Collisional excitation of formaldehyde in 'hot' interstellar molecular regions

Earlier calculations for rotational excitation of formaldehyde by collisions with He atoms are extended to include the lowest 81 rotational levels and kinetic temperatures to 300 K. Calculations are generally based on Green et al. (1978) but coupled differential equations are solved via a modified log-derivative propagator at low energies (Manolopoulos, 1986) and an R-matrix propagator at higher energies (Stechel, Walker, and Light, 1978). Consideration is given to the expected accuracy of the rates for excitation of formaldehyde by collisions with He atoms which are calculated in the present paper and to the expected differences between these rates and those for excitation by H2 molecules which comprise a larger fraction of the interstellar gas. The comparisons performed indicate that, for excitation by He, calculated state-to-state rates are likely to be 50 percent accurate, and total excitation rates, better than 20 percent. For excitation by H2, the rates should be increased by a factor of 2.2.