The interstellar methanol masers and their environments

To promote the understanding of massive star formation processes, we have studied the 6.6 GHz methanol (CH 3 OH) masers and their environments—the dense cores and the outer regions of the molecular cloud. The physics of the CH 3 OH maser or the thermal emission formation region is studied by fitting the observational data of the 6.6 GHz 5 1 –6 0 A + and the 107 GHz 3 1 –4 0 A + CH 3 OH maser emission, using the radiative transfer calculations. The type II characteristics of the 6.6 GHz CH 3 OH maser are confirmed by the calculation results. A greater intensity of the radiation field leads to an increase in the peak intensity of the maser; however, high densities tend to turn off the maser. The calculation results show that to be a maser the 6.6 GHz CH 3 OH emission needs a radiation field of 150–300 K and a density not higher than 10 7 cm −3 , while the 107 GHz emission requires a radiation field of 210–300 K and a density not higher than 3×10 6 cm −3 . The 6.6 GHz line is maser towards all six studied sources, while the 107 GHz line is maser towards Cep A only. Moreover, the former’s intensity is much stronger than the latter. The radiative transfer calculations also indicate that the 6.6 GHz maser emission is so strong that the requirements of its formation (e.g. the radiation field, the density and the kinetic parameters) can only be satisfied at a certain stage of the processes of the massive star formation. Therefore it is often used as one of the most prominent tracers for the massive star formation regions. The calculation results of the simutaneous observations of (1,1) through (4,4) inversion lines of the ammonia (NH 3 ) indicate that both the temperature and the density in the 6.6 GHz CH 3 OH maser formation regions are higher than that of the NH 3 line formation regions. Furthermore, the common fact of |V isr (CO)| > |V isr (NH 3 )|>|V isr (CH 3 OH 6.6GHz maser)| in all six sources implies the ongoing developing trends of those gas flows driven by the masers.