| Abstract : | Methanol masers at 6.7 GHz-the brightest of class II methanol masers-can be used as an ideal tool to probe the early phases of massive star formation. The primary goal of my thesis was to investigate the evolutionary stage of the YSOs that excite 6.7 GHz methanol masers. This work has made use of the entire sample from the Methanol Multibeam Survey (MMB), FIR data from Hi-GAL and millimeter wave spectroscopic data from the MALT90 survey. To investigate the physical properties of the maser sources, we obtained the spectral energy distributions (SEDs) from 870 to 70 μm for 320 6.7 GHz methanol maser sources, and used the best-fit parameters of the SED fits to derive the maser clump properties. A comparison of the mass–luminosity diagram of the sample with evolutionary tracks from the turbulent core model suggests that over 90 percent are in the early accreting phase. We also investigated the chemical properties of the maser hosts, using the MALT90 data, for a sample of 68 out of the 320 methanol masers of the first study. We used the line intensities and abundances of four molecular transitions: N2H+(1-0), HCN (1-0), HNC (1-0) and HCO+(1-0) since they are bright and are good tracers of dense gas. The HCN/HNC, N2H+/ HCO+, HNC/ HCO+ and N2H+/ HNC ratios of column density and integrated intensity suggest that methanol masers are at an earlier evolutionary state -- much in agreement with previous dust continuum studies. My thesis work thus gives strong evidence that along a timeline for massive star formation, the 6.7 GHz methanol maser phase originates in massive young stellar objects that are more evolved than IRDCs and is quenched by the time the sources evolve into ultracompact HII regions. |
