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Objects Included and Omitted

We believe all objects classified as PNe and published until the end of 1999 are included in this version CGPN(2000). It contains PNe from CGPN(1967) as well as PNe discovered later and published in the supplements. These objects appear in Table 2: List of PNe (n=1510). Objects which have been removed from the list of PNe are given in Table 3: List of misclassified PNe (n=245). Objects from both tables are included in a general list of PNe (Table 1) (n=1755): all objects once classified as PNe, having therefore the PK designation, i. e.
n(Table 1) = n(Table 2) + n(Table 3).
Exceptions are Cn 1-1, MaC 1-1, PC 11 and M 1-27, which were once removed from PNe but later included again in this list.

We include in this catalogue only objects classified as PNe and given in published lists (at least in lists which appear in preprints) and not just given in lists in papers which are `` in preparation ''.

At present we include in the list of PNe several objects which also appear in the lists of symbiotic stars (SS). In our opinion it is not advisable to classify the objects either as SS or as PNe, and therefore some of them can appear simultaneously in both lists. We point out that the hot components of symbiotic stars resemble the central stars of PNe (luminosity, temperature, mass, diameter), being also on the post-AGB tracks, which means in the region of the HR diagram occupied by PNe. We would also like to emphasize the existence of the bipolar nebulae He 2-104 (Southern Crab, Schwarz et al., 1989) and BI Cru (Schwarz, Corradi, 1992), the classification of which as PNe seems to be probable already from their morphology, although these objects were included in the main list of SS (Allen, 1984). Allen's list of SS contains 144 objects and 44 of them are simultaneously classified as PNe! Also the new catalogue of SS (Belczynski et al., 2000) contains 53 objects (24% of known SS), which are simultaneously classified as PNe. It seems that bipolarity as a result of an anisotropic mass-outflow is a phenomenon common for young PNe as well as for some SS, so that there exist emission-line objects having high-velocity bipolar nebulae and high core densities, which can be called ``symbiotic proto-PNe''. Probably there is a link between bipolar PNe and SS (Corradi, 1995). Also the observed continuum in the IR and the presence of TiO bands, which give evidence for a late-type (G-K-M or carbon) star and which is a strong criterium for SS, cannot be a reason for excluding such objects from the list of PNe, because PNe having binary (symbiotic) central stars may also exist. We emphasize the binary system BE UMa which consists of the components sdO+K5. Incidentally there exists a small group of so-called ``yellow'' symbiotic stars, named so because their cool components are of spectral type of about F. As noted in Elementary Statistics the percentage of known binary and variable stars among the PN central stars is noticeably smaller compared with that of the ``common'' stars, so that a large number of binaries have probably not yet been detected. As for visual binaries a considerable contribution was made by using the Hubble Space Telescope (Ciardullo et al., 1999); the question of their physical association still remains open in many cases.

Our criteria for excluding objects from the list of PNe as SS are more narrow than the conventional definition of SS is. We exclude only such objects from the list of PNe which show

1) strong HeII$ \lambda$4686 emission line and simultaneously no (or nearly no) [OIII]$ \lambda$5007, $ \lambda$4959 lines, and
2) the presence of a 6825A emission feature, as in M 1-21 (006+07.1), He 2-417 (012-07.1) and He 2-374 (009-02.1).

Both criteria are empirical and reflect the appearance of some bright emission features in the optical region (visible also for faint objects). As to absorption features and the presence of the continuum of a late-type star, these are according to our opinion not sufficient criteria which distinguish necessarily between the SS and the (binary) central stars of PNe.
ad 1) The appearance of both bright HeII$ \lambda$4686 and N1, N2 lines is evidence for a PN of high or very high excitation class. But if we observe very weak N1, N2 and simultaneously bright HeII$ \lambda$4686 lines it would be strange for a PN even if we take into account different excitation zones.
ad 2) The appearance of the emission bands at $ \lambda$$ \lambda$6825A, 7088A, for a long time regarded as unidentified and even a little mysterious, is explained as a Raman scattering by neutral hydrogen of the OVI doublet $ \lambda$$ \lambda$1032A, 1038A (Schmid, 1989). It is proposed that OVI photons from a hot radiation source are absorbed by hydrogen near the cool giant atmosphere. These bands are observed only in the spectra of SS and not of PNe.

To find the spectral properties distinguishing between PNe and SS is of course useful, because there evidently exist some differences between them. We believe that the largest spectral differences between SS and PNe are given in the criteria presented above.

As in CGPN(1967) we left in the list of PNe several doubtful objects and objects having no emission lines detected at present. We have to accept the fact that some of them will be removed later from the list of PNe.

As written already in the Introduction we also present two lists of objects, which have not been classified as regular PNe, but which are in their evolution close to them: pre-PNe (Table 5), objects being between post-AGB and very young PNe, and post-PNe (Table 6), where the nebulae in most cases have already disappeared and the central stars are on the evolutionary way to WD.

There exists moreover a large group of objects which can be called PN candidates. These are partly objects from internal (unpublished) lists, but mainly objects from already published lists, which have not sufficient data to be classified as PNe. Before further observations and their interpretation are available, such objects can only be considered as PN candidates. In the past the objects were sometimes included in the list of PNe rather carelessly and without further confirmation. Also this fact led to a relatively large number of misclassified PNe. The classification as a PN should be made more thoroughly in future, and the category PN candidates should be reserved for the objects having insufficient data.

This also concerns distant objects in regions of large interstellar extinction, which is in the galactic equator, especially in the direction towards the galactic centre. Such objects are not visible in the optical region; only measurements made in IR- and radio-regions are at our disposal. This means that the common criteria for PNe (mainly the presence of the emission lines [OIII]$ \lambda$5007,$ \lambda$4959 or [NII] near H$ \alpha$) cannot be applied.

Pottasch and collaborators (Pottasch et al., 1988) found that PNe appear in a certain region of the colour-colour diagram for IRAS sources. On the other hand, IRAS objects from the same region of this diagram (i.e. with far-IR colours that are typical for PNe) may be considered as PN candidates. Preite-Martinez (1988) using very similar criteria gave a classification of 388 IRAS sources from the IRAS PSC as possible new PNe. For some PN candidates radio measurements of continuum emission were made which show the presence of ionized gas. The detection of radio continuum emission provides strong evidence that the objects may indeed be PNe. However even this fact is at present not sufficient for classifying such objects as PNe.

Helfand and his group (Becker et al., 1994; Kistiakowsky, Helfand, 1995) classified many objects as PN candidates using VLA. Sometimes nothing at all is visible at the position of such PN candidates in the optical region, but many candidates have a bright line [SIII]$ \lambda$9532 stronger than [OIII]$ \lambda$5007.

Let us remind the reader that so far all known PNe have been recognized optically. Nevertheless it may be that in future many new additions to the group of PNe will be made as a result of radio and infrared studies only. For such a purpose clear criteria for classifying the objects as PNe according to their properties in IR and radio regions should be postulated. At present such objects were not included in this catalogue.


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Nächste Seite: Explanation of Tables Aufwärts: Contents Vorherige Seite: Scope of the Catalogue
Lubos Kohoutek
2001-04-02