Notes on the formaldehyde masers

TL;DR

This paper investigates the rarity and non-detection of certain formaldehyde masers using theoretical models of hyper-compact HII regions, revealing conditions under which specific maser transitions are inverted or suppressed.

Contribution

It introduces a detailed theoretical framework for understanding formaldehyde maser inversion and explains observational non-detections through modeling of free-free radiation fields and geometric effects.

Findings

Some hyper-compact HII regions have ineffective free-free radiation for 4.8 GHz inversion.

Only the 4.8 GHz transition can be inverted in certain regions.

Attenuation in molecular envelopes can significantly suppress maser signals.

Abstract

The questions of the rarity of the 4.8 GHz formaldehyde masers and the non-detection of 14.5 GHz formaldehyde masers are addressed from a theoretical point of view. The pumping free-free radiation fields were obtained using the photo-ionization code Cloudy to simulate hyper-compact HII regions. Implementation of these free-free radiation fields in solving the rate equations shows that the free-free radiation fields of some hyper-compact HII regions are ineffective to invert the 4.8 GHz transition. Investigation of the variation of the inversion of the 4.8 GHz and 14.5 GHz transitions with radial distance revealed that there are regions where only the 4.8 GHz transition is inverted. It is also shown that the projection of the masing region toward the edge of a hyper-compact may explain the non-detection of 14.5 GHz masers. The attenuation of the 4.8 GHz and 14.5 GHz masers in the molecular envelope is investigated. It is found that attenuation can be significant. Comparison of the velocities of the 4.8 GHz masers with the centre velocities of associated 4.8 GHz absorption features shows that the maser emission lies at the edge of the absorption features. This suggests that the masers may be associated with kinematic structures such as rotating toroids.