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Three-dimensional (3D) radiative hydrodynamic model atmospheres of metal-poorlate-type stars are characterized by cooler upper photospheric layers thantheir 1D counterparts. This property of 3D models can dramatically affectelemental abundances derived from temperature-sensitive spectral lines. Weinvestigate whether the cool surface temperatures predicted by metal-poor 3Dmodels can be ascribed to the approximated treatment of scattering in theradiative transfer. We use the Bifrost code to test three different ways tohandle scattering in 3D model atmospheres of metal-poor stars. First, we solveself-consistently the radiative transfer equation for a source function with acoherent scattering term. Second, we solve the radiative transfer equation fora Planckian source function, neglecting the contribution of continuumscattering to extinction in the optically thin layers; this has been thedefault mode in previous models of ours. Third, we treat scattering as pureabsorption everywhere, which is the standard case in CO5BOLD models. We findthat the second approach produces temperature structures with cool upperphotospheric layers very similar to the correct coherent scattering solution.In contrast, treating scattering as pure absorption leads to significantlyhotter and shallower temperature stratifications. The main differences intemperature structure between our published models and those generated with theCO5BOLD code can be traced to the different treatments of scattering.Neglecting the contribution of continuum scattering to extinction in opticallythin layers provides a good approximation to the full radiative transfersolution for metal-poor stars. Our results demonstrate that the cooltemperature stratifications predicted for metal-poor late-type stellaratmospheres by previous models of ours are not an artifact of the approximatedtreatment of scattering.

Three-dimensional surface convection simulations of metal-poor stars -- The effect of scattering on the photospheric temperature stratification