http://arxiv.org/abs/1402.5471
We present large scale observations of C$^{18}$O (1–0) towards four massive star forming regions for MON R2, S156, DR17/L906 and M17/M18. The transitions of H$_2$CO (1$_{10}$–1$_{11}$), C$^{18}$O (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 H$_2$CO absorption line is affected by strong background continuum emission. From the comparison of H$_2$CO and C$^{18}$O maps, we found that the extent of H$_2$CO absorption is broader than that of C$^{18}$O emission in the four regions. Except for the DR17 region, the H$_2$CO absorption maximum is located at the same position with the C$^{18}$O peak. The good correlation between intensities and widths of H$_2$CO absorption and C$^{18}$O emission lines indicate that the H$_2$CO absorption line can trace dense and warm regions of the molecular cloud. Finding that N(H$_2$CO) was well correlated with N(C$^{18}$O) in the four regions and that the average column density ratio is $<$N(H$_2$CO)/N(C$^{18}$O)$>$ $\sim$ 0.03.
X. Tang, J. Esimbek, J. Zhou, et. al.
Tue, 25 Feb 14
48/59