Top row: curves of the [HNC]/[HCN] abundance ratio output by the Meudon PDR modeling code are plotted against extinction, for a representative slab of gas with integrated A_V = 1.0 of a representative slab of gas. Points are colored based on kinetic gas temperature, which peaks toward the edges of the slab. Bottom row: average abundance ratio for each model curve is marked with the corresponding symbol (light blue), outlining the predicted trend with respect to displacement from the UV flux source. Additional points (red, green, blue) mark the IRAM 30 m observed line ratios for the three modeled positions (Globules, Rim, Plumes), respectively. Left: Model Set 1. HNC/HCN of three model slabs with increasing distances from the UV source, thus decreasing incident UV flux. Pressure and density remain fixed. Right: model Set 2. UV flux, pressure, and density are all decreased with increasing distance from the CSPN. See Sect. 5.2.