The power law index of the mass opacity coefficient, (3, provides important infomation about the dust evolution in the protoplanetary disks. Theoretically, Miyake & Nakagawa [7] suggested that the index ¡3 decrease as the dust particles coagulate. Observationally, it is reported that (3 of protostar is larger than that of T Tauri stars [7,8].

Our results show no apparent correlation between (3 and the stellar ages. HL Tau and GM Aur show that (3 ~ 1, which is typical value of T Tauri stars. Whereas, DO Tau shows a significantly small (3 compared to other sources. In order to explain the small ¡3 of DO Tau, the mm emission would have to be either optically thick, or optically thin with f3 ~ 0.3. It is unnatural that the dust particles around DO Tau have already coagulate into significantly bigger sizes. So we are in favour of interpreting the observed continuum emission as optically thick emission from DO Tau circumstellar disk.

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