JWST reveals cosmic ray dominated chemistry in the local ULIRG IRAS 07251$-$0248

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 \textit{James Webb Space Telescope}/Mid-Infrared Instrument Medium Resolution Spectrometer spectrum (4.9--27.9\,$\upmu$m) of the local ultra luminous 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 (LTE) 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 (IR) 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$90--330\,$M_\odot$\,yr$^{-1}$), which might be the base of the larger scale cold molecular outflow detected in this source. Finally, the elevated abundance of molecular cations can be explained by a high cosmic ray ionization rate, with log($ζ_{\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.