Near-infrared surface brightness observations of the Thumbprint Nebula and determination of the albedo of interstellar grains

We have made near-infrared J, H and K bandpass imaging of a small, highly symmetric globule called the Thumbprint Nebula. At all these wavelengths the globule shows a surface brightness higher than the adjacent sky due to the radiation scattered by the globule. The intensity of the observed surface brightness is interpreted in terms of the scattering properties, i. e. albedo a and the form of the scattering function (phase function asymmetry parameter g) of the dust particles. Monte Carlo calculations for a spherical cloud have been used to solve the radiation transfer problem for different values of a and g. The all-sky measurements of the DIRBE (Diffuse InfraRed Background Experiment) instrument aboard the COBE(1) (COsmic Background Explorer) satellite were used to calculate the galactic radiation field incident on the globule at 1.26 and 2.16 mu m, while at 1.66 mu m an interpolation has been used. The values of a and g can not be determined separately, but the value of a is nearly independent of g for probable values of g (0.0 < g < 0.8). If it is assumed that the observed surface brightness is due solely to the scattered light then the grain albedo is 0.7 at 1.26 and 1.66 mu m, and 0.6 at 2.16 mu m, for 0.0 < g < 0.8. The reddening of the stars shining through the globule is used to determine the radial dust density distribution. The dust density distribution is found to be more centrally peaked than the density distribution of molecular gas.