Every cloud has a translucent region

The physical and chemical processes in translucent clouds are controlled by the interstellar radiation field (ISRF) which can penetrate through the whole cloud although at a reduced rate into the deepest parts. A well known example of these clouds are the ubiquitously present high-latitude IRAS cirrus. It is noteworthy that these cirrus had already been discovered before IRAS on high quality IIIa-J plates taken at UKST in course of the deep ESO/SERC southern sky survey (e.g. King et al. 1979).

Optical surface brightness images provide important information for the investigation of the internal structure of translucent clouds or cloud regions. It is important to realise that every cloud has a translucent layer: the edges of dark clouds, Giant Molecular Clouds, and in the ultimate sense the edge of every clump is a translucent region. An important chemical aspect of such regions is that they trace the regime where the carbon transformation C C CO takes place (e.g. van Dishoeck & Black 1988; Stark & van Dishoeck 1994). This transition is very sensitive to the physical parameters, and small variations in e.g. temperature, total hydrogen density and column density can cause large variations in the observed CO emission. In a detailed study of a series of isolated high-latitude cirrus clouds it was found that a smooth blue surface brightness distribution corresponds often to a filametary CO emission without significant CO emission (Stark 1995). On the other hand when brightness condensations are visible on the B-image they coincide often with clumps in the CO emission where also CO emission becomes significant. The optical surface brightness images are therefore an important tool to distinguish between volume density and column density variations. There is a very good correspondence between the optical and FIR surface brightness (Paley et al. 1991; Stark 1995). The optical images reveal the diffusely backscattered radiation from stars in the galactic plane (Sandage 1976) while the absorbed radiation is re-radiated at far-infrared wavelengths. Comparison provides direct information on the energy balance.

Optical surface brightness images will be indispensable in identifying spurious effects in future FIR-CBR surveys. First results of a project to study the carbon content in galactic cirrus clouds through comparison of the lowest CO rotational transitions and observations of the fine structure line of [Cá ] at 492 GHz (Stark & van Dishoeck 1994; Ingalls et al. 1994, 1997) indicate that most of the carbon is not yet in the form of CO, . Moreover, recent observations of the [Cá ] 158 m line with the Infrared Space Observatory (ISO) shows this line to have a strength comparable to [Cá ] which indicates a low incident radiation field, (Stark et al. in preparation). The clouds are therefore cold ( K) and radiate much of their energy at wavelengths longward of 200 m. Understanding small-scale surface brightness variations of foreground cirrus at optical wavelenghths will therefore be of vital importance for projects searching for variations of the FIR cosmic background radiation with submillimetre bolometer arrays.

á© Copyright Astronomical Society of Australia 1997