  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.
Continuum flux levels, spectral line profiles and curves of growth for
different species are compared with calculations that treat scattering as
absorption. Photons may escape from deeper, hotter layers through scattering,
resulting in significantly higher continuum flux levels beneath a wavelength of
5000 A. 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 A for Fe
II lines in 3D with excitation potential 2 eV at [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.
