2014A&A...562A..68V

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. We perform numerical calculations to investigate under which conditions the 1₁₀-1₁₁ transition (4.8GHz) of ortho-H₂CO is inverted in order to address the question of the correlated flaring of the 6.7GHz CH₃OH and 4.8GHz H₂CO masers in G37.55+0.20. 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. 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 HII 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 (14.5GHz) and 3₁₂-3₁₃ (28.9GHz) transitions are inverted only in the presence of the free-free continuum radiation field of a very compact HII region. 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.