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

摘要

We studied the effects of surface composition and thermohaline mixing caused by secular instability on the accreting components for low--mass binaries and applied the results on a short-orbital-period BS F190 in the old cluster M67. 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 HRD and CMD. The change of surface composition makes the gainer bluer and smaller than the ones 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. Our simulation of F190 shows that the primary's mass $M_{\rm 1i}$ of the appropriate models is located in the range of 1.40 to 1.45$M_{\odot}$ with initial mass ratio $q_{\rm i}=1.5$ and initial orbital period $P_{\rm i}=0.8$ days, 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.