Extended Data Figure 3. Excitation models for different gas temperatures and densities.

a, CO J=3-2 line peak (for N(CO)=10¹⁸ cm^-2). b, c and d HCO⁺ J=4-3 integrated line intensity. Each curve represents a different electron abundance model: x_e = 0 (blue) and 10^-4 (red). Continuous curves are for N(HCO⁺)=5·10¹³ cm^-2 and dotted lines for N(HCO⁺)=2·10¹⁴ cm^-2 (appropriate for deeper inside the Bar, δx>30’’). The horizontal green dashed line represents the average $T_{peak}^{CO3-2}$ (a) and $W_{4-3}^{HC{O}^{+}}$ (b, c, and d) with their standard deviation (grey shaded) towards the dissociation front (at δx≈15’’).