6. Spatial model
Although rare, the peculiar morphology of NGC 1501 is not unique amongst PNe. A quick look at the main imagery catalogues allowed us to identify three more candidates: A 72 and NGC 7094 are faint, high excitation PNe presenting a complex filamentary structure (Manchado et al., 1996a); IC 4642 is quite bright, at high excitation, decidedly tetra-lobed in both the H and [OIII] images published by Schwarz et al. (1992).
Moreover, Manchado et al. (1996b) introduced the morphological class of quadrupolar PNe, containing five compact objects (M 2-46, K 3-24, M 1-75, M 3-28 and M 4-14); these nebulae present two pairs of lobes, each pair symmetric with respect to a different axis. A sixth candidate, NGC 6881, was added by Guerrero & Manchado (1998). Following these authors, a quadrupolar PN can be formed by precession of the rotation axis of the central AGB star, possibly in the presence of a binary companion, associated with multiple shell ejection.
Finally, the general spatial structure derived for NGC 1501 (i.e. two pairs of bipolar lobes on axes having different orientations and intersecting at the central star) presents noticeable analogies with the model suggested by Balick & Preston (1987, their Fig. 4) for NGC 6543.
In the case of NGC 1501, the modest departures from the spherical symmetry can be explained in terms of small inhomogeneities occurred during the super-wind ejection (for instance, due to stellar rotation), later enhanced by photoionization and winds interaction (see Dwarkadas & Balick, 1998 and Garcia-Segura et al., 1999). The introduction of a close companion (as suggested for NGC 6543 by Balick & Preston, 1987, and Miranda & Solf, 1992), or even of a binary companion (as proposed by Manchado et al., 1996b, for quadrupolar PNe) appears unnecessary for NGC 1501.
Clearly, the tetra-lobed shape is a simplification of the true spatial structure of this nebula; a close inspection to the observational data indicates that each macro-lobe is constituted of a heap of small components, that some morphological differences exist amongst the lobes associated to the major axis of the ellipsoid and those connected with the intermediate axis, and that these lobes are only roughly aligned with the axes of the central figure. An extended spectroscopic coverage of the nebula is in progress, in order to obtain the detailed spatial structure of the ionized gas.
The most intriguiging characteristic of the matter distribution in NGC 1501 is the presence of an inwards tail in the radial density trend; this tail, particularly evident in the directions of the lobes, can be the result of hydrodynamic processes in the nebular shell. Following Capriotti (1973; see also Breitschwerdt & Kahn, 1990, Kahn & Breitschwerdt, 1990, and Garcia-Segura et al., 1999), in the first evolutionary phases, when the PN is still ionization bounded, Rayleigh-Taylor instabilities occur at the ionization front, forming a series of knots, condensations and radially arranged fingers (see Dyson, 1974, and Bertoldi & McKee, 1990) which expand slower than the ionization front.
Similar instabilities are produced also by the interaction of the fast stellar wind with the low velocity nebular material (Vishniac, 1994, Garcia-Segura & Mac Low, 1995, and Dwarkadas & Balick, 1998); in this case the optical thickness of the nebula is unimportant. If confirmed, the extremely large value of ( ) derived for the WC4/OVI nucleus of NGC 1501 by Koesterke & Hamann (1997a) using the standard atmosphere model for Wolf-Rayet stars, would indicate winds interaction as the main responsible of both the radial density distribution and the large expansion velocity observed in this nebula.
To be noticed that, besides the dynamical effects on the nebular gas, an intense and lasting mass-loss of hydrogen depleted and He, C and O enriched photospheric material would produce enhanced ionic and chemical composition gradients across the nebula (as recently observed by Sabbadin et al., 2000 in NGC 40, a low excitation PN powered by a WC8 star).
Moreover: if winds interaction and/or Rayleigh-Taylor instabilities are the sources of the inwards tails detected in the density distribution of NGC 1501, these tails are essentially constituted of knots and condensations; if they survived ionization and/or heating by conduction, we expect , where is the local filling factor, as previously defined. In other words: the true electron densities in the blobs and condensations forming the inwards tail are larger by the factor than the values shown in Fig. 4 and Fig. 5, where we assumed .
Unfortunately, our observational material is inadequate to transform the previous speculations into quantitative results to be compared with the theoretical predictions. Detailed, deep studies at higher spatial and spectral resolution of this (overlooked so far) PN are needed to answer the stimulating questions here excited.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000