Matthew Bailey
Berea College

REU program-Summer 2008
Univ. of Wisconsin - Madison
Madison, WI 53706

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Investigating the Creation of Blue Straggler Stars

Preliminary Topics - An Introduction to Open Clusters & CMD's

Open Clusters

Open clusters are physically related groups of stars held together by mutual gravitational attraction. Therefore, they populate a limited region of space, typically much smaller than their distance from us, so that they are all roughly at the same distance and the same age. On size, open clusters typically contain from 10 to 10,000 stars, unlike the globular clusters that contain hundreds of thousands to millions of stars, therefore in density, open clusters are comparatively diffuse. Consequently, the extent of cluster dynamics on Blue Straggler star (BSs) production in open clusters naively seems less applicable than in the dense globular clusters. In addition to star density, an extremely old open cluster (like NGC 188) may have an age of 7 Gyrs whereas globular clusters typically range from 10 to 20 Gyrs, and can actually be used to approximate the age of the universe.

Explaining the CMD

A Color Magnitude Diagram (CMD) is a plot of the absolute magnitude (a measure of the intrinsic brightness of stars) versus their B-V, or Blue minus Visual (green), color. The reason for this, as it turns out, is that stars with a given B-V color have the same absolute magnitude, and thus fall along the same line in the CMD, called the Main Sequence (MS). Keep in mind, smaller magnitudes mean intrinsically brighter stars, so brighter stars lie at the top of the CMD and the B-V color increases for cooler temperatures for the same reason a blue flame is hotter than a red one.

Observed CMD of NGC 188
Observed CMD of NGC 188
Observational Results Provided by Geller et al. 2008

Introduction & Motivation

As studies of open (and even globular) clusters amounted, certain stars that were in fact members, strangely appeared younger than the rest of the cluster. Their position above and blueward of the main sequence made these members the more baffling. The reason is because had they been single stars, then standard stellar evolution showed that there was no reason that they should appear so young, and therefore should not lie above and blueward of the main sequence. These so called Blue Straggler Stars (BSs), must have therefore undergone some process that hence rejuvenated, or made younger, their apparent ages.

Purpose of Study

The purpose of this project is to use a binary evolution simulation that evolves stars from birth to a given age to determine whether dynamical interactions between cluster stars are necessary to produce the variety of BSs observed. By comparing the nature of simulated BSs to observation and correlating these characteristics with specific formation pathways of the BSs, we will be more apt to resolve whether BSs can form without considering cluster dynamics.

Open Cluster NGC 188
Open Cluster NGC 188
Photograph by Martin Germano

The cluster chosen to model is NGC 188 because its old age offers the chance to observe the effects of interactions between stellar, binary, and cluster evolution. In addition, old clusters have a rich population of evolved stars. Finally,NGC 188 is well studied and its Blue Straggler Star (BSs) populations reveals markedly diverse natures, as not only are its BSs found in a mixture of short and long period binaries but also a variety of eccentricities and as single BSs.

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