A Monte Carlo simulation study of the excitation of molecules in high-mass star forming regions

Abstract

Astronomical maser emission occurs in various astrophysical environments and can be used to infer the physical properties of the regions where they are excited. A key component of interpreting the presence of maser emission associated with a specific astrophysical environment is determining the pumping mechanism (radiative or collisional) for a particular maser. Baan et al. (2017) recently concluded that the extragalactic 4.8GHz formaldehyde megamasers are radiatively pumped, based on calculations, using the online Radex facility. The aforementioned is contrary to the conclusion of van der Walt (2014) that formaldehyde masers associated with high mass star forming regions are collisionally pumped. Since much of the interpretation of the maser emission depends on the pumping mechanism, we revisited the pumping of the formaldehyde masers to try to understand the results obtained by Baan et al. (2017). A Monte Carlo model was built to investigate the pumping mechanism of this molecule. A single molecule is modelled as a system, and the rest of the molecules, H2 molecules and the radiation represent the reservoir. The molecule is followed in time. The Monte Carlo simulation models the random walk of the molecule into various energy states according to the probabilities of radiative and collisional excitation processes. Two distributions are constructed as a result: the holding time distribution and the distribution of the number of visits. The calculations show that understanding the roles of collisional and radiative excitation processes helps to determine the pumping scheme of a maser. The pumping scheme proposed explains why the collisional excitation process plays a prominent role in establishing an inversion and why external radiation alone cannot lead to an inversion. The results of this study suggest that 4.8 GHz formaldehyde masers are not pumped by radiation just because it is dominant but more precisely require a path and an excitation process, which in this case is collisions, which is the corresponding process of the dominant radiative process.