Discovery of Rubidium, Cadmium, and Germanium Emission Lines in the
  Near-Infrared Spectra of Planetary Nebulae
  We identify [Rb IV] 1.5973 and [Cd IV] 1.7204 micron emission lines in
high-resolution (R=40,000) near-infrared spectra of the planetary nebulae (PNe)
NGC 7027 and IC 5117, obtained with the IGRINS spectrometer on the 2.7-m
telescope at McDonald Observatory. We also identify [Ge VI] 2.1930 $\mu$m in
NGC 7027. Alternate identifications for these features are ruled out based on
the absence of other multiplet members and/or transitions with the same upper
levels. Ge, Rb, and Cd can be enriched in PNe by s-process nucleosynthesis
during the asymptotic giant branch (AGB) stage of evolution. To determine ionic
abundances, we calculate [Rb IV] collision strengths and use approximations for
those of [Cd IV] and [Ge VI]. Our identification of [Rb IV] 1.5973 $\mu$m is
supported by the agreement between Rb3+/H+ abundances found from this line and
the 5759.55 A feature in NGC 7027. Elemental Rb, Cd, and Ge abundances are
derived with ionization corrections based on similarities in ionization
potential ranges between the detected ions and O and Ne ionization states. Our
analysis indicates abundances 2-4 times solar for Rb and Cd in both nebulae. Ge
is subsolar in NGC 7027, but its abundance is uncertain due to the large and
uncertain ionization correction. The general consistency of the measured
relative s-process enrichments with predictions from models appropriate for
these PNe (2.0-2.5 M_sun, [Fe/H]= -0.37) demonstrates the potential of using PN
compositions to test s-process nucleosynthesis models.
