Constraints on temperature vs. hydrogen density from the relative population of C I fine-structure levels. The contours show 1–3 sigma confidence regions (dark to light shading) from three different line ratios (J = 1 to J = 0, dashed blue; J = 2 to J = 0, dotted red; and J = 2 to J = 1, solid black). All constraints assume an excitation temperature of 71 K derived from H₂ rotational levels.