Abstract We have studied the influence of different choices of core-envelope transition point on the final merger of contact binaries with two main-sequence components. A binary of 1.00+0.90 M⊙ with an initial orbital period of 0.35 d is examined. The mass fraction of the primary mixed with the matter of the secondary, qmix, determined by the chosen core-envelope transition point, ranges from 0.04 to 1.00 in our analysis. If as qmix<0.8, none of the helium-rich matter in the center of the primary is mixed into the envelope, and there is little distinction in the evolutionary tracks of the mergers. The timescales of the mergers remaining on the main sequence, tBS, are very similar (~6.2×108 yr) if qmix<0.71, since no hydrogen-rich matter of the secondary is mixed into the core of the mergers; for qmix>0.71, the larger qmix is, the greater the mixing, hence the longer the blue straggler lifetime, tBS, and also the greater the luminosity. For qmix = 1.00, tBS~8.5×108 yr. Estimation by ∇r−∇a = 0.0 shows that the point at which tBS begins to increase is about qmix = 0.68. In comparison with the homogeneously mixed models, the merger with a helium profile similar to that of the primary is less luminous and has a shorter tBS.

Keywords stars: binaries --- stars: evolution --- stars: blue stragglers

https://doi.org/10.1088/1009-9271/5/1/008