FORMALDEHYDE OBSERVATIONS TOWARDS METHANOL MASER SOURCES
Observational works on 4.83 GHz Formaldehyde (H2CO) absorptions and 4.87 GHz H110α radio recombination lines towards 6.7 GHz Methanol (CH3OH) maser sources are presented. Methanol masers provide ideal sites to probe the earliest stages of massive star formation, while 4.8 GHz Formaldehyde absorptions are accurate probes of physical conditions in dense and low temperature molecular clouds towards massive star forming regions. Feature similarities between the formaldehyde absorptions and the methanol masers are studied to expand knowledge of events and physical conditions in massive star forming regions. A total of 176 methanol maser sources were observed for formaldehyde absorptions, and detections were made in 138 of them (corresponding to a detection rate of about 80%). Fifty-three of the formaldehyde absorptions are new discoveries. We observed strong correlation (r=0.89, ρ=3.78 × 10-47) between the formaldehyde and methanol source velocities, but a weak correlation (r=-0.03, ρ=0.72) between their intensities. This possibly indicates that the signals arise from about the same regions, but that the mechanisms that enhance their excitations are different. The strongest formaldehyde absorptions were associated with IRAS (Infrared Astronomical Satellite) sources and IRDCs (Infra-Red Dark Clouds) that have developed HII regions, and that do not have EGOs (Extended Green Objects).