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Gravitational wave observations need accompanying electromagnetic signals toaccurately determine the sky positions of the sources. The ejecta of neutronstar mergers are expected to produce such electromagnetic transients, calledmacronovae. Characteristics of the ejecta include large velocity gradients andthe presence of heavy $r$-process elements, which pose significant challengesto the accurate calculation of radiative opacities and radiation transport. Forexample, these opacities include a dense forest of bound-bound features arisingfrom near-neutral lanthanide and actinide elements. Here we investigate the useof fine-structure, line-smeared opacities that preserve the integral of theopacity over frequency, which is an alternative approach to theexpansion-opacity formalism. The use of area-preserving line profiles producesfrequency-dependent opacities that are one to two orders of magnitude greaterthan those obtained with the use of expansion opacities in the Sobolevapproximation. Opacities are calculated for four $r$-process elements (cerium,neodymium, samarium and uranium) using fully and semi-relativistic methods, aswell as different amounts of configuration interaction in the atomic structure,in order to test the sensitivity of the emission to the underlying atomicphysics. We determine the effect of these new opacities on simulated spectraand broad-band light curves by applying a multi-dimensional ray-trace method tothe ejecta predicted from 3-dimensional merger calculations. With oursubstantially larger opacities the simulations yield slightly lowerluminosities that peak in the mid-IR. These results suggest that thoseradioactively powered transients that are related to the very heaviest$r$-process material are more difficult to observe than previously believed.

A line-smeared treatment of opacities for the spectra and light curves from macronovae