Collisional Excitation of CaO and CaS by Helium: Cross Sections, Rate Coefficients, and Line Ratio Diagnostics

The first rotational (de)excitation rate coefficients for CaO–He and CaS–He are reported and computed from new CCSD(T)-F12B potential energy surfaces (PESs) with a core–valence basis set. The global PES minima are −159.54 cm ^−1 and −22.7 cm ^−1 , respectively. Close-coupling calculations of the (de)excitation cross sections among the first 21 rotational levels are performed for both systems for collision energies up to 1000 cm ^−1 . Rate coefficients are obtained by thermal averaging over a Maxwell–Boltzmann velocity distribution for temperatures ranging from 5 to 150 K. Driven by the distinct potential well depths, the rate coefficients maintain a strict propensity for Δ j = ±1 transitions in CaO–He, whereas the shallower CaS–He system exhibits a clear crossover to Δ j = ±2 dominance within this range. Comparison between CaO–He and CaS–He shows that the ratio k _CaS−He / k _CaO−He ranges from 0.52 to 1.35 over the entire temperature range. The CaO–He rate coefficients are scaled to CaO–H _2 for non-LTE radiative transfer calculations of the four transitions (4→3, 5→4, 9→8, and 10→9) recently tentatively observed toward G+0.693−0.027. Owing to the large dipole moment of CaO, the line ratios are mainly density probes and exhibit a steep, temperature-insensitive critical-density switch near 10 ^6 ∼10 ^7 cm ^−3 . These data provide essential tools for determining physical conditions in interstellar clouds where CaO has been tentatively detected.