Blue Straggler Binary Star Formation Mechanism

In recent years, blue stragglers (BSs) received more and more attentions. Because BSs are bright and blue, a few BSs may contribute remarkably spectral energy in the blue and ultraviolet, and affect the integrated spectrum of the host clusters. Hence, BSs are important in the reseach of clusters and groups. The characteristic of BSs, i.e. luminosity, effective temperature, gravity etc, are relevant to their formation channels. Researchers gradually pay more attention to binary origin for BSs as much evidence for the existence of binaries is found in many clusters. In the thesis, we summarize the observational and theoretical studies of BSs, especially for the study of binary origin, and present the results of our studies on the field.We have studied the properties of BSs from binary mass transfer in details. Thermohaline mixing is included as a time-dependant process in the code. The study shows that BSs from this way have abnormal surface composition, i.e. CN reversion, He enhancement and Li depletion. After mass transfer, the abnomality of surface composition may be lessened because of thermohaline mixing, but it will not disappear in a short timescale. Meanwhile, mass transfer (case A and case B) may only account for BSs in mid- and short orbital period binaries (P < 40 d).A typical observational short-orbital-period BS F190 in the old cluster M67 is examined. Our grid simulation shows that the primary's mass M_li of the appropriate models is located in 1.40 and 1.45M。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 study also shows that F190 is still in a slow stage of mass transfer, with a cool companion. As a consequence, obvious CNO abundance abnormality should be observered for the object. The mass ratio of F190 to its companion is about 5, which means that it is difficult to obtain the information of the companion except for a very high S/N.An important parameter — the core-envelope transition point in the primary of contact binaries with two main-sequence components, may significantly affect the eventually remnant of the binaries. We studied the influence of the choice of the point and found that, we may ignore the effect resulting from the different choices of the point on the colours, magnitudes and BS lifetime of the merger, if the point is outside the nuclear reaction region of the primary. However if the point is inside the nuclear reaction region, with the point moving closer to the center, the merger will be bluer and have a larger luminosity, meanwhile, a longer BS lifetime. Up to now, the nuclearreaction region is considered as a reasonable choice of the core-envelope transition point for a main-sequence star.We systematically studied the mergers of slow and rapid mass transfer from case A evolution — binary merge via W UMa. The core-envelope transition point is determined as that 99 per cent of total luminosity is produced by the core, and the matter from the secondary homogeneously mixed with the envelope of the primary. The mass of the primary M\\ is located in 0.89-2.00Mo. Followings are the initial results on the studies: (l)With the increase of mass, the central hydrogen abundance of the mergers does not decrease. It means that there isnot a blueward concentration with mass decreasing, which is different from the predict of Sandquist et al.(1997).(2) Both of the two channels cannot explaine the BSs near the turnoff of the cluster, except for a large mass loss during coalesence, or a large angular momentum loss for less mass binaries (Mh<0.89 Mo) , resulting in a short timescale from detact to contact. The BSs in this region may result from dynamical unstable mass transfer.