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Astron. Astrophys. 362, 1008-1019 (2000)

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3. Evolutionary sequence of H-deficient PNNi

Fig. 1 plots [FORMULA] versus the [WC] subclass of the central star. The last bin, labelled PG 1159, includes other H-deficient PNNi. Later on in this paper all the H-deficient PNNi which have not been classified as [WC] will be called, for simplicity, as PG 1159-type.

[FIGURE] Fig. 1. Nebular electron density versus spectral type of H-deficient PNNi.

Fig. 1 confirms the finding of Acker et al. (1996) that the nebular density tends to decrease as one goes from [WC 11] to [WC 2]. As can be seen from Fig. 1, the PG 1159-type objects clearly follow the sequence in the sense that on average they have nebulae of lower density than the earliest [WC] PNNi. Four [WC] objects have been marked with filled squares in Fig. 1. These are late-[WC] PNNi having significantly lower [FORMULA] than the other objects of this class. Acker et al. (1996) have suggested that these low density, late [WC] objects can have a different evolutionary status from the rest of the [WC] PNNi. We return to this point at the end of this section.

Fig. 2 shows the [FORMULA] surface brightness plotted against the central star spectral type. The notation is the same as in Fig. 1. Similarly as [FORMULA] in Fig. 1, [FORMULA] also decreases while going from late-[WC] to early-[WC] and PG 1159.

[FIGURE] Fig. 2. [FORMULA] surface brightness versus spectral type of H-deficient PNNi.

The same evolutionary sequence, even more pronounced than those in Fig. 1 and Fig. 2, can be seen in Fig. 3 which plots [FORMULA] versus the PNN spectral type.

[FIGURE] Fig. 3. [FORMULA] versus spectral type of H-deficient PNNi.

Simple evolutionary considerations show that all the observational parameters investigated in this section, i.e. [FORMULA], [FORMULA] and [FORMULA], decrease as the nebula and its central star evolve with time. Therefore Fig. 1, Fig. 2, and Fig. 3 give observational evidence that the general evolutionary sequence of the H-deficient PNNi is: late-[WC], early-[WC], PG 1159. This sequence is most clearly seen in Fig. 3 exploring [FORMULA]. As noted in Sect. 2, [FORMULA] is a parameter easily distinguishing between young and old objects.

The above evolutionary sequence is the same as that inferred from spectroscopic studies of the H-deficient nuclei (e.g. Hamann 1997). It is also worth noting that infrared studies of the [WC] type PNe suggest that the early-[WC] PNNi evolve from the late types (Górny et al. 1997b).

It should however be stressed that the evolutionary sequence resulting from the above consideration, i.e. late-[WC], early-[WC], PG 1159, should be regarded as a general evolutionary scheme for most of H-deficient PNNi. The available observational data are not good enough to investigate the evolutionary status of individual objects. Especially that we are lacking reliable theoretical evolutionary models for H-deficient PNNi. Therefore we cannot claim that all the H-deficient PNNi exactly follow the above sequence. In fact, from the discussion in Sect. 5.2, we expect that in the PG 1159 population there might be objects that have avoided evolution through the [WC] sequence.

Four objects of late [WC] have been distinguished in Fig. 1 (filled squares) as they have significantly lower [FORMULA] and do not seem to fit to the general evolutionary sequence, as suggested by Acker et al. (1996). However, in Fig. 2 and particularly in Fig. 3 these objects fit much better to the general trend. The electron density has been derived from the [S II ] lines and may refer to less dense nebular regions in these objects ([S II ] lines are collisionally de-excited at high [FORMULA]). Thus we conclude that the suggestion of Acker et al. (1996) that the discussed objects have a different evolutionary status is too far reaching. These central stars seem, however, to evolve somewhat slower than the rest of the population. This can be inferred from the fact that they tend to occupy lower parts of the distributions in Fig. 1, Fig. 2, and Fig. 3. Normally a slowly evolving PNN is interpreted as having a low mass. It seems that it is a likely interpretation in the case of the four late-[WC] PNNi marked with squares in Fig. 1, Fig. 2, and Fig. 3. However one has to remember that the presently avaiable PNN models, even those burning He, have been calculated with H-rich envelopes and therefore they do not correspond to the [WC] PNNi. For a [WC] PNN the effective photosphere is formed in the wind and it is the wind density which is the primary factor determining photospheric parameters and thus the observed spectral type. It is not clear if the PNN mass is the only important factor determining the evolution of the wind density in the [WC] phase.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000