In recent years, several groups have independently developed and matured
3D Radiation-Hydro models for the convection zones and the photospheres
of cool stars. A subsequent re-examination of the solar spectrum based
on these models has led to a drastic revision of the solar abundances of
e.g. Oxygen, which has spurred an ongoing and very controversial debate
on the validity of the new models and on the severe consequences of the
much lower Oxygen abundance.
Due to the lack of modern software for the spectrum synthesis these
studies have so far been limited to very short pieces of the spectrum,
i.e. single lines or small blends. In the first part of the talk I will
present the new Advanced Spectrum Synthesis 3D Tool (ASSET) that is
based on a full-fledged radiation transfer scheme in 3 dimensions.
ASSET features arbitrarily complex opacities with molecules, scattering,
and « by the book » higher-order interpolations and radiation
transfer. Moreover, ASSET imposes no principal limits on the wavelength
range covered by one calculation. The code is fully parallelized with
OpenMP and MPI and runs very efficiently on multi-CPU, multi-core
workstations as well as on clusters.
In the second part of my talk I will compare the results from ASSET with the results derived from a different approach that is based on 1D radiation transfer methods augmented by a scheme to rotate the input model to calculate intensities for arbitrary angles. I will discuss the limitations of the two approaches in terms of accuracy and speed, and present a comparison with observations.