Detection of vibrational emissions from the helium hydride ion (HeH$^+$)
  in the planetary nebula NGC 7027
  We report the detection of emission in the v=1-0 P(1) (3.51629 micron) and
P(2) (3.60776 micron) rovibrational lines of the helium hydride cation (HeH+)
from the planetary nebula NGC 7027. These detections were obtained with the
iSHELL spectrograph on NASA's Infrared Telescope Facility (IRTF) on Maunakea.
The confirm the discovery of HeH+ reported recently by Guesten et al. (2019),
who used the GREAT instrument on the SOFIA airborne observatory to observe its
pure rotational J=1-0 transition at 149.137 micron. The flux measured for the
HeH+ v=1-0 P(1) line is in good agreement with our model for the formation,
destruction and excitation of HeH+ in NGC 7027. The measured strength of the
J=1-0 pure rotational line, however, exceeds the model prediction
significantly, as does that of the v=1-0 P(2) line, by factors of 2.9 and 2.3
respectively. Possible causes of these discrepancies are discussed. Our
observations of NGC 7027, covering the 3.26 - 3.93 micron spectral region, have
led to the detection of more than sixty spectral lines including nine
rovibrational emissions from CH+. The latter are detected for the first time in
an astronomical source.
