Analysis of the December 6, 1997 comet. Top: cometary absorption spectrum of December 6, 1997 observation in the a⁴D Fe II lines around 2750 Å. The grey zone represents the radial velocity range [+12; +40] km s⁻¹ over which the cometary absorptions were measured. Bottom: curve of growth of the same Fe II lines, showing the mean absorption depths measured in the [+12; +40] km s⁻¹ range (blue dots), with one-sigma error bars computed from the STIS pipeline tabulated error bars and the two-parameter fitted model (red line) as a function of the theoretical line strength .