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Origins of Blue Straggler Stars in Open Clusters, Globular Clusters and Dwarf Spheroidal Galaxies Origins of Blue Straggler Stars in Open Clusters, Globular Clusters and Dwarf Spheroidal Galaxies 〓, 正〓 ザオ, ジェンシイ ZHAO, Zhengshi Blue straggler stars (BSSs) lie above the current main sequence turn off on the color-magnitude diagram of star cluster. Their positions on the color-magnitude diagram imply that they are more massive than current main sequence turn-off stars in the same cluster. Thus, their presence of such an upper main sequence was a challenge for standard stellar evolution theory, as normal stars with the same mass have already evolved into white dwarfs. To explain their extraordinary long lifespan on the main sequence, various mechanisms have been suggested, all involving in additional hydrogen material input to normal stars. Two of the most popular mechanisms are merger by direct collisions in single stars and coalescence or mass transfer in close binary systems. As products of dynamical processes in the host cluster and stellar evolution itself, studying BSSs can shed a light on the process of merger and/or mass transfer, and be a tracer of the dynamical evolution of the host cluster. The first discovery of BSSs in a globular cluster M3 dates back to six decades ago. Since then, many observational ndings suggest that BSSs are present in globular clusters, open clusters, as well as dwarf spheroidal galaxies (dSphs), and even in the elds and the bulge of the Milky Way and imply that dierent formation mechanisms may operate in different dynamical environments. Though we are still facing with problems in understanding these objects clearly and completely. The aim of this thesis is to derive a comprehensive interpretation for origins of BSS by investigating the BSS properties in dierent star systems, such as open clusters, globular clusters and dSphs based on observational data of the highest quality up to the present day. We have collected homogeneous deep photometric data for 9 dSphs taken from Subaru/ Suprime-Cam. The sample includes 3 classical dSphs, Draco, Ursa Minor (UMi) and Sextans and 6 ultra faint dwarf galaxies. For these classical dSphs, we have analyzed the BSSs' radial distributions and luminosity functions. BSSs radial distribution shows a lack of strong central peak and luminosity function is independent of radius. These results imply that in these dSphs, the blue plume stars are most likely to be real BSSs formed mainly by mass transfer in primordial binaries. The frequency of BSSs in the 9 galaxies does not correlate with galaxy parameters, such as half light radius rh, metallicity [Fe/H], central surface brightness μV;C, concentration parameter c, central density pc, velocity dispersion δV , half mass relaxation time log trh and collisional rate Γ , as well as integrated absolute magnitude, MV. This result suggests that the evolution of primordial binaries, which would form BSSs, was not affected significantly by dynamical interactions in such sparse environments. Thus, the observed BSS frequency is almost the same in these galaxies under the simple hypothesis that the primordial binary fraction was the same in all galaxy. We have also collected homogeneous deep photometric data for 51 GC obtained by ACS/HST. 8 GCs in our sample show bimodal BSSs frequency radial distributions. Among them, the bimodal distribution is reported for the first time in NGC 5286, NGC 5986, NGC 6362, NGC 6535, NGC6541 and NGC7099, in total 6 GCs. It has raised the number of this kind of GCs by twice the number known before. Moreover, the significant anticorrelation between BSS frequency and MV is confimed with our sample. BSS frequency is also significantly anti-correlated to central density, central velocity dispersion and collision rate. Furthermore, luminosity functions of BSSs in bright (MV < -8.8) and faint (MV > -8.8) clusters do not show significant differences. One possible interpretation for all these properties is that BSSs formation from binaries is the dominant mechanism in globular clusters. However, The effect of collision on BSS formation is still considerable because collisions destroy primordial binaries and/or accelerate evolution of binary systems. Finally, we have observed 30 BSSs and 9 main sequence turn-off candidates in three old Galactic open clusters, NGC 2682, NGC 2112 and NGC 6819. Among them, radial velocities (Vr) of all target stars in NGC 2682 and NGC 6819, and 5 BSSs and all TOs in NGC 2112 are consistent with mean Vr of member stars in the clusters. Rotational velocity (vsini ) of BSSs spans a wide range from few kms-1 to hundreds km_s-1, while those of turn-off stars vary in small range from few kms-1 to 25 kms-1. The high rotation velocity of current BSS is consistent with predictions both from mass transfer and collision formation mechanism. Some turn-off stars and a few of BSSs show normal Li abundances. For the other BSSs, only the upper limit of Li abundance is estimated. Interestingly, BS1371 in NGC 2112 and BS 10010 in NGC 6819, show very high Li abundance, A(Li) 3.94 dex and A(Li) 3.52 dex. Two BSSs show depletion in oxygen abundance, while the most of BSSs show similar oxygen abundance to turn-off stars. Some BSSs show depletion in carbon abundance, while other BSSs show the normal carbon abundance comparing to turn-off stars. These abundance anomalies in light elements found in some BSSs suggest that mass transfer from such highly evolved stars is the origin of these BSSs. On the other hand, the progenitor of other BSSs with normal carbon and oxygen abundances and low lithium abundances might be normal red giant stars. 総研大甲第1543号 eng thesis https://ir.soken.ac.jp/records/3573 博士（理学） 2012-09-28 総合研究大学院大学 https://ir.soken.ac.jp/record/3573/files/甲1543_要旨.pdf application/pdf 282.5 kB 2016-02-17