SIMBAD references

We present the result of photo-ionizing modelling of the three planetary nebulae (PNe) A 20, A 15 and MeWe 1-3. All three objects are roughly circular, highly excited and have a high Galactic z. The PNe display low densities in the shell, but relatively dense halos. A low metallicity and a relative high electron temperature are found. Comparisons with radio observations confirm the obtained properties. The objects very likely originate from thick disk stellar progenitors. The distances found investigating the PNe shells are somewhat lower than those derived spectroscopically for the central stars in the past.