2025MNRAS.542L.117S

We analyse the ro-vibrational absorption bands of various molecular cations (HCO$^+$, HCNH$^+$, and N$_2$H$^+$) and neutral species (HCN, HNC, and HC$_3$N) detected in the James Webb Space Telescope/Mid-Infrared Instrument Medium Resolution Spectrometer spectrum (4.9-27.9 μm) of the local ultraluminous infrared galaxy IRAS 07251-0248. We find that the molecular absorptions are blueshifted by 160 km s$^{-1}$ relative to the systemic velocity of the target. Using local thermal equilibrium excitation models, we derive rotational temperatures ($T_{\rm rot}$) from 42 to 185 K for these absorption bands. This range of measured $T_{\rm rot}$ can be explained by infrared radiative pumping as a by-product of the strength, effective critical density, and opacity of each molecular band. Thus, these results suggest that these absorptions originate in a warm expanding gas shell ($\dot{M}$$\sim$90-330 $\mathrm{ M}_\odot$ yr$^{-1}$), which might be the base of the larger scale cold molecular outflow detected in this source. Finally, the elevated abundance of molecular cations can be explained by a high cosmic ray ionization rate, with log($\zeta _{\text{H}_2}$/n$_{\rm H}\, [\text{cm}^3\, \text{s}^{-1}])$ in the range of -18.2 (from H$_3^+$) to -19.1 (inferred from HCO$^+$ and N$_2$H$^+$, which are likely tracing denser gas), consistent with a cosmic ray dominated chemistry as predicted by chemical models.