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Nächste Seite: Objects Included and Omitted Aufwärts: Contents Vorherige Seite: Introduction

Scope of the Catalogue and General Remarks

In order to answer the fundamental question

`` what is a planetary nebula? ''
we have compared the properties of various objects considering the current review literature and came to the following conclusion: A planetary nebula is a mainly gaseous object (also containing dust) expanding from its hot central star of intermediate mass in a late evolutionary phase on the way between red giants and white dwarfs. The central stars ionize and illuminate the respective nebulae. Although the parameters of the nebulae change very much during the rapid evolution of their nuclei and are therefore dependent on age, it is possible to summarize their typical values (extreme values are given in parenthesis).

N e b u l a :

Morphology: objects of mostly symmetrical shape (circular or elliptical discs or rings, sometimes bipolar structure with ``equatorial'' torus and ``pole'' condensations), with apparently sharp outer boundaries; often multiple shells (main nebula + faint outer structure or halo). The morphology depends on the wavelength (stratification); it also reflects the intrinsic absorption and the orientation in space.
Some objects have envelopes of neutral hydrogen and molecules.
Angular diameter: $ \thicksim$ 20"- 40", depending on the wavelength (limits stellar - $ \thicksim$ 20', Sh 2-216 even larger).
Spectrum: emission lines:
(a) recombination lines mostly of H and He;
(b) collisionally excited (forbidden) lines of C, N, O, Ne, Mg, Si, S, Cl, Ar;
(c) fluorescent lines (rare) of OIII and NIII.
continuum emission:
free-bound, free-free, two-quantum processes, emission from grains (dust). Spectrum depending on excitation conditions (exc. class), stratification, chemical composition.
Exc. classes: 0 - 10,
main criteria:
I([OIII]$ \lambda$5007 $ + \lambda$4959)/I(H$ \beta$),
I(HeII$ \lambda$4686)/I(H$ \beta$),
I([OII]$ \lambda$3727)/I([OIII])$ \lambda$4959).
(see also below)
IR-spectrum: nebular emission lines, dust continuum, IR-emission features.
IR-fluxes: F$ _{\nu}$(12$ \mu$m)/F$ _{\nu}$(25$ \mu$m) $ \leqq$ 0.35, F$ _{\nu}$(25$ \mu$m)/F$ _{\nu}$(60$ \mu$m) $ \geqq$ 0.3,
H2 (some objects).
Radio emission: continuum, mainly molecules CO, OH.
Dimension: diameter 0.1 pc - 0.2 pc (limits $ \thicksim$ 0.005 pc or even smaller, $ \thicksim$ 7 pc) depending on wavelength.
Electron density: 10$ ^{3}$ - 10$ ^{4}$cm$ ^{-3}$ (but also <10$ ^{3}$cm$ ^{-3}$ for old objects and >10$ ^{4}$cm$ ^{-3}$ for young objects possible).
Electron temperature: 9 000$ ^\circ$K - 15 000$ ^\circ$K (limits 8 000$ ^\circ$K - 23 000$ ^\circ$ K).
Total mass: ionized gas:
0.1 M$ _{\odot}$ - 0.2 M$ _{\odot}$ (limits $ \thicksim$ 0.001 M$ _{\odot}$, $ \thicksim$ 1 M$ _{\odot}$);
neutral gas + dust:
sometimes much higher, very different;
dust mass/gas mass $ \thicksim$ 2x10$ ^{-4}$ to 3x10$ ^{-2}$.
Expansion velocity:        non-isotropic, $ \thicksim$ 25 km/s (limits 4 km/s, 60 km/s, outer condensations up to $ \thicksim$ 300 km/s).
Age: 0 - $ \thicksim$ 100 000 years.

N u c l e u s :

Spectrum: WR, O, Of, WR+Of, OVI, sdO, cont., peculiar, sometimes variable.
Effective temperature: 40 000$ ^\circ$K - 100 000$ ^\circ$K (limits $ \thicksim$ 20 000$ ^\circ$K - $ \thicksim$ 250 000$ ^\circ$K).
Luminosity: $ \thicksim$ 5x10$ ^{3}$L$ _{\odot}$ (limits $ \thicksim$ 10 L$ _{\odot}$, $ \thicksim$ 10$ ^{4}$L$ _{\odot}$).
Radius: limits $ \thicksim$ 0.005R$ _{\odot}$, $ \thicksim$ 1.5R$ _{\odot}$.
Mass: $ \thicksim$ 0.6M$ _{\odot}$ (progenitors between 0.8M$ _{\odot}$ and 6-8M$ _{\odot}$).
Mass loss: var $ \thicksim$ 10$ ^{-10}$M$ _{\odot}$/yr - 10$ ^{-7}$M$ _{\odot}$/yr ($ \thicksim$ 10$ ^{-5}$M$ _{\odot}$/yr in late AGB).
Gravity: log g $ \thicksim$ 3.0 - 7.5.

The mean parameters of the nebulae and of their nuclei have been taken over mainly from Supplement 3 (with the extension given in Supplement 5). The summary of the criteria by which PNe are distinguished from several types of objects is given in Supplement 2.

Evolution of PNe
The main group of PNe is part of the so-called "blue-white sequence" which was already introduced by Vorontsov-Velyaminov (1947) for the description of the positions of PN nuclei in the HR diagram. This sequence was explained as the place of occurence of planetary nuclei in their evolution between red giants and white dwarfs (Vorontsov-Velyaminov, 1948 and 1953; Shklovsky, 1956; Harman, Seaton, 1964; and later). The actual best explanation of the expansion of the nebular envelopes was found in the model of the Interacting Stellar Winds (Kwok et al., 1978; Kwok 1982) based on the mass loss coming from AGB stars. The evolutionary track of central stars was calculated mainly by Paczynski (1971), Schönberner (1979) and Blöcker (1995).

There are two periods in the evolution of PNe which are still vaguely understood at present:
(a) The first period concerns the o r i g i n of PNe and the evolution of the new-born PN. It is generally believed, and the theoretical evolutionary tracks support this, that PNe evolve from red giants to post-AGB objects via stellar winds. Especially IRAS objects with colours similar to those of common PNe are highly suspected of accomplishing this evolution and they therefore deserve our particular attention. Nevertheless such objects are not yet classified generally as PNe; they are listed as  p r e - PNe in a separate table (Table 5). Exceptionally some of these objects are classified as PNe even if the well-known nebular lines [OIII]$ \lambda$5007,$ \lambda$4959 are missing; in these cases other classification criteria support the idea that they are after all early PNe.
(b) The second period concerns very old PNe, where the nebulae have already disappeared and the central stars are very faint and similar to common WD. It is very difficult to detect such objects with the present observational techniques. Again in a separate table (Table 6) we list these objects as  p o s t - PNe.

Excitation classes
The exc. classes were introduced in order to classify the spectra of planetary nebulae using the level of excitation (excitation potential of the emission lines). The classification criteria are based on the scheme which was developed mainly by Aller (1956), partly already by Page (1942) or even earlier. Only one criterion is sensitive over nearly the whole range of excitation: I([OIII]$ \lambda$5007+$ \lambda$4959)/I(H$ \beta$). Moreover the ratio I(HeII $ \lambda$4686)/I(H$ \beta$) is suitable for high and very high, whereas I([OII]$ \lambda$3727)/I([OIII]$ \lambda$4959) for low and medium excitation classes. Unfortunately this last criterion should be taken with caution because of possible strong interstellar absorption which might weaken the $ \lambda$3727 line. For low excitation classes the ratio I([NII]$ \lambda$6584)/I(H$ \alpha$) is also useful. We added exc. classes 0-1 and 1 for very-low-excitation (VLE-objects were introduced already by Sanduleak, Stephenson, 1973) mostly compact objects: in exc. class 1 [OIII]$ \lambda$5007 is very weak but still visible, exc. class 0 - 1 is reserved for objects showing no visible [OIII]$ \lambda$5007 line. In this case some additional indications for PNe (e.g. non-stellar angular diameter, continuous spectrum, infrared fluxes) should be given. The above exc. classes contain mainly objects the star temperatures of which are too low for producing the N1 line, but which are on the evolutionary way to common PNe.

The proposed scheme of excitation classes 0-10 is as follows:
exc. class exc. level
0-1 and 1 very low
2-3 low
4-5 medium
6-7 high
8,9 and 10 very high


Designation
We intentionally use the same designation of PNe as in the edition CGPN(1967), i.e. in the system: lll $\pm$ bb.n, where n=1,2 ... is the number of the object in the respective area 1$^\circ$x 1$^\circ$. This designation (and not the more detailed one) was used in CGPN(1967) also in order to avoid a possible confusion due to the sometimes approximate coordinates (the positional accuracy of 26 % of the objects in CGPN(1967) was not better than 1 arcmin). At present the positional accuracy has been improved, and the designation can therefore be more detailed. We also give (Table 2) the IAU designations of galactic planetary nebulae, PN G lll.l $ \pm$ bb.b, recommended by IAU Commission 5 (Astronomical Nomenclature) and also used in SECGPN. In order to avoid possible confusion we use the same PN G designation of objects as given in SECGPN; this is also in case our galactic coordinates would differ slightly from those of SECGPN due to improved equatorial coordinates.

We are of the opinion that the confusion concerning the PK designation which sometimes occurred in the literature was avoidable. We would have expected that the discoverer would publish the new objects either together with their galactic coordinates only, or with the first part of the designation, containing the galactic longitude and latitude and n o t with the number of the object in the respective area 1$ ^\circ$ x 1$ ^\circ$ (e.g. 255 - 15. only). If necessary it would have been possible to distinguish several objects in one area of the galactic longitude and latitude with letters A, B,.. (e.g. 255 -15.A, 255 - 15.B, ...). The definitive numbering of the objects in this area should have been reserved for the person who wrote the respective supplement to CGPN or the new updated version of this catalogue. As an example: there are several PK designations in the new catalogue of symbiotic stars (Belczynski et al., 2000, Table 8), which did not appear either in CGPN(1967) or in Supplements S1-S5 and which are not correct. To use them could therefore be confusing. - The above procedure has not been explicitely mentioned in CGPN(1967), but it was assumed to be self-evident; in reality it was sometimes not. For this reason we shall write about this matter in more detail now.


next up previous
Nächste Seite: Objects Included and Omitted Aufwärts: Contents Vorherige Seite: Introduction
Lubos Kohoutek
2001-04-02