Date of Award

8-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics

Advisor

Dr. Jeremy King

Committee Member

Dr. Sean Brittain

Committee Member

Dr. Bradley Meyer

Committee Member

Dr. Jens Oberheide

Abstract

A population of stars exists in the old, open cluster M67, whose photometry, color magnitude diagram locations and associated evolutionary states cannot be explained by current, standard single star evolution theory. These stars are often referred to as ``yellow straggler' stars. Yellow stragglers have been identified in multiple star clusters suggesting that these stars constitute a real population. Additionally, according to independent studies, at least some of the yellow straggler stars in M67 are likely cluster members. Therefore, cluster non-membership is not a sufficient explanation for the observed anomalous photometry of these stars. It is possible that the yellow stragglers occupy their precarious color magnitude diagram positions as a result of the evolution of mass transfer blue straggler stars. These are stars which have been formed by Roche Lobe overflow mass transfer in close binary systems. If this the case for the yellow stragglers, it is hypothesized that they could potentially exhibit two spectroscopic characteristics that can be indicative of this type of mass transfer system. Specifically, variable radial velocities can be used to indicate that the yellow stragglers exist in binary systems and enhancements of s-process elements in yellow stragglers can indicate Roche Lobe overflow mass transfer from a once asymptotic giant branch star which has since evolved into a white dwarf. This Dissertation details the radial velocity survey and the chemical abundance analysis that have been conducted to investigate the yellow stragglers with regard to this hypothesis. The radial velocity survey revealed that eight of the ten yellow stragglers studied exhibit variable radial velocities indicating that the yellow straggler population of M67 possess a high binary frequency. However, the chemical abundance analysis revealed that none of the yellow stragglers exhibited enhancements of the s-process elements Y and Ba. Therefore, a history which involves Roche Lobe overflow mass transfer cannot be confirmed for the yellow straggler stars.

Share

COinS