In Xin et al. (2015) we present binary evolution models, including case-B mass transfer and binary merging, specifically calculated for an old (13 Gyr) and metal-poor (Z=0.0001) star cluster as M30. The distribution in the color-magnitude diagram of synthetic blue straggler stars (BSSs) generated from these models: (i) well reproduces the location of the red sequence of BSSs discovered by Ferraro et al. (2009), and (ii) never attains the location of the blue BSS sequence. These results support the hypothesis that red BSSs formed through mass transfer in binary systems, while blue BSSs have been generated by a different process (likely collisions).
Here we provide the full version of Table 2 of the paper, listing the magnitude and colors of 13 Gyr old binaries characterized by different values of the progenitor masses and orbital separation.
GRID OF MASS-TRANSFER
BINARY MODELS
The figure below shows the evolutionary tracks for an illustrative example of binary system (0.9 Msol + 0.5 Msol, initial orbital radius = 2.7 Rsol) undergoing case-B mass transfer, in the color-magnitude diagram. The evolutionary tracks of the primary (donor) and secondary (accretor) stars are shown as dashed and dotted lines, respectively. The track of the binary system (thick solid line) has been obtained by combining the luminosity contributions of the primary and secondary components. Different symbols mark a few key events in the binary evolution: the beginning of mass transfer (open squares), the epoch at which the mass ratio is equal to one (open stars), the end of the mass transfer process (solid squares), and the 13 Gyr age (open circles). For the sake of comparison, the 7.54 and 13 Gyr isochrones at the appropriate metallicity of M30 (Z=0.0001) are also plotted in solid lines.
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