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Binary neutron star mergers are thought to be one of the dominant sites ofproduction for rapid neutron capture elements, including platinum and gold.Since the discovery of the binary neutron star merger GW170817, and itsassociated kilonova AT2017gfo, numerous works have attempted to determine thecomposition of its outflowing material, but they have been hampered by the lackof complete atomic data. Here, we demonstrate how inclusion of new atomic datain synthetic spectra calculations can provide insights and constraints on theproduction of the heaviest elements. We employ theoretical atomic data(obtained using GRASP$^0$) for neutral, singly- and doubly-ionised platinum andgold, to generate photospheric and simple nebular phase model spectra forkilonova-like ejecta properties. We make predictions for the locations ofstrong transitions, which could feasibly appear in the spectra of kilonovaethat are rich in these species. We identify low-lying electric quadrupole andmagnetic dipole transitions that may give rise to forbidden lines when theejecta becomes optically thin. The strongest lines lie beyond $8000\,\r{A}$,motivating high quality near-infrared spectroscopic follow-up of kilonovacandidates. We compare our model spectra to the observed spectra of AT2017gfo,and conclude that no platinum or gold signatures are prominent in the ejecta.From our nebular phase modelling, we place tentative upper limits on theplatinum and gold mass of $\lesssim$ a few $10^{-3}\,\rm{M}_{\odot}$, and$\lesssim 10^{-2}\,\rm{M}_{\odot}$, respectively. This work demonstrates hownew atomic data of heavy elements can be included in radiative transfercalculations, and motivates future searches for elemental signatures.

Constraints on the presence of platinum and gold in the spectra of the kilonova AT2017gfo