  Three-dimensional (3D) radiative hydrodynamic model atmospheres of metal-poor
late-type stars are characterized by cooler upper photospheric layers than
their 1D counterparts. This property of 3D models can dramatically affect
elemental abundances derived from temperature-sensitive spectral lines. We
investigate whether the cool surface temperatures predicted by metal-poor 3D
models can be ascribed to the approximated treatment of scattering in the
radiative transfer. We use the Bifrost code to test three different ways to
handle scattering in 3D model atmospheres of metal-poor stars. First, we solve
self-consistently the radiative transfer equation for a source function with a
coherent scattering term. Second, we solve the radiative transfer equation for
a Planckian source function, neglecting the contribution of continuum
scattering to extinction in the optically thin layers; this has been the
default mode in previous models of ours. Third, we treat scattering as pure
absorption everywhere, which is the standard case in CO5BOLD models. We find
that the second approach produces temperature structures with cool upper
photospheric layers very similar to the correct coherent scattering solution.
In contrast, treating scattering as pure absorption leads to significantly
hotter and shallower temperature stratifications. The main differences in
temperature structure between our published models and those generated with the
CO5BOLD code can be traced to the different treatments of scattering.
Neglecting the contribution of continuum scattering to extinction in optically
thin layers provides a good approximation to the full radiative transfer
solution for metal-poor stars. Our results demonstrate that the cool
temperature stratifications predicted for metal-poor late-type stellar
atmospheres by previous models of ours are not an artifact of the approximated
treatment of scattering.
