A survey of interstellar formaldehyde in dust clouds

This survey of formaldehyde in dust clouds by means of its absorption of the cosmic background radiation in the lio lii rotational transition at 4830 MHz consists of observations of 381 of the clouds listed in Lynds's Catalogue of Dark Nebulae. The observed line profiles are represented by functions that include the six hyperfine components of the line and allow for optical-depth effects. Parameters derived from this representation are examined statistically. The lines are all relatively weak, and the strength of the line is correlated with the visual opacity of the dust cloud in which it arises. Excitation temperatures of this H2CO transition are not much below 2.7 K and are correlated with the optical depth in the observed line. The clouds form a part of the general flow of gas near the Sun that, in the plane, resembles an expanding ring with the Sun near the center. Formaldehyde in dark clouds is compared with that observed in absorption in the spectra of bright radio sources. The abundance of formaldehyde compared to that of hydrogen is similar in the two cases. A more significant difference between them may lie in their differing turbulent velocities. A model of a typical formaldehyde cloud associated with a dust cloud is proposed. It has a radial gradient in both density and excitation temperature; the density decreases outward, while the temperature increases. The more opaque the dust cloud, the lower the central excitation temperature. The model supports the collisional mechanism for "cooling" the molecules. Subject headings: interstellar matter - molecules, interstellar - nebulae - radio lines