Publication: MOCCA code for star cluster simulations - I. Blue stragglers, first results

In February 2013 the first paper from the series of papers about the MOCCA code was published in the Montly Notices of the Royal Astronomical Society (Hypki et al. 2013).

The paper presents first results concerning blue straggler stars which were obtained with the new and improved version of the Monte Carlo code, called MOCCA. It combines the Monte Carlo method for star cluster evolution and the FEWBODY code to perform scattering experiments. The FEWBODY code was added in order to track more precisely dynamical interactions between objects which can lead to the creation of various exotic objects observed in star clusters (in example blue stragglers). The MOCCA code is currently one of the most advanced codes for simulating real size star clusters. It follows the star cluster evolution closely to N-body codes but is much faster. We show that the MOCCA code is able to follow the evolution of blue stragglers stars (BSs) with details. It is a suitable tool to perform full scale evolution of real star clusters and detailed comparison with observations of exotic star cluster objects like BSs.

This paper is the first one of a series of papers about properties of BSs in star clusters. This type of stars are particularly interesting today, because by studying them one can get important constraints on the link between the stellar and dynamical evolution of star clusters. We discuss here first results concerning BSs for an arbitrary chosen test model. We investigate properties of BSs which characterize different channels of formation like masses, semi-major axes, eccentricities and orbital periods. We show how BSs from different channels change their types, and discuss the initial and final positions of BSs, their bimodal distribution in the star cluster, lifetimes and more.

A few selected conclusions from the paper are presented here.

Mass ratios for long period Evolutionary Mass Transfer BSS (EMT) fit to the equation q=M{turn-off}/M{WD}. In the nominator there are masses of long period EMT, which are just slightly larger than M{turn-off} (typically 0.1*M{turn-off} after 500\302\240Myrs). WDs are companions in the long period EMT. Thus, in the denominator there are masses of WDs calculated based on Chernoff (1990, Tab. 1). It shows that mass ratios of long period EMT have a narrow range and predictable values through the entire simulation.

For many BSs there is a significant delay before the actual detection. The last merger or the last mass transfer can happen even several Gyrs before BSs actually exceeds the turn-off point. This effect was not expected in common scenarios for creation BSs. It was rather assumed, that mergers between stars create BSs immediately. Dormant BSs seem to be important, because there were created overall 112 dormant BSS from the total 476 BSs which gives 24%.