A mass transfer origin for blue stragglers in NGC 188 as revealed by half-solar-mass companions

TL;DR

This study provides evidence that blue stragglers in NGC 188 originate from mass transfer in binary systems, based on the observed companion masses, challenging the collisional formation hypothesis.

Contribution

The paper offers conclusive observational evidence supporting mass transfer as the primary formation mechanism for blue stragglers in NGC 188, dismissing collisional origins.

Findings

Companion masses are about half a solar mass with a narrow distribution.

Collisional origin is ruled out due to incompatible companion mass predictions.

Data supports mass transfer from white dwarf companions as the formation process.

Abstract

In open star clusters, where all members formed at about the same time, blue straggler stars are typically observed to be brighter and bluer than hydrogen-burning main-sequence stars, and therefore should already have evolved into giant stars and stellar remnants. Correlations between blue straggler frequency and cluster binary fraction, core mass, and radial position suggest that mass transfer or mergers in binary stars dominates the production of blue stragglers in open clusters. Analytic models, detailed observations, and sophisticated N-body simulations, however, argue in favor of stellar collisions. Here we report that the blue stragglers in long-period binaries in the old (7 Gyr) open cluster NGC 188 have companions with masses of about half a solar mass, with a surprisingly narrow mass distribution. This conclusively rules out a collisional origin, as the collision hypothesis predicts a companion-mass distribution with significantly higher masses. Mergers in hierarchical triple stars are marginally permitted by the data, but the observations do not favor this hypothesis. The data are closely consistent with a mass transfer origin for the long-period blue straggler binaries in NGC 188, in which the companions would be white dwarfs of about half a solar mass.