  We examine dust formation in macronovae (as known as kilonovae), which are
the bright ejecta of neutron star binary mergers and one of the leading sites
of r-process nucleosynthesis. We find that dust grains of r-process elements
are difficult to form because of the low number density of the r-process atoms,
while carbon or elements lighter than irons can condense into dust if they are
abundant, in light of the first macronova candidate associated with GRB
130603B. Dust grains absorb emission from ejecta with opacity even greater than
that of the r-process elements, and re-emit photons at infrared wavelengths.
Such dust emission can potentially account for the macronova without r-process
nucleosynthesis as an alternative model. This dust scenario predicts a more
featureless spectrum than the r-process model and day-scale
optical-to-ultraviolet emission.
