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

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 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 (14.5 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 Hii region. Conclusions. Due to the di erent 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