Two blue-straggler sequences discovered in globular cluster M30 provide a strong constraint on the formation mechanisms of blue stragglers .
We study the formation of the blue-straggler binaries through binary evolution , and find that binary evolution can contribute to the blue stragglers in both of the sequences .
Whether a blue straggler is located in the blue sequence or red sequence depends on the contribution of the mass donor to the total luminosity of the binary , which is generally observed as a single star in globular clusters .
The blue stragglers in the blue sequence have a cool white-dwarf companion , while the majority ( \sim 60 \% ) of the objects in the red sequence are the binaries that are still experiencing mass transfer , but there are also some objects that the donors have just finished the mass transfer ( the stripped-core stars , \sim 10 \% ) or the blue stragglers ( the accretors ) have evolved away from the blue sequence ( \sim 30 \% ) .
Meanwhile , W UMa contact binaries found in both sequences may be explained by various mass ratios , that is , W UMa contact binaries in the red sequence have two components with comparable masses ( e.g .
mass ratio q \sim 0.3-1.0 ) , while those in the blue sequence have low mass ratio ( e.g . q < 0.3 ) .
However , the fraction of blue sequence in M30 can not be reproduced by binary population synthesis if we assumed the initial parameters of a binary sample to be the same as that of the field .
This possible indicates that dynamical effects on binary systems is very important in globular clusters .