SIMBAD references

We describe a pseudo-3D photoionization code, nebu{lowbar}3d, and its associated visualization tool, visneb{lowbar}3d, which are able to treat a wide variety of nebular geometries easily and rapidly, by combining models obtained with a 1D photoionization code. The only requirement for the code to work is that the ionization source is unique and not extended. It is applicable as long as the diffuse ionizing radiation field is not dominant and strongly inhomogeneous. As examples of the capabilities of these new tools, we consider two very different theoretical cases. One is that of a high-excitation planetary nebula that has an ellipsoidal shape with two polar density knots. The other is that of a blister Hii region, for which we have also constructed a spherical model (the spherical impostor), which has exactly the same Hβ surface brightness distribution as the blister model and the same ionizing star.

We present and comment upon line intensity maps corresponding to different viewing angles. We also use the computed line intensities to derive physical properties of the model in the same way as an observer would do for a real object. For example, we derive the `apparent' value of N/O for the entire nebula and along spectral slits of different orientations. For this, we take the electron temperature and density derived from the [Nii]5755Å/[Nii]6583Åand [Oii]3726Å/[Oii]3729Åratios, respectively, and we adopt the common recipe: N/O = N⁺/O⁺. Interestingly, we find that, in the case of our high-excitation nebula, the derived N/O is within 10-20 per cent of the real value, even when the slit crosses the high-density knots. On the other hand, for the blister Hii region and its spherical impostor, we find that the apparent N/O is much smaller than the true one (about 0.68 and 0.5 of it, respectively).

These two examples warn against preconceived ideas when interpreting spectroscopic and imaging data of Hii regions and planetary nebulae. The tools nebu{lowbar}3d and visneb{lowbar}3d, which will be made publicly available in the future, should facilitate the performance of numerical experiments, to yield a better understanding of the physics of aspherical ionized nebulae.