Some young, massive stars can be found in the Galactic halo. As star formation is unlikely to occur in the halo, they must have been formed in the disk and been ejected shortly afterwards. One explanation is a supernova in a tight binary system. The
companion is ejected and becomes a runaway star. HD,271791 is the kinematically most extreme runaway star known (Galactic restframe velocity $725 pm 195, rm km,s^{-1}$, which is even larger than the Galactic escape velocity). Moreover, an analysis of the optical spectrum showed an enhancement of the $alpha$-process elements. This indicates the capture of supernova ejecta, and therefore an origin in a core-collapse supernova. As such high space velocities are not reached by the runaway stars in classical binary supernova ejection scenarios, a very massive but compact primary, probably of Wolf-Rayet type is required. HD,271791 is therefore a perfect candidate for studying nucleosynthesis in a supernova of probably type Ibc. The goal of this project is to determine the abundances of a large number of elements from the $alpha$-process, the iron group, and heavier elements by a quantitative analysis of the optical and UV spectral range. Detailed line-formation calculations are employed that account for deviations from local thermodynamic equilibrium (non-LTE). We intend to verify whether core-collapse supernova are a site of r-process element production. Here, we state the current status of the project.
The recent downward revision of the solar photospheric abundances now leads to severe inconsistencies between the theoretical predictions for the internal structure of the Sun and the results of helioseismology. There have been claims that the solar
neon abundance may be underestimated and that an increase in this poorly-known quantity could alleviate (or even completely solve) this problem. Early-type stars in the solar neighbourhood are well-suited to testing this hypothesis because they are the only stellar objects whose absolute neon abundance can be derived from the direct analysis of photospheric lines. Here we present a fully homogeneous NLTE abundance study of the optical Ne I and Ne II lines in a sample of 18 nearby, early B-type stars, which suggests log epsilon(Ne)=7.97+/-0.07 dex (on the scale in which log epsilon[H]=12) for the present-day neon abundance of the local ISM. Chemical evolution models of the Galaxy only predict a very small enrichment of the nearby interstellar gas in neon over the past 4.6 Gyr, implying that our estimate should be representative of the Sun at birth. Although higher by about 35% than the new recommended solar abundance, such a value appears insufficient by itself to restore the past agreement between the solar models and the helioseismological constraints.
The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined. High-resolution solar spectra were used to determine solar $log gfepsilon_{rm
Si}$ values by comparison with Si line synthesis based on LTE and NLTE level populations. The results will be used in a forthcoming paper for differential abundance analyses of metal-poor stars. A detailed analysis of silicon line spectra leads to setting up realistic model atoms, which are exposed to interactions in plane-parallel solar atmospheric models. The resulting departure coefficients are entered into a line-by-line analysis of the visible and near-infrared solar silicon spectrum. The statistical equilibrium of ion{Si}{i} turns out to depend marginally on bound-free interaction processes, both radiative and collisional. Bound-bound interaction processes do not play a significant role either, except for hydrogen collisions, which have to be chosen adequately for fitting the cores of the near-infrared lines. Except for some near-infrared lines, the NLTE influence on the abundances is weak. Taking the deviations from LTE in silicon into account, it is possible to calculate the ionisation equilibrium from neutral and ionised lines. The solar abundance based on the experimental $f$-values of Garz corrected for the Becker et al.s measurement is $7.52 pm 0.05$. Combined with an extended line sample with selected NIST $f$-values, the solar abundance is $7.52 pm 0.06$, with a nearly perfect ionisation equilibrium of $Deltalogepsilon_odot(ion{Si}{ii}/ion{Si}{i}) = -0.01$.
