The Electromagnetic Counterpart of the Binary Neutron Star Merger
  LIGO/VIRGO GW170817. IV. Detection of Near-infrared Signatures of r-process
  Nucleosynthesis with Gemini-South
  We present a near-infrared spectral sequence of the electromagnetic
counterpart to the binary neutron star merger GW170817 detected by Advanced
LIGO/Virgo. Our dataset comprises seven epochs of J+H spectra taken with
FLAMINGOS-2 on Gemini-South between 1.5 and 10.5 days after the merger. In the
initial epoch, the spectrum is dominated by a smooth blue continuum due to a
high-velocity, lanthanide-poor blue kilonova component. Starting the following
night, all subsequent spectra instead show features that are similar to those
predicted in model spectra of material with a high concentration of
lanthanides, including spectral peaks near 1.07 and 1.55 microns. Our fiducial
model with 0.04 M_sun of ejecta, an ejection velocity of v=0.1c, and a
lanthanide concentration of X_lan=1e-2 provides a good match to the spectra
taken in the first five days, although it over-predicts the late-time fluxes.
We also explore models with multiple fitting components, in each case finding
that a significant abundance of lanthanide elements is necessary to match the
broad spectral peaks that we observe starting at 2.5 d after the merger. These
data provide direct evidence that binary neutron star mergers are significant
production sites of even the heaviest r-process elements.
