The comparison of H2CO(110-111), C18O(1-0) and continuum towards molecular clouds

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.

The relation of H2CO, 12CO, and 13CO in molecular clouds

Aims. We seek to understand how the 4.8 GHz formaldehyde absorption line is distributed in the MON R2, S156, DR17/L906, and M17/M18 regions. More specifically, we look for the relationship among the H2CO, 12CO, and 13CO spectral lines. Methods. The four regions of MON R2 (60x90), S156 (50x70), DR17/L906 (40x60), and M17 /M18 (70x80)were observed for H2CO (beam 10), H110a recombination (beam 10), 6 cm continuum (beam 10), 12CO (beam 1), and 13CO (beam 1). We compared the H2CO,12CO,13CO, and continuum distributions, and also the spectra line parameters of H2CO,12CO, and 13CO. Column densities of H2CO,13CO, and H2 were also estimated. Results. We found out that the H2CO distribution is similar to the 12CO and the 13CO distributions on a large scale. The correlation between the 13 CO and the H2CO distributions is better than between the 12CO and H2CO distributions. The H2CO and the 13CO tracers systematically provide consistent views of the dense regions. T heir maps have similar shapes, sizes, peak positions, and molecular spectra and present similar centr al velocities and line widths. Such good agreement indicates that the H2CO and the 13CO arise from similar regions.