Pumping of the 4.8 GHz H₂CO masers and its implications for the periodic masers in G37.55+0.20

Center for Space Research, Potchefstroom Campus, North-West University, 2520 Potchefstroom, South Africa
e-mail: johan.vanderwalt@nwu.ac.za

Received: 20 August 2013
Accepted: 7 January 2014

Abstract

Context. Periodic or regular flaring of class II CH₃OH masers in nine high mass star forming regions is now a well established phenomenon. Amongst the nine star forming regions, G37.55+0.20 is the only case at present where apart from the presence of a periodic class II CH₃OH maser, correlated flaring of another masing species, H₂CO in this case, has been detected.

Aims. We perform numerical calculations to investigate under which conditions the 1₁₀−1₁₁ transition (4.8 GHz) of ortho-H₂CO is inverted in order to address the question of the correlated flaring of the 6.7 GHz CH₃OH and 4.8 GHz H₂CO masers in G37.55+0.20.

Methods. We developed a numerical code to study the population inversion of o-H₂CO. Equilibrium solutions for the level populations are found by integrating the rate equations using Heun’s method.

Results. It is found that collisional excitation with H₂ as well as radiative excitation by the free-free radio continuum radiation from a nearby ultra- or hyper-compact H ii region can invert the 1₁₀−1₁₁ transition. It is also found that radiative excitation by the dust infrared radiation field does not lead to an inversion of the 1₁₀−1₁₁ transition. The 2₁₁−2₁₂ (14.5 GHz) and 3₁₂−3₁₃ (28.9 GHz) transitions are inverted only in the presence of the free-free continuum radiation field of a very compact H ii region.

Conclusions. Due to the different pumping mechanisms of the H₂CO and CH₃OH masers it is unlikely that the near simultaneous flaring of the CH₃OH and H₂CO masers in G37.55+0.20 is due to changes in the pumping of the masers.