SCYON Abstract

Received on August 4 2006

The blue stragglers formed via mass transfer in old open clusters

AuthorsB. Tian(1,3), L. Deng(1), Z. Han(2) and X.B. Zhang(1)
(1) National Astronomical Observatories, CAS, Beijing 100012, P.R. China
(2) National Astronomical Observatories/Yunnan Observatory, CAS, Kunming, 650011, P.R. China
(3) Graduate University of Chinese Academy of Sciences, Beijing, 100049, P.R. China
Accepted byAstronomy & Astrophysics


In this paper, we present the simulations for the primordial blue stragglers in the old open cluster M67 based on detailed modelling of the evolutionary processes. The principal aim is to discuss the contribution to the blue straggler population in M67 from mass transfer between the components of close binaries. Firstly, we follow the evolution of a binary of 1.4M(sun) + 0.9M(sun). The synthetic evolutionary track of the binary system reveals that a primordial blue straggler can stay at the observed blue straggler region in Color-Magnitude diagram with a long lifetime to secure observable probability. Secondly, a grid of models of close binary systems experiencing mass exchanging are computed from 1Gyr to 6Gyr in order to account for primordial blue straggler formation in a time sequence. Based on such a grid, Monte-Carlo simulations are applied for the old open cluster M67. Adopting appropriate orbital parameters, 4 primordial blue stragglers are predicted by our simulations. This is consistent with the observational fact that only a few blue stragglers in M67 are binaries with short orbital periods. An upper boundary of the primordial blue stragglers in the Color-Magnitude diagram is defined and can be used to distinguish blue stragglers that are not formed via mass exchange. Using the grid of binary models, the orbital periods of the primordial BSs can be predicted. Compared with the observations, the predicted blue stragglers via mass exchange from the simulations is insufficient to fit the blue stragglers population in M67. There should be several processes to form the observed blue stragglers in M67.