The Simulation & the More Detailed Formation Pathways It Examines

Since the goal of this project was to test the feasibility of creating all of the Blue Straggler Stars (BSs) observed in open clusters through binary evolution only, the Binary Star Evolution code or BSE (Hurley, Tout, & Pols 2002) was used. The BSE is a comprehensive assemblage of current single and binary star evolution theory, packaged into a FORTRAN algorithm. In this form, the extent of cluster dynamics in the formation of BSs can measured by observing how clusters should evolve if only governed by the non-dynamical mechanisms of binary evolution by measuring the simulated results against observation.

Hypothesized Formation Pathways

While the hypothesized mechanism for the creation of BSs remains to be through binary mass transfer,the simulation examines a wider and more detailed range of formation Pathway, as Hurley implements within the BSE a treatment for documenting and classifying the types of mass transfers that can occur during the evolution of binary stars, they are;

• Merger: occurs when two Main Sequence Stars (MS) spiral in and subsequent merger producing a single BSs.

Types of Roche Lobe Overflow - Mechanisms of RLOF

• RGB-MS (or HG-MS) Wind: Denotes wind accretion from a Red Giant Branch (RGB) star sub-giant, or Hertzsprung Gap (HG) star onto a MS star

• Supergiant-MS Wind: Denotes wind accretion from a super-giant star to an MS star.

Since all the stars belonging to a given cluster are at approximately the same distance, then their positions on the CMD are governed by only one parameter - their mass. Therefore as one moves to smaller magnitudes and to bluer colors following the main sequence (MS), the mass of the stars tend to increase. Moreover, the stars at the MS turn-off represent the most massive stars remaining on the MS. From observation, there appears to be a significant number of BSs in binary systems that have a net mass greater than twice that of the stars present at the MS turn-off of the cluster. This observation violates a prediction made by McCrea (1964) for BSs created by binary evolution alone suggests that the mass of a BSs binary system should be below twice the turn-off mass. In other words, say two of the most massive MS stars happen to be in a binary that transfers mass and makes a BSs. The Resulting mass of the BSs binary system then cannot be greater than twice that of the turn-off. In order to explain such an observation, one must evoke dynamical encounters from within the cluster environment.

When considering dynamical mechanisms of the evolving cluster, Hurley et al. (2001) showed that the cluster environment is able to boost BSs production and that the diverse nature of the BSs observed in the open cluster M67 can be explained if allowed to form through various dynamical as well as evolutional mechanisms. As an example, M67 contains what is called a super blue straggler because it has a mass of more than twice that of the stars on the main sequence turn-off, and as previously mentioned by McCrea (1964), this should not be possible. Although standard binary evolution could not account for its existence, Hurleys N Body simulation indeed recreated a similar star as a result of a MS-MS star merger that made a single BS, which then interacted with other members of the cluster (dynamically). The result of this dynamical interaction was an exchange of orbits with yet another MS companion that was eccentric. Subsequently, a collision between the BSs and the new MS companion formed a super BS. Still, the model did not generate enough BSs in binaries at any single moment, as only 25% of the BSs where in binaries when the model neared the age of M67.