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On the Evolutionary Status of WR-Type Planetary Nebula Nuclei R

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On the Evolutionary Status of WR-Type Planetary Nebula Nuclei R ACTA ASTRONOMICA Vol. 43 (1993) pp. 389±396 On the Evolutionary Status of WR-type Planetary Nebula Nuclei by R. Tylenda and S.K. G o r n y Copernicus Astronomical Center, Chopina 12/18, PL-87100 ToruÂn, Poland ABSTRACT The planetary nebula nuclei showing WR-type spectra constitute a unique class of the central stars: they are He-burners. This work presents preliminary results of a study analyzing the observational characteristics of the WR-type nuclei and their nebulae. The existing He-burning models cannot account for the observations of the WR-type nuclei. In the discussion we consider two scenaria: (i) WR-type nuclei are single stars; (ii) WR phenomenon is an evolutionary phase of binary systems. 1. Introduction The planetary nebula nuclei (PNN) showing Wolf-Rayet features in their spectra constitue a unique class of the central stars. This class includes, at present, about 50 objects and all of them are of WC-type (Tylenda et al. 1993). Their strong winds imply that these stars have active shell sources. This and the fact that their atmospheres are hydrogen-poor (Mendez 1991) imply that the WR-type PNN are burning helium. It is, thus, the only class of PNN for which we know the nuclear burning mode. We have undertaken an extensive study of the WR-type PNN in order to better understand their evolutionary status. This paper presents some preliminary results. 2. Observed Samples From the list of WR-type PNN in Tylenda et al. (1993) we have selected a sample of 30 objects for which we have found necessary observational data (e.g. PNN magnitudes, H ¯uxes, nebular diameters) in the Strasbourg-ESO catalogue (Acker et al. 1992). Similarly from the list of H-rich PNN of Mendez (1991) we have selected another sample of 38 objects. An important part of the reported study is devoted to a comparison between these two samples. The names of the objects included in the two samples are given in Table 1. 390 A. A. Table1 Planetary nebulae included in the H-rich sample and in the WR-type sample. H-rich WR-type PK name PK name 9±5.1 NGC 6629 0±1.6 M 2-20 25+40.1 IC 4593 2+5.1 NGC 6369 34+11.1 NGC 6572 2±9.1 Cn 1-5 37±34.1 NGC 7009 3+2.1 Hb 4 43+37.1 NGC 6210 4+4.1 M 1-25 45±4.1 NGC 6804 11+4.1 M 1-32 54±12.1 NGC 6891 17±4.1 M 1-30 80±3.1 NGC 6853 29±5.1 NGC 6751 63+13.1 NGC 6720 61±9.1 NGC 6905 64+48.1 NGC 6058 64+5.1 BD+30 3639 66±28.1 NGC 7094 89+0.1 NGC 7026 83+12.1 NGC 6826 120+9.1 NGC 40 93+5.2 NGC 7008 130+1.1 IC 1747 96+29.1 NGC 6543 144+6.1 NGC 1501 123+34.1 IC 3568 146+7.1 M 4-18 166+10.1 IC 2149 161±14.1 IC 2003 197+17.1 NGC 2392 189+19.1 NGC 2371-7 206±40.1 NGC 1535 243±1.1 NGC 2452 215±24.1 IC 418 278+5.1 PB 6 220±53.1 NGC 1360 278±5.1 NGC 2867 261+32.1 NGC 3242 286+2.1 He 2-55 264±12.1 He 2-5 307±3.1 NGC 5189 285±14.1 IC 2448 309±4.1 He 2-99 292+4.1 PB 8 309±4.2 NGC 5315 294+43.1 NGC 4361 321+3.1 He 2-113 307±4.1 MyCn 18 327±2.1 He 2-142 315±13.1 He 2-131 332±9.1 He 3-1333 316+8.1 He 2-108 336±6.1 PC 14 320±9.1 He 2-138 337+1.1 Pe 1-7 325±12.1 He 2-182 358±21.1 IC 1297 326±6.1 He 2-151 327+10.1 NGC 5882 329+2.1 Sp 1 331±3.1 He 2-162 345+0.1 IC 4637 345±8.1 Tc 1 350+4.1 H 2-1 358±0.2 M 1-26 3. Results From the observational data we have calculated the blackbody Zanstra tem- T ( ) F ( ) T ( ) Z peratures: Z HI from the H to PNN continuum ¯ux ratio, and HeII Vol. 43 391 if the HeII 4686 AÊ line is observed in the nebular spectrum. Then we have Ê T = T ( ) T = T ( ) Z Z Z adopted Z HI if HeII 4686 A is not seen, and HeII if Ê T ( ) T ( ) Z Z HeII HI (this latter conditionis satis®edinall caseswhenHeII 4686 A T is observable). Fig. 1 plots Z against the WC subclass for the WR sample. As expected, the correlation between the two parameters is clear. Fig. 1. The Zanstra temperature against the WC subclass for the WR sample. T In Fig. 2 we display the histograms of Z for both samples. As can be seen the T H-rich sample tends to have somewhat lower Z than the WR sample. This is most probably due to observational selection. It is easier to identify a H-rich PNN at low T e when the Balmer lines are expected to be well seen in the spectrum. Note that the histogram for the WR sample does not show any signi®cant de®ciency of PNN T having middle Z as could have been expected from the observed lack of the WC 5 ± 8 subclasses (Tylenda et al. 1993). The ®rst difference between the samples which we have noted from the obser- V vational data concerns the nebular expansion velocities, exp . For the WR sample V the mean exp is 31.0 km/s with a standard deviation of 9.0 km/s. For the H-rich sample the ®gures are 18.7 km/s and 11.0 km/s, respectively. V T Z Fig. 3 shows the relation between exp and . Full circles represent the WR-type PNN while the open circles shows the objects from the H-rich sample. V Both samples follow a similar trend in the sense that exp tends to increase with T V exp the increasing Z . The obtained difference in the mean value of is partly due to selection effects in the samples. As can be seen from Fig. 3 objects having low T V exp Z and measured are underpopulated in the WR sample. Nevertheless, a part V of the difference in mean exp seems to be real. It can be seen that independently T V exp of Z the WR-type PNN tend to situate at higher than the H-rich PNN. 392 A. A. T Fig. 2. Top: The histogram of T for the H-rich sample. Bottom: The histogram of for the Z Z WR-type sample. Vol. 43 393 Fig. 3. The relation between the PN expansion velocity and the PNN Zanstra temperature. Full circles ± WR-type PNN; open circles ± H-rich PNN. f Fig. 4. TZ against the parameter . Full circles ± WR-type PNN. Open circles ± H-rich PNN. Dashed curves ± theoretical H-burning tracks. T f Fig. 4 plots Z against the parameter de®ned as 2 F V V exp = ( ) f 2 1 F V where V is the PNN ¯ux in the band and is the observed nebular radius. 394 A. A. This sort of diagrams, for the ®rst time used by Tylenda and Stasinska (1989), can be used for a comparison between observed PNN and theoretical models. Its great advantage is that it is independent of distances. Full symbols in Fig. 4 represent the WR-type PNN whereas the H-rich objects are shown as open symbols. Double V arrows mark objects for which exp is not known and we have adopted the mean V f exp for the WR sample while calculating . Dashed curves show the theoretical tracks of SchonbernerÈ (1981, 1983) and BlockerÈ and SchonbernerÈ (1991). These tracks are shown for orientation rather than for a comparison between the models and the observations. They represent H-burning PNN whereas the WR-type PNN are certainly He-burners. As can be seen from Fig. 4 the WR-type PNN tend to situate towards upper- left in the diagram in comparison with the H-rich sample. This would normally imply higher PNN masses. It is, however, premature to conclude like that since the nuclear burning mode may be different in the two samples. For most of the WR-type PNN in our sample, especially for those with low T f Z , we have obtained large values for the parameter (see Fig. 4). This indicates that their PN are rather compact and young. Therefore we can conclude that the H-poor layers in these stars have been exposed shortly after the PN formation at the tip of AGB. This excludes, from our considerations, scenaria like that of a ®nal helium shell ¯ash (Iben et al. 1983) which produce a H-poor, He-burning nucleus surrounded by an old, large nebula. He-burning PNN models have been calculated by a number of authors (e.g. Iben 1984, Wood and Faulkner 1986, Vassiliadis and Wood 1993). Unfortunately, most of them cannot be compared to the WR-type PNN since they conserve H-rich envelopes throughout the PN phase. The only set of He-burning PNN models in which the H-poor layers are quickly exposed after having left the AGB is the one of = Wood and Faulkner (1986) which leaves the AGB at phase 0 and adopts the type B mass loss.
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