Revisiting the Formaldehyde Masers II: Effects of an HII region and Beaming

TL;DR

This study uses numerical modeling to explore how free-free radiation fields from HII regions influence the inversion and pumping of Formaldehyde masers at 4.8 GHz and 14.5 GHz, addressing past criticisms and discussing factors affecting maser rarity.

Contribution

It provides new numerical insights into the pumping mechanisms of Formaldehyde masers and clarifies the role of free-free radiation fields and other astrophysical factors in maser occurrence.

Findings

Inversion occurs for multiple Formaldehyde transitions in free-free radiation fields.

The response of maser inversion depends on the emission measure and geometric dilution of HII regions.

Rarity of Formaldehyde masers is likely due to evolutionary and chemical factors, not the pumping mechanism.

Abstract

We present new results of a numerical study of the pumping of 4.8 GHz and 14.5 GHz maser of o-Formaldehyde in the presence of a free-free radiation field. It is shown that in the presence of a free-free radiation field inversion of not only the 4.8 GHz transition, but also the 14.5 GHz transition and other doublet state transitions occur. Further results are presented to illustrate how, as a consequence of the pumping scheme, the inversion of the 4.8 GHz and 14.5 GHz transitions respond to the free-free radiation fields associated with HII regions with different emission measures and levels of geometric dilution with respect to the masing region. We also discuss the criticism raised in the past by various authors against the pumping of the 4.8 GHz Formaldehyde masers by a free-free radiation field. It is argued that the rarity of the Formaldehyde masers is not to be ascribed to the pumping scheme but to other factors such as, e.g., the evolution of the associated HII region or the chemical evolution of the star forming region which determines the Formaldehyde abundance or a combination of both.