Determination of Hyperfine-Induced Transition Rates from Observations of a Planetary Nebula

Observations of the planetary nebula NGC3918 made with the STIS instrument on the Hubble Space Telescope reveal the first unambiguous detection of a hyperfine-induced transition [Formula presented] in the berylliumlike emission line spectrum of N IV at 1487.89 Å. A nebular model allows us to confirm a transition rate of [Formula presented] for this line. The measurement represents the first independent confirmation of the transition rate of hyperfine-induced lines in low ionization stages, and it provides support for the techniques used to compute these transitions for the determination of very low densities and isotope ratios.