Anomalous intensities in the infrared emission of CH$^+$ explained by
  quantum nuclear motion and electric dipole calculations
  The unusual infrared emission patterns of CH$^+$, recently detected in the
planetary nebula NGC 7027, are examined theoretically with high-accuracy
rovibrational wavefunctions and $ab$ $initio$ dipole moment curves. The
calculated transition dipole moments quantitatively reproduce the observed
$J$-dependent intensity variation, which is ascribed to underlying centrifugal
distortion-induced interference effects. We discuss the implications of this
anomalous behavior for astrochemical modeling of CH$^+$ production and
excitation, and provide a simple expression to estimate the magnitude of this
effect for other light diatomic molecules with small dipole derivatives.
