Context :

Aims : We investigate the effects of coherent isotropic continuum scattering on the formation of spectral lines in local thermodynamic equilibrium ( LTE ) using 3D hydrodynamical and 1D hydrostatic model atmospheres of red giant stars .

Methods : Detailed radiative transfer with coherent and isotropic continuum scattering is computed for 3D hydrodynamical and 1D hydrostatic models of late-type stellar atmospheres using the SCATE code .
Opacities are computed in LTE , while a coherent and isotropic scattering term is added to the continuum source function .
We investigate the effects of scattering by comparing continuum flux levels , spectral line profiles and curves of growth for different species with calculations that treat scattering as absorption .

Results : Rayleigh scattering is the dominant source of scattering opacity in the continuum of red giant stars .
Photons may escape from deeper , hotter layers through scattering , resulting in significantly higher continuum flux levels beneath a wavelength of \lambda \lesssim 5000 Å .
The magnitude of the effect is determined by the importance of scattering opacity with respect to absorption opacity ; we observe the largest changes in continuum flux at the shortest wavelengths and lowest metallicities ; intergranular lanes of 3D models are more strongly affected than granules .
Continuum scattering acts to increase the profile depth of LTE lines : continua gain more brightness than line cores due to their larger thermalization depth in hotter layers .
We thus observe the strongest changes in line depth for high-excitation species and ionized species , which contribute significantly to photon thermalization through their absorption opacity near the continuum optical surface .
Scattering desaturates the line profiles , leading to larger abundance corrections for stronger lines , which reach -0.5 dex at 3000 Å for Fe II lines in 3D with excitation potential \chi = 2 eV at \mathrm { [ Fe / H ] } = -3.0 .
The corrections are less severe for low-excitation lines , longer wavelengths , and higher metallicity .
Velocity fields increase the effects of scattering by separating emission from granules and intergranular lanes in wavelength .
1D calculations exhibit similar scattering abundance corrections for weak lines , but those for strong lines are generally smaller compared to 3D models and depend on the choice of microturbulence .

Conclusions : Continuum scattering should be taken into account for computing realistic spectral line profiles at wavelengths \lambda \lesssim 4000 Å in metal-poor giant stars .
Profile shapes are strongly affected by velocity fields and horizontal inhomogeneities , requiring a treatment based on 3D hydrodynamical rather than classical 1D hydrostatic model atmospheres .