DUSTY TORI OF SEYFERT NUCLEI PROBED BY THE WATER-VAPOR MASER EMISSION- HOW LARGE ARE THE DUSTY TORI

Geometrical and physical properties of dusty tori of Seyfert nuclei pr obed by the water vapor maser emission at 22 GHz are discussed. We ass ume that the dusty torus has a simple cylindrical form, and that the m aser emission can be detected only when we observe the torus from almo st edge-on views. The observed low frequency of occurrence of the wate r vapor maser emission (less than 10%) suggests that the torus is a ve rtically thin cylinder whose outer radius between a few parsecs and si milar to 10 pc. However, the observed masing regions are concentrated in the inner 1 pc regions of the torus. A possible explanation for thi s property is that only the inner a few pc regions have physical condi tions enough to cause the maser emission; the temperature is as high a s several hundred K, and the density is as high as similar to 10(10) c m(-3).