Implications for the Formation of Blue Straggler Stars from HST Ultraviolet Observations of NGC 188

TL;DR

This study uses HST ultraviolet observations to identify white dwarf companions to blue straggler stars in NGC 188, providing insights into their formation mechanisms, especially mass transfer, and estimating their formation frequency.

Contribution

It offers the first direct detection of WD companions to BSSs in NGC 188, quantifies the mass transfer formation frequency, and compares observational data with theoretical models.

Findings

Four BSSs have young, hot WD companions indicating recent mass transfer.

Approximately 67% of BSSs likely formed through mass transfer.

No clear CMD separation between different BSS formation mechanisms.

Abstract

We present results of a Hubble Space Telescope far-ultraviolet (FUV) survey searching for white dwarf (WD) companions to blue straggler stars (BSSs) in open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined binaries with properties suggestive of mass-transfer formation via Roche lobe overflow, specifically through an asymptotic giant branch star transferring mass to a main sequence secondary, yielding a BSS binary with a WD companion. In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a WD companion hot and luminous enough to be directly detected as a FUV photometric excess with HST. Comparing expected BSS FUV emission to observed photometry reveals four BSSs with WD companions above 12,000 K (younger than 250 Myr) and three WD companions with temperatures between 11,000-12,000 K. These BSS+WD binaries all formed through recent mass transfer. The location of the young BSSs in an optical color-magnitude diagram (CMD) indicates that distance from the zero-age main sequence does not necessarily correlate with BSS age. There is no clear CMD separation between mass transfer-formed BSSs and those likely formed through other mechanisms, such as collisions. The seven detected WD companions place a lower limit on the mass-transfer formation frequency of 33%. We consider other possible formation mechanisms by comparing properties of the BSS population to theoretical predictions. We conclude that 14 BSS binaries likely formed from mass transfer, resulting in an inferred mass-transfer formation frequency of approximately 67%.