Ro-vibrational excitation of AlCl induced by H2 collisions: The missing piece for accurate modeling of AlCl emission spectra from circumstellar gas

Abstract

Ro-vibrational transitions of metal-bearing molecules such as AlCl are key probes of warm and dense astrophysical environments such as circumstellar envelopes. Reliable non-local thermodynamic equilibrium (non-LTE) modeling of these regions requires accurate collisional data for both rotational and ro-vibrational excitation. This work presents the first comprehensive study of the ro-vibrational excitation of AlCl(X1Σ+) induced by collisions with H2. A new AlCl–H2 potential energy surface, which accounts for the vibrational motion of AlCl, has been calculated. Quantum scattering calculations were performed using the coupled states (CS) and vibrational close-coupling rotational infinite order sudden (VCC-IOS) approaches. These calculations yielded state-to-state cross sections for total energies up to 6000 cm−1, as well as rate coefficients for temperatures up to 1000 K for ro-vibrational transitions between rotational levels belonging to the $\mathit {v}$ = 0 - 2 vibrational levels. The VCC-IOS approximation is shown to be accurate for pure rotational transitions and for vibrational excitations with Δv = 1 at high kinetic temperatures above ~300 K. In such regimes, it offers a computationally efficient alternative to the CS approach, which remains essential for capturing low temperature regime accurately. To evaluate the impact of these new rate coefficients on the interpretation of AlCl observational spectra, we perform radiative transfer calculations. We found that non-LTE excitation occurs for physical conditions representative of circumstellar gas. Finally, we reinterpret AlCl emission spectra from the W Aquilae circumstellar enveloppe using the new collisional data. Our results show that actual AlCl-H2 collisional data are needed for correctly modeling AlCl observations.