The First Detection of Blue Straggler Stars in the Milky Way Bulge

TL;DR

This paper reports the first detection of Blue Straggler Stars in the Milky Way bulge, using proper motions to identify a small but significant population distinct from young stars, and compares their frequency to other stellar systems.

Contribution

It provides the first observational evidence of Blue Straggler Stars in the Milky Way bulge and estimates their population relative to horizontal branch stars.

Findings

At least 18 blue stragglers identified among 42 candidates.

The BSS fraction in the bulge is different from that in stellar clusters.

The young stellar population in the bulge is at most 3.4%.

Abstract

We report the first detections of Blue Straggler Stars (BSS) in the bulge of the Milky Way galaxy. Proper motions from extensive space-based observations along a single sight-line allow us to separate a sufficiently clean and well-characterized bulge sample that we are able to detect a small population of bulge objects in the region of the color-magnitude diagram commonly occupied young objects and blue strgglers. However, variability measurements of these objects clearly establish that a fraction of them are blue stragglers. Out of the 42 objects found in this region of the color-magnitude diagram, we estimate that at least 18 are genuine BSS. We normalize the BSS population by our estimate of the number of horizontal branch stars in the bulge in order to compare the bulge to other stellar systems. The BSS fraction is clearly discrepant from that found in stellar clusters. The blue straggler population of dwarf spheroidals remains a subject of debate; some authors claim an anticorrelation between the normalised blue straggler fraction and integrated light. If this trend is real, then the bulge may extend it by three orders of magnitude in mass. Conversely, we find that the genuinely young (~5Gy or younger) population in the bulge, must be at most 3.4% under the most conservative scenario for the BSS population.