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 μm) and P(2) (3.60776 μm) 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 on Maunakea. They confirm the discovery of HeH+ reported recently by Güsten et al., who used the GREAT instrument on the SOFIA airborne observatory to observe its pure rotational J = 1 - 0 transition at 149.137 μm. 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 μm spectral region, have led to the detection of more than 60 spectral lines including nine rovibrational emissions from CH+. The latter are detected for the first time in an astronomical source.