Analysis Of The Chemical Abundance Of The Milky Way And Magellanic Clouds

The Milky Way and its neighbouring galaxies,the Large and Small Magellanic Clouds,are the only typical galaxies where single stars can be distinguished,as thus they are the best laboratory to study the formation of the large-scale structure of the Universe in depth.The chemical abundances of stars in the Milky Way and its neighbouring galaxies are not only traces to probe the nucleosynthetic processes of various elemnts and chemical evolution of the Milky Way and its neighbouring galaxies,but also to interpret the early merger history of galaxy formation and to trace the chemical enrichment history of the entire Universe at different structural levels.This will provide observational constraints on the theory of galaxy formation and evolution.Our work is to analyse the stellar elemental abundances in the Milky Way and the Large and Small Magellanic Clouds,to trace the chemical evolution of galaxies through the chemical abundance signatures of different elements,and to analyse the formation sites and histories of different star families and the mechanisms of elemental nucleosynthesis using certain elemental abundance ratios and isotopic abundance ratios.The specific work consists of the following two parts.In the first part,we analyse the stellar chemical abundances of the Magellanic Clouds（MCN）and the similarities and differences between the chemical evolution of the Large and Small Magellanic Clouds（LMC）and the Milky Way,and explore the interactions between the LMC and the Milky Way.This part of the work is based on the stellar elemental abundance data obtained from the APOGEE high-resolution spectroscopic survey,from which 7121 red giants belonging to the LMC and SMC members（Sample A）were selected,and the abundance characteristics of 12 elements in Sample A were analysed,including the lighter elements C and N,theαelements（O,Mg,Si,Ca,Ti）,the odd Z elements Na,K and Al,and the iron group elements（Cr,Mn,Co,Ni）.A sample of Galactic red giants,comprising 20,839 red giants（Sample B）,was also selected from the stellar elemental abundance data given by the aforementioned APOGEE high-resolution spectroscopic survey,and the chemical abundance distributions of the aforementioned 14 elements were also given and compared to the abundance distributions of individual elements in Sample A.The results show that:（1）Elemental abundance characteristics of the Magellanic Clouds:The alpha elemental abundance distribution of the Magellanic Clouds differs from that of the Milky Way,with[α/Fe]being lower in the Magellanic Clouds than in the Milky Way.The abundances of allαelements are fairly consistent at the metal-rich end,and the[Mg/Fe]and[Ca/Fe]abundances are in good agreement between severalαelements.The Magellanic Clouds do not have the same C and N abundance patterns,but all are lower than the Milky Way at the metal-rich end.the abundance distributions of K and Al elements in the LMC and SMC are similar to those of theαelements.In addition,the abundance distribution of Fe-group elements in the LMC and SMC is consistent.In the Magellanic Nebula,some elements（C,N,Cr,Co）have differences and a large scatter.It indicates that the chemical enrichment history of the Magellanic Clouds is not uniform,especially for the SMC.（2）Relative to the Milky Way:among theαelements,both the Magellanic Clouds and the Milky Way show an inconsistent abundance distribution of[Ti/Fe]with the otherαelemental abundances.At[Fe/H]>-1.5,[Ti/Fe]both increase with[Fe/H].In the range-2.6<[Fe/H]<-1.5,the alpha,C+N,K,Fe peak and Al elements of the Magellanic Nebula are similar to those of Galactic halo stars.The Mn abundance distribution in the Magellanic Clouds is generally consistent with the Galactic trend,with the Mn abundances found to vary around[Fe/H]=-1.5.The[Ni/Fe]abundance of the Magellanic Clouds shows a deficit relative to the thin and thick disk of the Milky Way,below the position of the thin and thick disk of the Milky Way.The[Al/Fe]-[Mg/Fe]abundance of the Magellanic Clouds is lower than that of the Galactic stars.The SMC and disk stars can be roughly distinguished among the three abundance patterns[Al/Fe]-[Fe/H],[Al/Fe]-[Mg/Fe],[Al/Fe]-[N/Fe]and[Al/Fe]-[Mg/Mn].The above results indicate that:at[Fe/H]>-1,the LMC shows an increasing[α/Fe]-[Fe/H]abundance pattern,suggesting a more intense stellar outburst at this time compared to the Milky Way,when many type II SNe drove the abundance ofαelements.the low[C/N]abundance at the metal-rich end of the LMC suggests that at the same metal abundance,star formation at the LMC is more recent than that of the Milky Way.Furthermore,the pattern of[（C+N）]suggests that the Magellanic Clouds contribute more to the AGB enrichment than the Galactic high-alpha sequence.The lack of[Al/Mg]abundance in the Magellanic Clouds relative to the Milky Way may be a consequence of the overall lower type II SNe occurring in dwarf galaxies.The lower star formation efficiency may result in a large’lag’between the metal abundance of interstellar material and that of stars with enriched interstellar matter.The second part is an analysis of the Ba and Eu abundances of 48 halo stars in the Milky Way,including 29 high-αand 19 low-αhalo stars,with the aim of analysing the neutron capture abundances and nucleosynthesis mechanisms of high-αand low-αhalo stars,and thus tracing the formation and evolution of these two different halo stars.The observations were taken from the Fiber Optic Step Spectrometer（FIES）of the Nordic Optical Telescope（NOT）at La Palma and the UVES high-resolution spectrograph attached to the ESO Very Large Telescope（VLT）,with spectral resolutions of 55,000 and40,000,respectively.（NLTE）,the elemental abundances of the two neutron-capture elements Ba and Eu were obtained analytically for 48 halo stars.The hyperfine structure and isotopic shifts of the Ba II resonance line 4554?are also considered,and the relative proportions of the odd(¹³⁵Ba,¹³⁷Ba)and even(¹³⁴Ba,¹³⁶Ba,¹³⁸Ba)isotopes fodd are determined by best fitting the Ba II 4554?spectral line profile.and compared the abundance ratios[Eu/Ba]and the parity isotopic ratios of Ba for two different halo groups of stars.The results show that:（1）The NLTE correction for Ba abundances is negative,decreasing by-0.1 dex on average compared to LTE results,and this correction increases with increasing stellar metal abundance[Fe/H],while the NLTE correction for Eu abundances is positive,increasing by 0.1～0.18 dex compared to LTE calculations,and the correction is independent of stellar metal abundance.（2）There is no clear distinction between[Ba/Fe]and[Eu/Fe]for high-αand low-αhalo stars,but most low-αhalo stars have higher[Eu/Fe].（3）There is a good positive correlation between[Eu/Ba]and f_odd,i.e.f_oddincreases with[Eu/Ba],for low-αhalo stars,but this correlation does not exist for high-αhalo stars.These results suggest that low-αhalo stars may come from Galactic close dwarf galaxies and have high[Eu/Fe],while high-αhalo stars may be intragalactic stars with different Ba and Eu abundances due to their inconsistent formation environments.