Comparison of the three detected line series to the Case B model predictions. The solid black lines show the observed line fluxes for transition with lower energy levels (from top to bottom) 6, 7, and 8 relative to the reference line HI (9-6). The lines with different colors are the predicted values by the Case B model for different densities and temperatures of the emitting gas. The model densities are 10⁸ cm⁻³ (dotted line), 10¹⁰ cm⁻³ (slashed line), and 10¹² cm⁻³. The temperatures are 500 K (red), 5000 K (green), and 10 000 K (blue).