  The coalescence of compact objects are a promising astrophysical sources of
gravitational wave (GW) signals. The ejection of r-process material from such
mergers may lead to a radioactively-powered electromagnetic counterpart which,
if discovered, would enhance the science return of a GW detection. As very
little is known about the optical properties of heavy r-process elements,
previous light curve models have adopted opacities similar to those of iron
group elements. Here we report that the presence of heavier elements,
particularly the lanthanides, increase the ejecta opacity by several orders of
magnitude. We include these higher opacities in time dependent,
multi-wavelength radiative transport calculations to predict the broadband
light curves of one-dimensional models over a range of parameters (ejecta
masses from 0.001 to 0.1 solar masses and velocities from 0.1 to 0.3c). We find
that the higher opacities lead to much longer duration light curves which can
last a week or more. The emission is shifted toward the infrared bands due to
strong optical line blanketing, and the colors at later times are
representative of a blackbody near the recombination temperature of the
lanthanides (T ~ 2500K). We further consider the case in which a second mass
outflow, composed of 56Ni, is ejected from a disk wind, and show that the net
result is a distinctive two component spectral energy distribution, with a
bright optical peak due to 56Ni and an infrared peak due to r-process ejecta.
We briefly consider the prospects for detection and identification of these
transients.
