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Published online by Cambridge University Press: 07 August 2017
The ages of PNNe in our sample (Hu et al., 1990) were derived from the inner radius of the dust shell using the radiative transfer model fitting with SED and expansion velocities of envelopes obtained from radio molecular lines observations (if without radio data a Vexp of 15 km/sec was assumed). The objects are classified into the following groups:
1. invisible;
2. M-type;
3. F (or G)-type; and
4. F (or G)-type with Ra emission.