Direct N-body Modelling of Stellar Populations: Blue Stragglers in M67

Jarrod R. Hurley, Christopher A. Tout, Sverre J. Aarseth and Onno R. Pols

We present a state-of-the-art N-body code which includes a detailed treatment of stellar and binary evolution as well as the cluster dynamics. This code is ideal for investigating all aspects relating to the evolution of star clusters and their stellar populations. It is applicable to open and globular clusters of any age. We use the N-body code to model the blue straggler population of the old open cluster M67. Preliminary calculations with our binary population synthesis code show that binary evolution alone cannot explain the observed numbers or properties of the blue stragglers. On the other hand, our N-body model of M67 generates the required number of blue stragglers and provides formation paths for all the various types found in M67. This demonstrates the effectiveness of the cluster environment in modifying the nature of the stars it contains and highlights the importance of combining dynamics with stellar evolution. We also perform a series of N = 10 000 simulations in order to quantify the rate of escape of stars from a cluster subject to the Galactic tidal field.

The N-body simulation of M67 starts at an evolution age of 2 500 Myr and contains 5 000 binaries and 5 000 single stars initially. The Hertzsprung-Russell diagram and the spatial distribution in the XY-plane are shown at various epochs. In each figure main-sequence stars (black points), blue stragglers (filled blue circles), sub-giants and giants (filled red circles), naked helium stars (cyan points) and white dwarfs (magenta points) are distinguished. Binary stars are denoted by overlapping filled circles of colours appropriate to the stellar type of the components, with main-sequence binary components shown as green.

  • 2 933 Myr H-R diagram and XY-plane
  • 3 077 Myr H-R diagram and XY-plane
  • 3 653 Myr H-R diagram and XY-plane
  • 3 798 Myr H-R diagram and XY-plane
  • 4 014 Myr H-R diagram and XY-plane
  • 4 302 Myr H-R diagram and XY-plane

  • This work is to appear in the Monthly Notices of the Royal Astronomical Society.

    To obtain a preprint of the paper contact Christopher Tout.

    4th August, 2000.