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Material ejected during (or immediately following) the merger of two neutronstars may assemble into heavy elements by the r-process. The subsequentradioactive decay of the nuclei can power electromagnetic emission similar to,but significantly dimmer than, an ordinary supernova. Identifying such eventsis an important goal of future transient surveys, offering new perspectives onthe origin of r-process nuclei and the astrophysical sources of gravitationalwaves. Predictions of the transient light curves and spectra, however, havesuffered from the uncertain optical properties of heavy ions. Here we considerthe opacity of expanding r-process material and argue that it is dominated byline transitions from those ions with the most complex valence electronstructure, namely the lanthanides. For a few representative ions, we run atomicstructure models to calculate radiative data for tens of millions of lines. Wefind that the resulting r-process opacities are orders of magnitude larger thanthat of ordinary (e.g., iron-rich) supernova ejecta. Radiative transportcalculations using these new opacities indicate that the transient emissionshould be dimmer and redder than previously thought. The spectra appearpseudo-blackbody, with broad absorption features, and peak in the infrared (~1micron). We discuss uncertainties in the opacities and attempt to quantifytheir impact on the spectral predictions. The results have importantimplications for observational strategies to find and study the radioactivelypowered electromagnetic counterparts to compact object mergers.

Opacities and Spectra of the r-process Ejecta from Neutron Star Mergers