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Astron. Astrophys. 347, 169-177 (1999)

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1. Introduction

The central stars of planetary nebulae (CSPN) play an important role in our understanding of post-AGB (AGB = asymptotic giant branch) stellar evolution: They display directly the formation of white dwarfs (WD). After the ejection of a planetary nebula (PN) still on the AGB, their effective temperatures ([FORMULA]) increase at almost constant luminosity. Depending on the stellar mass, very high [FORMULA] can be reached ([FORMULA]). At [FORMULA] [FORMULA] their nebulae are ionized. The excitation class (EC) of a PN can be regarded as an indicator of the ionizing spectrum, i.e. [FORMULA] of its exciting CSPN. After the end of nuclear burning, temperatures and luminosities of the CSPN decrease and effects of gravitational settling dominate their further evolution into WD. Thus, CSPN of old (large) PN are found at the beginning of the WD cooling sequence.

Although many PN with very high EC are known, and nebula spectra are available, most of their CSPN have been disregarded so far in spectral analyses because they are very faint in the optical wavelength range.

Possibly due to the lack of CSPN studied by quantitative spectral analysis a discrepancy was found in the ratio of H- / He-rich objects in the case of cool WD and luminous PN ([FORMULA]) and in the case of hot WD / CSPN ([FORMULA]) and it seemed likely that all WD had passed a He-rich phase in their evolution ("one-channel-hypothesis", e.g. Fontaine & Wesemael 1987). In a recent investigation of old PN Napiwotzki & Schönberner (1995) analyzed 39 northern CSPN by means of NLTE (non-Local Thermodynamic Equilibrium) model atmosphere techniques and found a ratio of H- / He-rich CSPN of about 4.5 in the range [FORMULA]. However, only few very hot CSPN were detected in this sample and it seems that the discrepancy still exists for the hottest stars. It is worthwhile to note that the investigation of CSPN of old PN is a promising way for the discovery of H-deficient PG 1159 stars (in their sample, Napiwotzki & Schönberner discovered seven of them and three so-called "hybrid" stars, see below).

In the last decade many spectral analyses of very hot ([FORMULA] [FORMULA]) post-AGB stars by means of state-of-the-art NLTE model atmospheres have been performed (Rauch et al. 1994, 1996, 1998, 1999; Werner 1991; Werner & Rauch 1994; Werner et al. 1991, 1995) and our picture of post-AGB evolution has been improved. These analyses include the extremely hot He-rich PG 1159 stars which are characterized by broad and shallow He and C absorption lines. Their photospheric abundances can be explained by the "born-again post-AGB star" scenario of Iben et al. (1983): A late helium flash at already declining luminosity brings these stars back to the AGB and they start a second, He-burning phase (on a three times longer time scale than in the H-burning phase) of post-AGB evolution. In this phase they can completely lose their H-rich and most of their He-rich envelope and the intershell layers of the former thermally pulsing and double-shell burning AGB stars become visible at the stellar surface. The fact that about half of them show multiperiodic nonradial g-mode pulsations (Werner et al. 1991) allows the determination of photospheric parameters within small error ranges, making them to fixed points within evolutionary theory. Moreover, about every other PG 1159 star has an associated PN which permits to test predictions of evolutionary theory, e.g. hydrogen-deficient parts in the inner nebula should result of a second super-wind phase as predicted by Iben et al. (1983). However, in spite of the large variety of their spectral appearance the spectral analysis of previously unobserved, very hot CSPN is highly desirable.

In recent analyses of the related O(He)-type stars (Rauch et al. 1994, 1996, 1998) which in contrast to the PG 1159 stars exhibit an almost pure He II absorption line spectrum, evidence was found that these objects may also be "born-again" stars and thus might be progenitors of a PG 1159 stars.

The detection of "hybrid" CSPN (Napiwotzki & Schönberner 1991) which additionally show, in contrast to the PG 1159 stars, strong H lines contradicts the existence of two distinct evolutionary paths of pre-WD evolution (H-rich and H-deficient) and their unique connection to the H- / He-rich WD cooling tracks.

If the "gap" between the hottest known DA WD and hot H-rich CSPN is a selection effect as suggested by the results of Napiwotzki & Schönberner (1995) and their ratio of H- / He-rich stars of 4.5 is valid for very hot CSPN, too, then we can expect to discover very hot H-rich CSPN as well as PG 1159 stars in an investigation of faint, hitherto unobserved CSPN.

[FIGURE] Fig. 1. PN G214.9+07.8 has a perfectly circular shape, a central hole with radial "cartwheel" structures and weak polar lobes. A thin shell is visible just outside the main nebula. H [FORMULA] = left, [O III ] [FORMULA] 5007 Å = right, North up, East right, field (this image!) [FORMULA] (cf. Table 2)

[FIGURE] Fig. 2. PN G231.8+04.1 is a quite circular multiple-shell PN with a prominent central hole and polar lobes. It exhibits a distinct faint outer shell as well as a large very faint halo (see lower panel). All three shells are fairly regularly shaped. However, the brighter rim from the NS to the SE side and the slightly wedge-shaped inner nebula suggest an interaction with the interstellar matter. The two panels differ in the cuts for the intensity scale: top 0:15 000, bottom 0:200

[FIGURE] Fig. 3. PN G249.3-05.4 is a circular PN with prominent central hole

[FIGURE] Fig. 4. PN G253.5+10.7 is an elliptical PN with a strong central hole and "ansae" which are quite prominent in [O III ] (right). One notes two thin jet-like features aligned with the central star, 60o off the axis connecting the ansae and the two main [O III ] emission blobs (cf. Bell & Pollacco 1997)

[FIGURE] Fig. 5. PN G257.5+00.6 is a highly amorphous PN with two lobes that have point symmetric extensions. Further out, one notes many faint nebulosities, filaments, and loops. The three panels differ in the intensity cuts: top 0:1 200, middle 0:450, bottom 0:100. The lower panel shows the S-W quadrant of the PN: nebula emission is detected 225" to the West, 310" to the South. Thus we estimate [FORMULA]" and [FORMULA]" (Table 2)

[FIGURE] Fig. 6. PN G277.1-03.8 is a non-circular PN with two lobes. Its shape reminds of a torus seen in the equatorial plane, with large lobes of diffuse emission at the poles. The tubular shape is not straight, but has a kink. The lower panel shows a detail from the inner nebula: two stars are found here very close to each other, the easterly one is the central star. Intensity cuts: top and bottom 0:20 000, middle 0:200

[FIGURE] Fig. 7. PN G283.6+25.3 is a circular PN with two lobes without a central hole

[FIGURE] Fig. 8. PN G293.6+10.9 is a circular PN with weak lobes without a central hole. There appears a distinct secondary shell just outside the main nebula

[FIGURE] Fig. 9. PN G324.1+09.0 is a circular PN without a central hole

[FIGURE] Fig. 10. Comparison of our synthetic line profiles to the observation

[FIGURE] Fig. 11. Spectrum of the supposed central star of PN 277.1-03.8 compared to a Kurucz model (dotted curve, [FORMULA], [FORMULA]). Obviously, the spectrum arises most likely from a F-type star which suggests a composite binary spectrum which is dominated in the optical by the cool component

[FIGURE] Fig. 12. Spectra of the central stars of the PN from Table 1. The positions of identified lines are marked. Note that only in the top four spectra He II [FORMULA]Å is clearly detectable. Synthetic spectra with parameters from Table 4 are overplotted. In the case of PN G277.1-03.8, left and right denote two objects which are very close together at the center of the PN (Fig. 6). In the case of PN G257.5+00.6, 1st and 2nd denote the two objects described in Sect. 2.2. Neither the spectra of the PN G257.5+00.6 stars nor those of the PN G277.1-03.8 resemble the spectra of the exciting stars but rather F-type companions. A close inspection of the spectrum of the central star of PN G283.6+25.3 shows at [FORMULA] similar absorption features like in the supposed F-type spectra of PN G257.5+00.6 and PN G277.1-03.8 (marked by dashed lines). This suggests the presence of a cool companion which was recently identified on Hubble Space Telescope images by Bond et al. (1997), [FORMULA] away from the central star. PN G253.5+10.7 shows the (reflection) spectrum of the binary system (cf. Livio 1997)

[FIGURE] Fig. 13. Positions of our CSPN (with respective error bars) in the [FORMULA] - [FORMULA] diagram compared to theoretical evolutionary tracks of hydrogen burning stars (Schönberner 1983, Blöcker & Schönberner 1990). The tracks are labeled with the respective stellar masses (in [FORMULA])

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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