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 2s2p ³P₀ᵒ→2s² ¹S₀ in the berylliumlike emission line spectrum of N IV at 1487.89 {}. A nebular model allows us to confirm a transition rate of 4×10^-4 sec^-1±33% 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.