Modelling Barium Isotopes in Metal-Poor Stars

The principal theory concerning the origin of the elements heavier than the Fe-peak, such as Ba, strongly suggest that for old, metal-poor environments, the rapid (r-) process is the most likely path taken in their synthesis, while the slow (s-) process becomes more substantial in younger, more metal-rich stellar populations. In this work I test this theory by evaluating the isotope ratios of Ba. It is understood that Ba consists of seven stable isotopes, five of which are synthesised by the two neutron-capture processes. The two odd isotopes, Ba, as well as Ba are synthesised via both the rand s-processes while two of the even isotopes, Ba are synthesised via the s-process only. The relative contribution of the rand s-process to these isotopes can be understood via nucleosynthesis calculations and is described using the parameter fodd, where fodd = [N ( Ba) +N (Ba)] /N (Ba). Low values of fodd (∼ 0.11) indicate an s-process regime, while high values of fodd (∼ 0.46) indicate an r-process regime. In the Ba II 4554 A line the even isotopes lie close to the line centre, while the odd isotopes, which are hyperfine split because of their non-zero nuclear spin, lie in the wings of the line. From an analysis of the line profile shape, one can determine whether Ba has been synthesised primarily through the r-process or s-process; a broad, asymmetric line would indicate a high r-process contribution, while a line with a deeper core and shallower wings would indicate a high s-process contribution. Using the radiative transfer code ATLAS, which assumes local thermodynamic equilibrium (LTE) and employs 1-dimensional (1D) KURUCZ06 model atmospheres, I synthesised line profiles for six metal-poor stars: HD 140283, HD 122563, HD 88609, HD 84937, BD−04◦ 3208 and BD+26◦ 3578 for a range of isotope ratios. All six are of sufficiently low metallicity that Ba was expected to have an r-process origin. These were fit to high resolution (R ≡ λ/∆λ = 90 000 − 95 000), high signal-to-noise iv (S/N = 550 − 1100) observed stellar spectra using a χ code. It was found that the Ba isotopic ratios were extremely sensitive to a star’s macroturbulent velocity, which is a measure of the velocity distributions in the gas of a star’s atmosphere. So that the star’s macroturbulence could be measured independently of the Ba line, I fit synthetic Fe lines to a few dozen observed Fe lines for each star adopting a Gaussian macroturbulent profile. Through fitting fully-resolved synthetic Ba profiles to the observed stellar spectra, I found that the Ba II line profiles indicated an s-process signature, which disagrees with current theoretical expectations. As a confirmation of the isotopic ratio derived from the Ba II 4554 A line, the Ba II 4934 A line was examined for one star. However, blends with other lines made it difficult to accurately determine the isotopic ratio. Nevertheless, it seemed to confirm the isotope ratio derived from the 4554 A line, but with a significant increase in uncertainty. Two further types of symmetric profiles were examined to model the stellar macroscopic broadening, in addition to the Gaussian profiles already used. It was found that fitting the observed lines with synthetic lines broadened by v sin i profiles, which are used to model the effect of rotation on line broadening, produced poorer fits than the Gaussian macroturbulent broadening profiles. The third profile used to model stellar macroturbulence was a radial-tangential (ζRT) broadening profile. Improvements to the statistical fits (as measured by their χ minima) were seen when compared to the Gaussian profiles. However, it was found that using a 1D LTE radiative transfer code and 1D model atmospheres produced poor fits to the observed spectra, regardless of the type of profile used to model the stellar macroturbulence. In particular, the observed Fe profiles showed greater absorption in the red wing than in the blue wing, particularly ∼ 140 mA redward of the line centre. It was suspected that this could be attributed to the simplistic assumptions that 1D atmospheres and radiative transfer codes make when synthesising synthetic spectra, leaving them unable to model asymmetries caused by convection, which are visible in the high quality stellar spectra used in this thesis. In an attempt to fit these lines better, two different techniques were examined using the Fe lines. The tests used Fe lines as they are relatively simple to model, compared v to the Ba II 4554 A line which has multiple components. In the first test, synthetic spectra were computed using the non local thermodynamic equilibrium (NLTE) radiative transfer code MULTI. The synthetic line profiles were fit to a number of lines in HD 140283. Although this technique might have improved the fit in the line core, it was found that such a treatment did not improve upon fitting errors associated with the best fit 1D LTE synthetic profiles. The second test used a 3-dimensional (3D) radiative transfer code (LINFOR3D) that employed 3D, time-dependent atmospheres produced with COBOLD. The 3D synthetic profiles were fit to a selection of Fe lines and improvements over the poor fits produced by the 1D LTE synthesis were seen. It was found that the 3D synthesis could almost completely reproduce the line asymmetries seen in the observed stellar spectrum. This result suggests that further work to refine the 3D calculations and synthesis code would be valuable.