Blue stragglers




Ouellette, John Anders

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Blue stragglers are enigmatic stars which appear to have undergone some form of rejuvenation, bringing them near the zero-age main sequence of the cluster in which they reside. The most likely explanation for the existence of these stars is that they have formed recently, through the merger of two stars, either through a direct stellar collision, or through binary mass-transfer and coalescence. This thesis presents models of the remnants of these processes, and a comparison of the predictions of these models with observed blue stragglers in several clusters. The predictions of smoothed particle hydrodynamic simulations of colliding stars have been used to create models appropriate for input into a stellar evolution code. Since these models develop only thin, short-lived, convective envelopes, angular momentum loss via a magnetically driven stellar wind is unlikely to be a viable mechanism for slowing the rapidly rotating blue stragglers predicted by the collisional scenario. Angular momentum transfer to either a circumstellar disk (possibly collisional ejecta) or a nearby companion remain plausible mechanisms for explaining the low rotation velocities observed for most blue stragglers. In addition to these models of collisional mergers, simplistic models of the remnants of binary coalescence and mass-transfer were also developed. The predictions of both sets of models were compared with the observed blue straggler populations of six globular clusters (NGC 104, NGC 2419, NGC 5024, NGC 6809, NGC 7099). While most of the clusters' blue stragglers appear to be well matched by the predictions of the collisional mergers, the blue stragglers in the cluster with the highest central density, NGC 7099, appear to be a hybrid population of both collisional and binary mergers. The blue stragglers of NGC 2419—the least dense of the clusters studied here—are well matched solely by the predictions of the collisional mergers of equal mass stars. However, due to the low density of this cluster, it is likely that some fraction of these blue stragglers are being formed via binary mergers and that the mergers and that the simple models of binary mergers used here are inadequate.



Galaxies, Clusters, Cosmology