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 (60' × 90'), S156 (50' × 70'), DR17/L906 (40' × 60'), and M17/M18 (70' × 80') were observed for H₂CO (beam 10'), H110α 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 ¹³CO 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. Their maps have similar shapes, sizes, peak positions, and molecular spectra and present similar central velocities and line widths. Such good agreement indicates that the H₂CO and the ¹³CO arise from similar regions.

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