Kilonova light curves from the disc wind outflows of compact object mergers

We study the radioactively powered transients produced by accretion disc winds following a compact object merger. Based on the outflows found in two-dimensional hydrodynamical disc models, we use wavelength-dependent radiative transfer calculations to generate synthetic light curves and spectra. We show that resulting kilonova transients generally produce both optical and infrared emission, with the brightness and colour carrying information about the merger physics. In those regions of the wind subject to high neutrino irradiation, r-process nucleosynthesis may halt before producing high-opacity, complex ions (the lanthanides). The kilonova light curves thus typically has two distinct components: a brief (∼2 d) blue optical transient produced in the outer lanthanide-free ejecta, and a longer (∼10 d) infrared transient produced in the inner, lanthanide line-blanketed region. Mergers producing a longer lived neutron star, or a more rapidly spinning black hole, have stronger neutrino irradiation, generate more lanthanide-free ejecta and are optically brighter and bluer. At least some optical emission is produced in all disc wind models, which should enhance the detectability of electromagnetic counterparts to gravitational wave sources. However, the presence of even a small amount (10−4 M⊙) of overlying, neutron-rich dynamical ejecta will act as a ‘lanthanide-curtain’, obscuring the optical wind emission from certain viewing angles. Because the disc outflows have moderate velocities (∼10 000 km s−1), numerous resolved line features are discernible in the spectra, distinguishing disc winds from fast-moving dynamical ejecta, and offering a potential diagnostic of the detailed composition of freshly produced r-process material.