SIMBAD references

Context. The 4.8 GHz formaldehyde (H₂CO) masers are one of a number of rare types of molecular masers in the Galaxy. There still is not agreement on the mechanism responsible for the inversion of the 1₁₀-1₁₁ transition and the conditions under which an inversion can occur, and therefore how to interpret the masers.
Aims. The aim of the present calculations is to explore a larger region of parameter space to improve on our previous calculations, thereby to better understand the range of physical conditions under which an inversion of the 1₁₀-1₁₁ transition occurs. We also aim to understand recently published results that H₂CO masers are radiatively pumped.
Methods. We solve the rate equations of the first 40 rotational levels of o-H₂CO using a fourth-order Runge-Kutta method. We consider gas kinetic temperatures between 10 and 300 K, H₂ densities between 10⁴ and 10⁶ cm^–3, and a number of different dust temperatures and grey-body spectral energy density distributions.
Results. We show that when using a black body radiation field the inversion of any transition will disappear as the kinetic temperature approaches the black-body radiation temperature since the system, consisting of the gas and radiation field, approaches thermodynamic equilibrium. Using a grey-body dust radiation field appropriate for Arp 220 we find that none of 1₁₀-1₁₁, 2₁₁-2₁₂, and 3₁₂-3₁₃ transitions are inverted for kinetic temperatures less than 100 K. Our calculations also show that in theory the 1₁₀-1₁₁ transition can be inverted over a large region of explored parameter space in the presence of an external far-infrared radiation field. Limiting the abundance of H₂CO to less than 10^–5, however, reduces the region where an inversion occurs to H₂ densities ≥10⁵ cm^–3 and kinetic temperatures ≥100 K. We propose a pumping scheme for the H₂CO masers which can explain why collisions play a central role in inverting the 1₁₀-1₁₁ transition, and therefore why an external radiation field alone does not lead to an inversion.
Conclusions. Collisions are an essential mechanism for the inversion of the 1₁₀-1₁₁ transition. Our results suggest that 4.8 GHz H₂CO megamasers are associated with hot and dense gas typical of high mass star forming regions rather than with cold material. Although limiting the H₂CO abundance to less than 10^–5 significantly reduces the region in parameter space where the 1₁₀-1₁₁ is inverted, it still is not clear whether this is the only reason why these masers are so rare.