Table 2: Expansion time scales from proper-motion measurements. The data are either from Palen et al. (2002), optical measurements, or from the compilation of Terzian (1997), radio measurements. The angular radii, $\theta _{\rm rim}$, and their expansion rate, $\dot{\theta}_{\rm rim}$, refer to the semi-minor axes. The effective temperatures are spectroscopic values (Méndez 1992) or He  II Zanstra temperatures (Gorny, priv. comm.)

Object
$T_{\rm eff}$ $\theta _{\rm rim}$ $\dot{\theta}_{\rm rim}$ $\hspace{3mm}\theta_{\rm rim}/\dot{\theta}_{\rm rim}$ comment reference
  [K] [mas] [mas yr-1] [yr]    

IC 2448
65 000 4500 $2.25\pm0.6$ $2000\pm\ \ 600 $ optical Palen et al. (2002)
NGC 6578 67 000 3360 $1.60\pm0.4$ $2100\pm\ \ 600 $ optical Palen et al. (2002)
NGC 6884 87 000 2280 $1.14\pm0.3$ $2000\pm\ \ 600 $ optical Palen et al. (2002)

NGC 3242
75 000 8800 $13.2\pm4.7$ $700\pm\ \ 300 $ radio Hajian et al. (1995)
NGC 7662 100 000 6200 $5.6\pm 5.0$ $1100\pm1000 $ radio Hajian & Terzian (1996)
NGC 7027 200 000 4000 $4.7\pm1.0 $ $900\pm\ \ 200$ radio Hajian et al. (1993), Masson (1989)


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