We present large scale observations of C18O(1-0) towards four massive star forming regions for MON R2, S156, DR17/L906 and M17/M18. The transitions of H2CO(110-111), C18O(1-0) and 6 cm continuum were compared towards the four regions. Analysis of observation and Non--LTE model results shows that the brightness temperature of the formaldehyde absorption line is strongest in background continuum temperature range of about 3 - 8 K. The excitation of the H2CO absorption line is affected by strong background continuum emission. From the comparison of H2CO and C18O maps, we found that the extent of H2CO absorption is broader than that of C18O emission in the four regions. Except for the DR17 region, the H2CO absorption maximum is located at the same position with the C18O peak. The good correlation between intensities and widths of H2CO absorption and C18O emission lines indicate that the H2CO absorption line can trace dense and warm regions of the molecular cloud. Finding that N(H2CO) was well correlated with N(C18O) in the four regions and that the average column density ratio is <H2CO/N(C18O)> ~ 0.03.
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