The central stars of A43 and NGC7094 are so-called "hybrid'' PG1159 stars (Napiwotzki 1992; Werner 1992) which have a significant amount of hydrogen in their photospheres which shows that there is no sharp delimitation between the H-normal and H-deficient post-AGB stars (cf. Werner et al. 1997).
At first glance, one can have the impression to see "Galactic footballs'' (Figs. 5, 6), i.e. a kind of spherical shell with denser matter at the "seams'' where the pentagons and hexagons meet - a foot(soccer)ball is made out of 12 pentagons and 20 hexagons (Kroto et al. 1985). The penta- and hexagons are more prominent in Fig.5. However, this interpretation is somewhat subjective, e.g. Dwarkadas & Balick (1998) mentioned A43 with "filamentary structure with radial orientation'', NGC7094 with "prominent filamentary structure at the surface''. Velocity measurements taken in order to derive the distribution of nebula matter in the line of sight would constrain a deprojection and help to achieve a realistic picture in this case.
|Figure 6: NGC7094. The PN is apparently similar to A43 (Fig.5) but, in contrast, the extension of its shell is about 10% smaller towards the SE and thus, not perfectly circular|
The most likely explanation for the instabilities in these nebulae's surfaces might be that the old, slow AGB wind matter is swept up to a thin shell by the fast central star wind. While the invisible inner, high-pressure bubble is expanding due to the released energy of the stellar wind, instabilities in the dense, moving shell may appear (Vishniac 1983), effective enough to produce filament-like surface structures of the shell matter. As these filaments form, the intrafilament region can expand out ahead of the filaments, giving rise to a somewhat "lumpy'' outer edge on the shell. This is quite obvious on the image of Abell43 (Fig.5).
Similar PNe are known, e.g. NGC6894, NGC7048, or NGC7139 (Balick 1987) but the edges of their shells appear smooth and round in projection. Thus, Abell43 is an excellent test case for hydrodynamical simulations!
In any case, a reliable attempt to explain the morphologies of A43 and NGC7094 and the mentioned instabilities requires a detailed kinematic and spectroscopic study along with three-dimensional (magneto-) hydrodynamical models which account for the properties of the stellar wind.
Copyright The European Southern Observatory (ESO)