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Astron. Astrophys. 358, 1058-1068 (2000)

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6. Conclusions

We have presented the first study of expansion velocities and dynamical ages of Bulge and halo PNe, using expansion velocity profiles. A constant expansion velocity is found not to be a good approximation, but a linear velocity field fits the observed [OIII ] 5007 Å line shapes relatively well. Such a velocity field is in qualitative agreement with predictions from hydrodynamical models, allowing for the fact that the [OIII ] traces the inner regions only.

Expansion velocities determined from line splitting are shown to underestimate the true velocities. The FWHM gives a better approximation. We note that expansion velocities measured by different methods or from different lines cannot be easily compared.

A relation between expansion velocity and outer radius, as claimed by several previous papers, is not confirmed by our data. The only weak correlation is that Bulge PN with a very small radius ([FORMULA]) may show low velocities. No clear correlation is seen for expansion velocities against other stellar parameters, nor are the halo and Sgr PNe systematically different.

Dynamical ages are calculated using an approximation for the effects of acceleration and non-uniform velocities. We estimate the original AGB outflow velocities for our stars, and show that the nebulae have undergone acceleration by about a factor of 2. The inner regions of the nebulae show little memory of the original velocities, with the possible exception of a single object with very low metallicity.

Finally we derived core masses of PNe from our sample. This was accomplished by comparing our dynamical ages with the timescales of theoretical evolutionary tracks of Schönberner (1983) and Blöcker (1995). The average core masses are in the range of 0.60-0.625 [FORMULA], with slightly higher values for the disk objects than for those from Galactic Bulge. The resulting masses show good agreement with Gorny et al. (1997) and Stasinska et al. (1997) who apply a different method but use the same evolutionary tracks.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000