| 其他摘要 | Globular clusters (GCs) were thought as the ideal simple stellar populations, which means all of the stars in a given GC share the same age and same initial chemical composition. However, a large number of spectroscopic and photometric studies provided evidence for multiple stellar populations in GCs. For example, the splits (or a broadening) in the main sequence (MS), red giant branch (RGB), and horizon branch have been found in the color–magnitude diagram (CMD). In addition, the abundances of light elements show inhomogeneities among stars within the same GC.Binary systems play an important role in the evolution of GCs. Many exotic stars can be produced through binary interactions, such as blue stragglers, cataclysmic variable stars, millisecond pulsars, X-ray binaries and so on. Besides, Binary interactions can also produce stars with anomalies abundances, such as Ba star, CH star, carbonenhanced-metal-poor star and so on. But it remains uncertain whether the mass transfer in binaries can reproduce the abundance anomalies observed in GCs. What’s more, it is important to understand what fraction of stars on the red giant branch of GCs is the result of binary interactions, which may provide clues on the formation of multiple stellar populations in GCs.In this thesis, we focus on the contribution of binary interactions on the multiple stellar populations in GCs. We use the MESA (MODULES FOR EXPERIMENTS IN STELLAR ASTROPHYSICS) code to calculate binary evolutionary models, and investigate the contribution of stable Roche lobe overflow (RLOF) mass transfer of low-mass binaries on chemical anomalies among unevolved stars in GCs. In addition, we performed a binary population synthesis study to track the number of post-binaryinteraction (post-BI) stars that appear on the red giant branch, with particular emphasis on the evolved blue straggler stars (E-BSSs). The main results are summarized as follows:(1) The results show that in some low-mass binary systems, the accretors exhibit peculiar chemical patterns when they are still unevolved stars, e.g. C and O depletion; Na and N enhancement; and constant Mg, Al, and C + N + O. The abundance patterns of the accretors are significantly different from their initial abundances (or that of normal single stars), and can match the observed populations.(2) The surface abundances of the accretors are directly related to the abundance profiles of the donor stars, the amount of material transferred, and the dilution on the accretors. Our results show that the initial parameters of the binary system (the donor mass, initial mass ratio, and initial orbital period) and various assumptions (mass-transfer efficiency and magnetic braking) play important roles in the abundance anomalies of the accretors. The surface of the accretors shows a stronger enhancement of Na in binary systems with more massive donors, lower mass ratio, and shorter initial orbital period under the same mass-transfer episodes. The accretors in binary systems with magnetic braking and larger mass-transfer efficiency show stronger Na enhancement.(3) Assuming an initial binary fraction of nearly 50%, we find that about half of the objects on the RGB underwent the binary interactions, and a large fraction (∼42.98%) of giants formed from the wind accretion channel. E-BSSs account for around 10% of the giants in our standard simulation set.Our results show that the stable RLOF scenario in low-mass binary systems is able to reproduce main-sequence stars or early subgiant stars with peculiar chemical patterns, and probably alleviate the mass budget problem of multiple generation scenarios to a certain extent. In addition, we find that nearly half of these giants underwent the binary interactions, which means that the binary interactions strongly affect the evolution of giants in GCs. |
修改评论