Effects of chemical composition and thermohaline mixing on the accreting components for low-mass close binaries: application to blue stragglers

Abstract

Blue stragglers (BSs) are important objects in cluster populations because of their peculiar properties. The colours and magnitudes of these objects are critical parameters in the population synthesis of the host cluster and may depend remarkably on the surface composition of BSs. Observations show that some BSs are short-orbital-period binaries, which may be accounted for by mass transfer in low-mass binaries. We have therefore studied the effects of surface composition and thermohaline mixing, caused by secular instability, on the accreting components for low-mass binaries, and we have applied the results to a short-orbital-period BS F190 in the old cluster M67. We examine thermohaline mixing in a low-mass accreting main-sequence star and find that, except for the redistribution of composition under the surface, the mixing affects the accretor very little during Roche lobe overflow unless thermohaline mixing is treated as an instantaneous process. A series of calculations is then carried out for low-mass binaries under different assumptions. The results indicate no distinction in surface composition between the models with and without thermohaline mixing during Roche lobe overflow, but we still see the divergences of evolutionary tracks on the Hertzsprung-Russell and colour-magnitude diagrams. The change of surface composition makes the gainer bluer and smaller than those with original surface composition, while thermohaline mixing lessens the effect slightly. If thermohaline mixing were to act instantaneously, the effect would be lessened more. Our calculation shows that case A and case B mass transfer may produce BSs in short- or relatively short-orbital-period binaries (including Algol systems), and that CNO abundance abnormalities could be observed in these products. This is consistent with the results of Monte Carlo simulations by previous studies. Our simulation of F190 shows that the primary's mass M-1i of the appropriate models is located in the range of 1.40-1.45 M-circle dot with initial mass ratio q(i) = 1.5 and initial orbital period P-i = 0.8 d, indicating that case A is a more likely evolutionary channel than case B to form this object. The simulation also shows that it is very likely that F190 is still in a slow stage of mass transfer. As a consequence, obvious CNO abundance abnormalities should be observed for the object.