Upper left panel: HCN (2 − 1) (red line); H¹³CN (1 − 0) (blue line filled yellow, × 16); HCN (1 − 0) (black line, ×1.5). Upper right panel: HCO⁺ (2 − 1), H¹³CO⁺ (1 − 0)×18, HCO⁺ (1 − 0)×1.3. Middle left panel: HNC (2 − 1), HN¹³C (1 − 0)×35, HNC (1 − 0)×1.5. Middle right panel: HOC⁺ (2 − 1), HOC⁺ (1 − 0)×4. Lower left panel: HCN (2 − 1) (red line, ×3.2); HNC (2 − 1) (black line filled yellow, ×16); HCO⁺ (2 − 1) (black line, ×1.8); the black line curve is the Gaussian fitting line of HNC (2 − 1). Lower right panel: HCN (2 − 1) (red line); HOC⁺ (2 − 1) (black line filled yellow, × 20); HCO⁺ (2 − 1) (black line, ×1.8); the black line curve is the Gaussian fitting line of HOC⁺ (2 − 1).