Two distinct sequences of blue straggler stars in the globular cluster M30

TL;DR

The paper identifies two separate sequences of blue straggler stars in globular cluster M30, linking their origins to core collapse processes that enhance stellar collisions and binary mass transfer.

Contribution

It reveals the existence of two distinct blue straggler sequences in M30, associating them with different formation mechanisms triggered by core collapse.

Findings

Two parallel blue straggler sequences identified in M30

Bluer sequence linked to stellar collisions

Redder sequence linked to binary mass transfer

Abstract

Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy's history. 'Blue stragglers' are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently, the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in M30. This globular cluster is thought to have undergone 'core collapse', during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences arise from the cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.