Primordial binary evolution and blue stragglers

Blue stragglers can significantly enhance the spectral energy towards short wavelengths, especially in ultraviolet and blue bands. Much evidence shows that blue stragglers are relevant to primordial binaries. In this paper, we systematically studied blue stragglers produced from primordial binary evolution via a binary population synthesis approach, and examined their contribution to the integrated spectral energy distributions of the host clusters. The mass transfer efficiency, beta, is an important parameter for the final products (then blue stragglers) after mass transfer, and it is set to be 0.5 except for case A binary evolution. The study shows that primordial binary evolution may produce blue stragglers at any given times, and that different evolutionary channels correspond to blue stragglers in different visual magnitude regions ( in V band) on the colour-magnitude diagram (CMD) of clusters. The specific frequency of blue stragglers obtained from primordial binary evolution decreases with time first, and then increases again when the age is larger than 10 Gyr, while that from angular momentum loss induced by magnetic braking in low-mass binaries increases with time and exceeds that of primordial binary evolution in a population older than 3 Gyr. Meanwhile, blue stragglers resulting from primordial binary evolution are dominant contributors to the integrated spectral energy distributions (ISEDs) in ultraviolet and blue bands in a population between 0.3 and 2.0 Gyr. The value of beta significantly affects the final results, e. g. the specific frequency of blue stragglers decreasing with beta, blue stragglers produced from a high value of beta being more massive, then contributing more to the ISEDs of the host clusters. For old open clusters, the assumption of beta = 1 when the primary is in HG at the onset of mass transfer matches the observations better than that of beta = 0.5 from the locations of blue stragglers on the CMDs. Our study also shows that, for most Galactic open clusters, the specific frequency of blue stragglers obtained from our simulations is lower than that of observations, which is puzzling. Nevertheless, primordial binary evolution cannot account for all blue stragglers observed in old clusters.