The Effect of Pre-Main Sequence Evolution on Star Cluster Dynamics

Abstract

The effects of adding pre-main sequence stellar evolution to a stellar dynamics program is investigated. Based on available stellar evolution tracks, pre-main sequence evolution from birth to the zero age main sequence was implemented into the popular dynamics code Starlab. Medium-sized star clusters were modeled under different circumstances, paying special attention to the differences in stellar population. In all, 3 sets of simulations were used. The first was a control set with all stars starting at the main sequence. The second used similar parameters as the first, but with stars beginning their evolution at the pre-main sequence. Because pre-main sequence stars have such large radii, a large number of the binary stars were in contact. For the third set, the binary parameters were adjusted to ensure that all of the binary stars were detached. The second set of simulations produces a luminosity profile that is dominated by high magnitude stars in the early years of the clusters. It also experiences a large number of mergers, which affect a number of dynamical properties of the models. The mergers lower the binary function of the clusters, which slightly affect the behaviour of its core. More intermediate mass stars abound in the clusters, which leads to higher mass loss through stellar evolution and more high velocity escaping star systems. Fewer blue stragglers are observed since many of the close binaries merge very early on in their existence. The third set ofsimulations yields similar results, but mostly for different reasons. There are very few mergers in this implementation, but since there are few hard binaries and more soft binaries many of the multiple systems break up, yielding a similay binary fraction to that of the second set of simulations. Very few of the binaries in these models circularize in stark comparison to the first two sets of models, iii which experience circularization in a fraction of its binaries. These models also end up having a slightly higher concentration at the end of the simulation, with a core density of roughly 3 times that of the other sets after 1.5 Gyr. In general, adding pre-main sequence evolution to star cluster simulations decreases the binary fraction and the number of hard binaries in the cluster. Thus pre-main sequence evolution should be computed for high detail simulations.