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One of the most promising electromagnetic signatures of compact objectmergers are kilonovae: approximately isotropic radioactively-powered transientsthat peak days to weeks post-merger. Key uncertainties in modeling kilonovaeinclude the emission profiles of the radioactive decay products---non-thermalbeta- and alpha-particles, fission fragments, and gamma-rays---and theefficiency with which they deposit their energy in the ejecta. The totalradioactive energy and the efficiency of its thermalization sets the luminositybudget and is therefore necessary for predicting kilonova light curves. Weoutline the uncertainties in r-process decay, describe the physical processesby which the energy of the decay products is absorbed in the ejecta, andpresent time-dependent thermalization efficiencies for each particle type. Wedetermine the net heating efficiency and explore its dependence on r-processyields---in particular, the production of translead nuclei that undergoalpha-decay---and on the ejecta's mass, velocity, composition, and magneticfield configuration. We incorporate our results into new time-dependent,multi-wavelength radiation transport simulations, and calculate updatedpredictions of kilonova light curves. Thermalization has a substantial effecton kilonova photometry, reducing the luminosity by a factor of roughly 2 atpeak, and by an order of magnitude or more at later times (15 days or moreafter explosion). We present simple analytic fits to time-dependent netthermalization efficiencies, which can easily be used to improve light curvemodels. We briefly revisit the putative kilonova that accompanied gamma rayburst 130603B, and offer new estimates of the mass ejected in that event. Wefind that later-time kilonova light curves can be significantly impacted byalpha-decay from translead isotopes; data at these times may therefore bediagnostic of ejecta abundances.

Radioactivity and thermalization in the ejecta of compact object mergers and their impact on kilonova light curves