The Abundances And Nucleosynthesis Of Ba And Eu In High-? And Low-? Halo Stars

The stellar elemental abundances and abundance distributions are good tracers to probe the formation and chemical evolution of stars.Heavy elements Ba and Eu are the typical neutron-capture elements of s-process and r-process respectively,and thus the abundance ratio [Eu/Ba] can be used as a tracer of the nucleosynthesis feature.On the other hand,the odd isotopes of Barium ¹³⁵Ba and ¹³⁷Baare mainly synthesized through the r-process,while the even isotopes ¹³⁴Ba,¹³⁶Ba and ¹³⁸Baare mainly synthesized through the s-process,the even-to-odd isotope ratio apparently represent the relative contributions from the r-and s-process to the Ba abundance in a way.Therefore,we can use the abundance ratio [Eu/Ba] and the even-to-odd isotope ratio of Ba to investigate the nucleosynthesis history and star formation as well as stellar chemical evolution.In this paper,based on the MAFAGS opacity sampling?OS?atmospheric models,we analyze the abundances of Ba and Eu elements in 40 halo stars?including 13low-? halo stars and 27 high-? halo stars?under the hypothetical conditions of local thermodynamic equilibrium?LTE?and non-LTE?NLTE?respectively.Some stellar spectra were observed with ESO VLT/UVES and are available in the ESO/ST-ECF Science Archive,and the others were observed with the FIbre fed Echelle Spectrograph?FIES?at the Nordic Optica Telescope?NOT?.Taking into consideration of the hyperfine splitting of the Ba? resonance line l4554? result from the isotopes of Ba,and the contributions from the odd?135,137?or even?134,136,138?isotopes to the line formation are different,we can determined the relative contribution of rand s-processes to the Ba abundances from the odd-to-even ratio of Ba isotopes by fitting the profiles of the Ba ? resonance line at 4554? using the least value of ?².Our analysis results are as follows :?1?The abundance of Ba has a negative NLTE effect with an average the correction of-0.06 dex in [Ba/Fe] compared to the LTE abundance.However,the NLTE effects are different for different subordinate lines of Ba II.Such as the line of?6496? has the strongest NLTE effect with a correction of-0.15 dex compared to theLTE [Ba/Fe].The line at ?5853? only has a small NLTE correction of-0.03 dex,and the line of ?6141? almost have none NLTE effect.?2?The abundance of Eu has an positive NLTE effect with the correction from0.03 dex to 0.12 dex in [Eu/Fe] compared to the LTE abundance.?3?There is no obvious distinction in [Ba/Fe] abundances for the two different halo stars of the so-called low-? and high-? halo stars,while the [Eu/Fe] is more higher in the low-? halo stars than that in the high-? halo stars.?4?For most of the low-? halo stars,the abundance of Ba is synthesized both from the r-and s-processes,but the contribution from the r-process is greater than the s-processes.Furthermore,for a few of the low--? halo stars,their Ba abundances are fully produced by a pure r-process.For the high-? halo stars,the nucleosynthesis mechanism of Ba is more complex,there is both the conditions of pure r-process or pure s-process,while in the most case,Ba abundances come from both the two processes with a random relative contribution.In conclusion,we think that the high-? and low-? halo stars have different origins,both of them are not originated from only one formation sit.They are more likely formed in multi situations,such as the “in situ”,“kicked-out”,and “accreted”,as well as some other origins.