Institute of Astronomy

Barium Enhanced Blue Stragglers in NGC 6819

SpeakerTalk DateTalk Series
Katelyn Milliman29 July 2016Binary Stars Talks


Possible formation pathways for blue straggler stars include mergers in hierarchical triple systems, stellar collisions during dynamical encounters, and mass transfer from a giant companion. Extensive work on the blue stragglers in the old open cluster NGC 188 (7 Gyr) indicate that mass transfer is the dominant mechanism for blue straggler formation in open clusters. Such mass transfer events should pollute the surface abundance of the blue straggler with nucleosynthesis products from the evolved donor. The other formation pathways, mergers and collisions, are predicted to produce no such enhancements. I will present the results of a surface abundance study of the blue stragglers in the intermediate-aged open cluster NGC 6819 (2.5 Gyr). This work identified five blue stragglers with significant barium excess that indicates formation via mass transfer from an AGB donor, but surprisingly four of these stars show no radial-velocity evidence for a companion and the last star has a luminous secondary. I will discuss the conclusions we can draw from these results and our attempts to explain the origin of these blue straggler systems.