Temperature fluctuations and the size distribution of interstellar grains

The present analysis indicates that a bimodal size distribution of interstellar grains may develop naturally within diffuse interstellar clouds if the absorption energies of accreted compounds onto small grains are about 0.1 eV and the photodesorption yields are less than .000001. It is shown that, depending on the photodesorption yield, the depletion of cooling agents and expansion of the gas within an isolated cloud lead to a balance between photodesorption and accretion after about 10 to 100 million years. The dust grains are then characterized by a normal dust-to-gas ratio of about 0.1 micron.