We present large scale observations of C18O(1–0) towards four massive star forming regions: MON R2,S156,DR17/L906 and M17/M18. The transitions of H2CO(110–111),C18O(1–0) and the 6 cm continuum are compared in these four regions. Our analysis of the observations and the results of the Non–LTE model shows that the brightness temperature of the formaldehyde absorption line is strongest in a background continuum temperature range of about 3 – 8 K. The excitation of the H2 CO absorption line is affected by strong background continuum emission. From a comparison of H2 CO and C18 O maps,we found that the extent of H2 CO absorption is broader than that of C18 O emission in the four regions. Except for the DR17 region,the maximum in H2 CO absorption is located at the same position as the C18 O peak. A good correlation between intensities and widths of H2 CO absorption and C18 O emission lines indicates that the H2 CO absorption line can trace the dense,warm regions of a molecular cloud. We find that N(H2CO) is well correlated with N(C18O) in the four regions and that the average ratio of column densities is N(H2CO)/N(C18O) ～ 0.03.
The 110-111 formaldehyde (H2CO) absorption line and H110α radio recombination line (H RRL) are observed towards the giant HII region in cloud complex W43. The observations are obtained using the Nanshan 25 m single dish operated by Urumqi Observatory, National Astronomical Observatories of China. A region about 30 × 30 is observed, which covers the whole HII region in W43. Except for the central 10 region, all the other seven points are first observed with the H2CO 110-111 absorption. The column density of the H2CO is calculated, and the H2CO density contours show some differences with the infrared image. Multiple features appear in the H2CO and H RRL which indicate complex structure. The intensities of the H2CO and the velocities of the H110α seem to present a linear correlation, which illustrates that the sphere of influence of the central WR/OB cluster may be much more extended than presently known, since the size is nearly 50 pc.
We collect 3249 OH maser sources from the literature published up to April 2007, and compile a new catalog of OH masers. We look for the exciting sources of these masers and their infrared properties from IRAS and MSX data, and make a statistical study. MSX sources associated with stellar 1612 MHz OH masers are located mainly above the blackbody line; this is caused by the dust absorption of stellar envelopes, especially in the MSX A band. The mid-IR sources associated with stellar OH masers are concentrated in a small region in an [A]-[D] vs.[A]-[E] diagram with a small fraction of contamination; this gives us a new criterion to search for new stellar OH masers and distinguish stellar masers from unknown types of OH masers. IR sources associated with 1612 MHz stellar OH masers show an expected result: the average flux of sources with F60 > F25 increases with increasing wavelength, while those with F60 < F25 vary little with wavelength, because the sources with F60 < F25 are much hotter than those with F60 > F25.
