Chemistry and ro-vibrational excitation of CH+ in the planetary nebula NGC 7027

Small carbon hydride cations, such as methylidynium ( ), are important in the chemistry of the interstellar medium. They participate in a network of gas-phase reactions with a range of molecular and atomic species and lead to the formation of diverse hydrocarbon products that, in turn, act as precursors to more complex carbon-chain and organic molecules. CH+ CH+ is known to be a reactive ion that is quickly destroyed by H, , and free electrons, which makes its excitation challenging to model because chemical formation and destruction rates should be considered along with the usual radiative and inelastic rates when solving the statistical equilibrium equations. This so-called chemical pumping or excitation effect, already evidenced in the literature, is examined here with the first set of ab initio state-resolved ro-vibrational (reactive and inelastic) collision data to model the observed line intensities and better constrain the physico-chemical conditions of the environment. H2 CH+ Multiple rotational and ro-vibrational transitions of detected toward the planetary nebula NGC 7027 are analyzed in this work. The chemical structure of in NGC 7027 was modeled with the photoionization code using updated formation and destruction reaction rate coefficients for . The electron temperature and atomic and molecular gas densities were modeled as a function of position within the nebula. The nonlocal thermodynamic equilibrium analysis of the observed emission lines was then performed with the 1D code and single-zone code using an accurate and comprehensive set of spectroscopic and inelastic collisional data. In a second approach, chemical formation and destruction rate coefficients of were implemented in . This code was combined with a Markov chain Monte Carlo sampling (performed in the -parameter space) in order to extract the best-fit column density and physical conditions from the observed line fluxes. CH+ CH+ Cloudy CH+ CH+ Cloudy RADEX CH+ RADEX RADEX CH+ Our model reproduces the observed line fluxes to within a factor of 1.3 on average, with a maximum deviation of a factor of 3. It also suggests that the rotational and ro-vibrational lines originate from physically distinct regions within NGC 7027 that differ mostly in kinetic temperature. Our Cloudy CH+ CH+ RADEX models show that chemical pumping significantly enhances the population of all levels above $(̆psilon=0, J=1)$, with a strong increase in the ro-vibrational line intensities within the ̆psilon=2 1 band. A single-zone model, however, remains limited, and we strongly encourage using a full 1D model, which consistently incorporates all excitation processes, with the rate coefficients we accurately determined here.