M. Peña

Galactic planetary nebulae with Wolf-Rayet nuclei II. A consistent observational data set ?;??

We present high resolution spectrophotometric data for a sample of 34 planetary nebulae with (WC) spectral type central stars (WRPNe) in our Galaxy. The observed objects cover a wide range in stellar charac- teristics: early and late (WC) type stars, as well as weak-emission line stars (WELS). Physical conditions in the nebulae (electron density and temperatures) have been obtained from various diagnostic line ratios, and chemical abundances have been derived with the usual empirical scheme. Expansion velocities were estimated in a con- sistent manner from the line proles for most objects of the sample. A statistical study was developed for the derived data in order to nd fundamental relationships casting some light on the evolutionary status of WRPNe. We found evidence for a strong electron temperature gradient in WRPNe which is related to nebular excitation. Such a gradient is not predicted in simple photoionization models. Abundance ratios indicate that there seems to be no preferential stellar mass for the Wolf-Rayet phenomenon to occur in the nucleus of a planetary nebula. Two objects, M 1-25 and M 1-32, were found to have a very small Ne/O ratio, a property dicult to understand. We reexamined the relation between the nebular properties of the WRPNe and the spectral types of the central stars. Our data conrm the trend found by other authors of the electron density decreasing with decreasing spectral type, which was interpreted as evidence that (WC) stars evolve from late to early (WC) types. On the other hand, our data on the expansion velocities do not show the increase of expansion velocity with decreasing spectral type, that one might expect in such a scenario. Two objects with very late (WC) type central stars, K 2-16 and PM 1-188, do not follow the general density sequence, being of very low density for their spectral types. We suggest that the stars either underwent a late helium flash (the born again scenario) or that they have had a particularly slow evolution from the AGB. The 6 WELS of our sample follow the same density vs. (WC)-type relation as the bona de WRPNe, but they tend to have smaller expansion velocities. Considerations about the evolutionary status of WELS must await the constitution of a larger observational sample. The analysis of the dierences between the WRPNe in the Magellanic Clouds (distribution of (WC) spectral types, N/O ratios) and in the Galaxy indicates that metallicity aects the (WR) phenomenon in central stars of planetary nebulae.

Planetary nebulae and H ii regions in the spiral galaxy NGC 300 - Clues on the evolution of abundance gradients and on AGB nucleosynthesis

We have obtained deep spectra of 26 planetary nebulae (PNe) and 9 compact H ii regions in the nearby spiral galaxy NGC 300, and analyzed them together with those of the giant H ii regions previously observed. We have determined the physical properties of all these objects and their He, N, O, Ne, S and Ar abundances in a consistent way. We find that, globally, compact H ii regions have abundance ratios similar to those of giant H ii regions, while PNe have systematically larger N/O ratios and similar Ne/ Oa nd Ar/O ratios. We demonstrate that the nitrogen enhancement in PNe cannot be only due to second dredge-up in the progenitor stars, since their initial masses are around 2–2.5 M� . An extra mixing process is required, perhaps driven by stellar rotation. Concerning the radial abundance distribution, PNe behave differently from H ii regions: in the central part of the galaxy their average O/H abundance ratio is 0.15 dex smaller. Their abundance dispersion at any galactocentric radius is significantly larger than that shown by H ii regions and many of them have O/H values higher than H ii regions at the same galactocentric distance. This suggests that not only nitrogen, but also oxygen is affected by nucleosynthesis in the PN progenitors, by an amount which depends at least on the stellar rotation velocity and possibly other parameters. The formal O/H, Ne/ Ha nd Ar/He abundance gradients from PNe are significantly shallower than those from H ii regions. We argue that this indicates a steepening of the metallicity gradient in NGC 300 during the last Gyr, rather than an effect of radial stellar motions, although the large observed dispersion makes this conclusion only tentative.

The central star of the planetary nebula PB 8: a Wolf-Rayet-type wind of an unusual WN/WC chemical composition

A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. As a rule, these CSPNe exhibit a chemical composition of helium, carbon, and oxygen with the majority showing Wolf-Rayet-like emission line spectra. These stars are classified as CSPNe of a spectral type [WC]. We perform a spectral analysis of CSPN PB 8 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. The source PB 8 displays wind-broadened emission lines from strong mass loss. Most strikingly, we find that its surface composition is hydrogen-deficient, but not carbon-rich. With mass fractions of 55% helium, 40% hydrogen, 1.3% carbon, 2% nitrogen, and 1.3% oxygen, it differs greatly from the 30–50% of carbon which are typically seen in [WC]-type central stars. The atmospheric mixture in PB 8 has an analogy in the WN/WC transition type among the massive Wolf-Rayet stars. Therefore we suggest to introduce a new spectral type [WN/WC] for CSPNe, with PB 8 as its first member. The central star of PB 8 has a relatively low temperature of T∗ = 52 kK, as expected for central stars in their early evolutionary stages. Its surrounding nebula is less than 3000 years old, i.e. relatively young. Existing calculations for the post-AGB evolution can produce hydrogen-deficient stars of the [WC] type, but do not predict the composition found in PB 8. We discuss various scenarios that might explain the origin of this unique object.

The central star of the planetary nebula N 66 in the Large Magellanic Cloud: A detailed analysis of its dramatic evolution 1983–2000

The central star of the planetary nebula Nu200966 (alias WSu200935, SMPu200983 and HVu20095967) in the Large Magellanic Cloud enhanced its brightness dramatically in 1993 and 1994. Within the subsequent four years it returned to the previous level. Its spectrum resembles that of a Wolf-Rayet star of the nitrogen sequence (WN4.5). We monitored the object intensively from ground and with the Hubble Space Telescope. Now we present the complete set of spectroscopic observations from the different epochs before, during and after the brightness outburst ofxa0Nu200966. The stellar spectra from the different epochs are analyzed in detail by means of most advanced non-LTE models for expanding stellar atmospheres. The main results are: the luminosity,

A high resolution spectroscopic study of the extraordinary planetary nebula LMC-N66

log L/L_odot = 4.6

Comprehensive modelling of the planetary nebula LMC-SMP 61 and its [WC]-type central star

, before and after the outburst is exceptionally high for a central star of a planetary nebula. During the outburst in 1994, it even climbed up to

Planetary nebulae as observational constraints in chemical evolution models for NGC 6822

log L/L_odot = 5.4

Kinematic study of planetary nebulae in NGC 6822

for about one year. The effective temperature of about 112u2009kK remained roughly constant, i.e.u2000the luminosity mainly increased because of a larger effective stellar radius. The mass loss rate increased from

A two-mode planetary nebula luminosity function

10^{-5.7} ~M_{odot},{rm yr^{-1}}

Kinematic and chemical study of planetary nebulae and H II regions in NGC 3109

in the quiet state to