| dc.description.abstract | 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, 135,137Ba, as well as 138Ba are
synthesised via both the r- and s-processes while two of the even isotopes, 134,136Ba are
synthesised via the s-process only. The relative contribution of the r- and s-process
to these isotopes can be understood via nucleosynthesis calculations and is described
using the parameter fodd, where fodd = [N (135Ba) + N (137Ba)] /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: HD140283, HD122563, HD88609,
HD84937, 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\equiv \lamda/\Delta\lamda = 90 000 - 95 000), high signal-to-noise 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
HD140283. 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 CO5BOLD. The 3D synthetic
pro les 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. | en_US |
