The scattering properties and density distribution of dust in a small interstellar cloud

CCD observations have been used to derive surface-brightness profiles and the extinction profile for an isolated globule which is illuminated by the interstellar radiation field. A radial density distribution proportional to exp(-5.67r) was found to represent the observations best. The required dust albedo ranges from 0.80 at 469 nm to 0.58 at 856 nm wavelength, and the scattering phase function is strongly forward directed, with an asymmetry parameter about 0.7 for this spectral range. This implies dust grains with an upper size limit of 1 micron, a real index of refraction n less than 1.5, and only a small imaginary component. The cloud mass was derived as 14 + or - 3 solar mass. No evidence for dust fluorescence was found in this cloud; the increase of the cloud's optical depth with decreasing wavelength was close to that expected from the average interstellar extinction curve. If gas pressure and self-gravity are the two dominant effects governing the cloud structure, the object is in hydrostatic equilibrium at a temperature T = 11 K. 32 refs