formation mechanisms.

Abstract

This chapter describes the current state of models of individual blue stragglers. Stellar collisions, binary mergers (or coalescence), and partial or ongoing mass transfer have all been studied in some detail. The products of stellar collisions retain memory of their parent stars and are not fully mixed. Very high initial rotation rates must be reduced by an unknown process to allow the stars to collapse to the main sequence. The more massive collision products have shorter lifetimes than normal stars of the same mass, while products between low mass stars are long-lived and look very much like normal stars of their mass. Mass transfer can result in a merger, or can produce another binary system with a blue straggler and the remnant of the original primary. The products of binary mass transfer cover a larger portion of the colour-magnitude diagram than collision products for two reasons: there are more possible configurations which produce blue stragglers, and there are differing contributions to the blended light of the system. The effects of rotation may be substantial in both collision and merger products, and could result in significant mixing unless angular momentum is lost shortly after the formation event. Surface abundances may provide ways to distinguish between the formation mechanisms, but care must be taking to model the various mixing mechanisms properly before drawing strong conclusions. Avenues for future work are outlined.