What Does It Mean to be a "Depleted" Comet? High Spectral Resolution Observations of the Prototypical Depleted Comet 21P/Giacobini-Zinner from McDonald Observatory

We present high spectral resolution optical observations of comet 21P/Giacobini-Zinner from six nights in 2018. The observations were obtained with the Tull coude spectrograph on the 2.7m Harlan J. Smith Telescope of McDonald Observatory. This comet's spectrum shows strong depletions in C$_2$, C$_3$, CH, and NH$_2$ relative to CN. We explore what it means for a comet to be depleted and show that all of the species are present in the spectrum at similar relative line intensities within a a molecular band compared with a typical comet. The depletions represent a much lower abundance of the species studied.