| From the study of the characteristics of the stellar chemical abundances,we can know the astrophysical origins of the stellar elements and the physical scenarios of formatting of the star.Recent work has found that the metal-poor star HD 94028 shows interesting abundance features.The s-process material(e.g.,Ba,La,Ce,and Pb)and r-process material(e.g.,Eu,Os,Pt)are mildly overabundant while the element C is not enhanced.However,the observed super-solar ratio [As/Ge](=0.99 ± 0.23)and sub-solar ratio [Se/As](=-0.16 ± 0.22)could not be fitted by the combination of s-and r-process abundances.In order to explain the abundance pattern of HD 94028,the intermediate neutron-capture process(i-process)was suggested.Up to now,the astrophysical site(or sites)of the i-process has(or have)not been confirmed.Observationally,both the main s-process and the i-process are closely associated with AGB stars.Obviously,a detailed study of the abundances is still needed to investigate the astrophysical origins of the elements in HD 94028.In this work,adopting the abundance decomposition method,we investigate the astrophysical origins of the elements in this star.We find that the primary process of massive stars dominates the production of the light elements and iron-group elements.The lighter neutron(n)-capture elements from Ge to Te mainly come from the weak r-process.The elements from Ba to Pb are synthesized dominantly by the main s-and main r-process.The observed abundance features of Ge,As,and Se in HD 94028 are mainly produced by the weak r-process,which possesses the features of the super-solar ratio[As/Ge] and sub-solar ratio [Se/As].Because Ge is not synthesized solely by the neutron-capture process,As should be the lightest neutron-capture element.Although the i-process has been suggested in several astrophysical environments,it should be superfluous to explaining the abundances of the lighter n-capture elements in HD 94028.The object J053253.51-695915.1(J053253)in the Large Magellanic Cloud(LMC)is reported as a young stellar object(YSO).Its chemical abundances reflect the initial composition of the gas cloud in which the star formed.However,the discovery that this star shows the enhancement of the n-capture elements and the higher ratio of the heavier n-capture elements relative to the lighter n-capture elements is puzzling.From the comparison between the observed ratios [X/Fe] in J053253 and the averaged ratios [X/Fe] in normal LMC stars,we find that the abundance ratios of the n-capture elements are higher in J053253 than the averaged ratios in the normal LMC stars,particularly,[La/Fe] and [Eu/Fe] are about 0.6 and0.4 dex respectively higher in J053253 than in the normal LMC stars.The abundance ratios[X/Eu] in J053253 lie between the solar s-process and solar r-process ratios and near the solar total [X/Eu] ratios,which indicates the n-capture elements of this star might come from the combination of the s-and r-processes.From the comparisons of the observed [X/Eu] ratios in J053253 with in the post-AGB stars and in the extreme r-II stars stars,we find that,for the most observed n-capture elements,[X/Eu] ratios in this star are lower than in post-AGB stars about 0.6 dex(averaged)and higher than in the r-II stars about 0.5 dex(averaged).Using an abundance decomposed method,we explore the astrophysical origins of the n-capture elements in this star.We find that the abundance characteristic of the higher ratio of the heavier n-capture elements to the lighter n-capture elements can be explained by the pollution of the r-process event and the contamination of the s-process material.Additionally,the abundance ratios of J053253 are close to those of the i-process stars,which means that if J053253 is a post-AGB star or an extrinsic object,the i-process scenario might provide an explanation for the observed characteristics of this star.It is significant to note that the Sr abundances of the extremely metal-poor(EMP)stars are related to both the main r-process and the weak r-process,so the Sr abundance ratios contain the abundant information about n-capture nucleosynthesis and enrichment history of the early Galaxy.Furthermore,[Sr/Fe] ratios also contain the information about the astrophysical origin of element Fe.Because the lower limit of [Sr/Fe] ratios of the weak r-process stars is about 0.3 ± 0.2 dex and the upper limit of [Sr/Fe] ratios of the main r-process stars is about0.8 ± 0.2 dex,considering the observed uncertainties,the Sr abundances of the very metal-poor stars within the range of-0.3<[Sr/Fe]<1.0 should be explained by the combined contributions of the weak r-process and main r-process.Ba is believed to mainly originate from the s-process of the low and intermediate mass AGB stars in the solar system.However,for the lowest metallicity,Ba dominantly comes from the main r-process.Because the abundances of Ba have been measured for almost all stars with lower metallicity,we take Ba(instead of Eu)as the representative element of the main r-process and study the abundance characteristics of Sr and Ba for the EMP stars in the [Sr/Ba] vs.[Ba/Fe] space.We find that there are three boundaries for the distribution region of the EMP stars.The weak r-process star CS 22897-008 lies on the upper end and the main r-process stars lie on the right end of the region.Near the right boundary of the distribution region,there is a Fe-normal belt.For the EMP stars in the belt,element Fe dominantly originates from the normal massive stars.The low-Sr stars([Sr/Fe]?-0.3)distribute in the region of lower left of the Fe-normal belt and their Fe should originate partly from the prompt inventory.We find that the formation of the lower boundary of the distribution region is due to the pollution of the main r-process material and the formation of the right boundary could be explained by the combination of the weak r-and main r-process material.Furthermore,the formation of the left boundary is due to the pollution of the weak r-process material.Although the [Sr/Ba] ratios are related to the relative importance of the weak r-process material,the scatter of [Sr/Ba] ratios for the EMP stars mainly depends on the abundance ratio of the weak r-process.In the early Galaxy,the elemental abundances of metal-poor stars contain abundant information to investigate the chemical enrichment history and set important constraints on the neutron-capture nucleosynthesis.To reveal the r-process characteristics,it is important to study the abundances of the metal-poor stars with various r-process enhancements.In this work,we study the abundance characteristics of three metal-poor stars: strongly r-enhanced J0858-0809([Eu/Fe]=+1.44),moderately r-enhanced J2005-3057([Eu/Fe]=+0.94)and mildly r-enhanced J1432-4125([Eu/Fe]=+0.23).We find that the abundance continua between the patterns of the weak r-process star and the main r-process star can be explained by the abundance combinations of the main and the weak r-processes with various fractions.On the other hand,the solar r-process pattern can be fitted by the abundance mixing of the two r-process components.The results mean that the mixing of the two r-processes should be a common phenomenon.Through the analysis of the abundance trends,we find that,for the mildly r-enhanced star and the weak r-process stars,Zr abundances are correlated with the abundances of ? elements,which indicates that the weak r-process occurs in Fe core-collapse SNe with the progenitor of M > 11 M.For the strongly r-enhanced star and the main r-process stars,Zr abundances are uncorrelated with the abundances of ? elements.Furthermore,the abundances of the weak r-process elements are uncorrelated with those of the main r-process elements.The abundance relations also imply that the main r-process and the weak r-process are two independent components.From the display of the very metal-poor stars in the[Y/Eu]vs.[Eu/Fe] space,we find that the characteristics of the r-process enhancement for most metal-poor stars are due to the mixing pollution of the weak r-and main r-process material.For the formation scenario of the scatter tendency of the [Y/Eu] ratios for the very metal-poor stars,the pollution of the initial gas cloud the stars formed from the weak r-process material is more important than that from the main r-process material. |
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