Nächste Seite: Objects Included and Omitted
Vorherige Seite: Introduction
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 :
|| 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
it also reflects the intrinsic absorption and the orientation in space.
Some objects have envelopes of neutral hydrogen and molecules.
|| 20"- 40", depending on the wavelength (limits stellar - 20',
Sh 2-216 even larger).|
(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.
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,
(see also below)
|| nebular emission lines, dust continuum, IR-emission features.|
||F(12m)/F(25m) 0.35, F(25m)/F(60m) 0.3,
H2 (some objects).
||continuum, mainly molecules CO, OH.|
||diameter 0.1 pc - 0.2 pc (limits 0.005 pc or even smaller, 7 pc)
depending on wavelength.|
|| 10 - 10cm (but also <10cm for old objects and >10cm
for young objects possible).|
||9 000K - 15 000K (limits 8 000K - 23 000 K).|
0.1 M - 0.2 M (limits 0.001 M, 1 M);
neutral gas + dust:
sometimes much higher, very different;
dust mass/gas mass 2x10 to 3x10.
||non-isotropic, 25 km/s (limits 4 km/s, 60 km/s, outer
condensations up to 300 km/s).|
|| 0 - 100 000 years.|
N u c l e u s :
WR, O, Of, WR+Of, OVI, sdO, cont., peculiar, sometimes variable.|
40 000K - 100 000K (limits 20 000K - 250 000K). |
5x10L (limits 10 L, 10L).|
limits 0.005R, 1.5R.|
0.6M (progenitors between 0.8M and 6-8M).|
Mass loss: ||
var 10M/yr - 10M/yr ( 10M/yr in late AGB).|
log g 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]5007,4959 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.
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]5007+4959)/I(H). Moreover the
ratio I(HeII 4686)/I(H) is suitable for high and very high,
whereas I([OII]3727)/I([OIII]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 3727 line.
For low excitation classes the ratio
I([NII]6584)/I(H) 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:
exc. class 1 [OIII]5007 is very weak but
exc. class 0 - 1 is reserved for objects showing no visible
[OIII]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:
|0-1 and 1
|8,9 and 10
We intentionally use the same designation of PNe as in the edition CGPN(1967), i.e. in the system: lll bb.n, where n=1,2 ... is the number of the object in the respective area 1x 1. 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 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
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 x 1 (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.
Nächste Seite: Objects Included and Omitted
Vorherige Seite: Introduction