Study On The Fraction Of Odd Ba Isotopes In R-? Stars

The astrophysical origin of the r-process is still relatively unclear until now.It is still a very important task to study the nucleosynthesis history of stellar and its formation mechanism.Current theory regarding heavy element nucleosynthesis in metal-poor environments states that the r-process would be dominant.We provide measurements of the Ba isotopic fractions for metal-poor stars derived with a non-local thermodynamic equilibrium(NLTE)analysis,which can provide more stringent observation constraints for our theoretical studies.Studies of the r-process enhanced stars are important for understanding the nature and origin of the r-process better.We present a detailed Ba even-to-odd isotope abundance ratio(fodd,Ba)analysis of very metal-poor giant stars discovered in the HERES(The Hamburg/ESO R-process Enhanced Star survey)project,CS29491-069,HE1219-0312,HE2252-4225 and HE2327-5642,which exhibit overabundances of the r-process elements with [r/Fe]=+1.1,+1.5,+0.80 and +0.99,respectively.We use high resolution and high signal-to-noise spectra and take advantage of the hyperfine structure(HFS)to determine the fraction of odd Ba isotopes by measuring subtle asymmetries in the profile of the BaII line at 4554?.The fractional abundance of the odd isotopes of Ba is derived from a requirement that Ba abundances from the resonance line 4554? and subordinate lines 5853? and 6496? must be equal.The resonance spectrum is fitted by adjusting the macroturbulence value when the abundance is constant,and the optimal parity ratio of barium is obtained by chi-square test.In order to maintain consistency of measurement,lines of iron in the two ionization stages(FeI and Fe II)are used to derive a homogeneous set of effective temperatures,surface gravities,iron abundances and microturbulence velocities.The stellar parameters are derived with NLTE analysis.Effective temperature and microturbulence velocitie parameters are adjusted by requiring that the FeI and FeII abundances be independent of the excitation potential and absorption strengths,respectively.The surface gravity logg is measured through the ionization equilibrium of Fe I and FeII.The measured fodd,Ba values are 0.46±0.08,0.51±0.09,0.48±0.12 and 0.50±0.13 for CS29491-069,HE1219-0312,HE2252-4225 and HE2327-5642,respectively,which corresponding to pure r-process contribution to Ba production for those four stars.The high r-process signatureof barium suggests that the majority of the heavy elements in this kind of stars were synthesised via an r-process path.These conclusions are consistent with studies based on the metal-poor star model calculations to fit their abundance distributions.