Formaldehyde Observations towards Methanol Maser Sources

Abstract

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).