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

Astronomy & Astrophysics A&A 367, 983–994 (2001) DOI: 10.1051/0004-6361:20000497 c ESO 2001 Galactic planetary nebulae with Wolf-Rayet nuclei II. A consistent observational data set?,?? M. Peña1 , G. Stasińska2 , and S. Medina1 1 Instituto de Astronomı́a, Universidad Nacional Autónoma de México, Apdo. Postal 70 264, México D.F. 04510, México e-mail: 2 DAEC, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France e-mail: Received 21 September 2000 / Accepted 5 December 2000 Abstract. 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 characteristics: 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 consistent manner from the line profiles for most objects of the sample. A statistical study was developed for the derived data in order to find 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 difficult to understand. We reexamined the relation between the nebular properties of the WRPNe and the spectral types of the central stars. Our data confirm 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 fide 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 differences between the WRPNe in the Magellanic Clouds (distribution of [WC] spectral types, N/O ratios) and in the Galaxy indicates that metallicity affects the [WR] phenomenon in central stars of planetary nebulae. Key words. planetary nebulae: general – stars: Wolf-Rayet – planetary nebulae: individual: K 2-16, PM 1-188, M 1-32, M 3-15 1. Introduction Since the pioneering work of Paczyński (1971), the basic scheme for the production of planetary nebulae (hereSend offprint requests to: M. Peña, e-mail: ? Based on data obtained at the Observatorio Astronómico Nacional, SPM, B.C., México ?? Tables 2 and 3 are only available at http://www.edpsciences.org inafter PNe) and the evolution of their nuclei is relatively well understood. Planetary nebulae are formed from stars of initial masses between 1 and 8 M , which have completely burnt hydrogen and helium in their cores, have twice climbed the giant branch and lost most of their mass through intense winds. At this stage, the stars consist of dense carbon-oxygen cores of about 0.6 M surrounded by an envelope composed of two thin shells. The innermost one is composed of helium, and the outermost of hydrogen. When the strong winds cease, the stars evolve Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20000497 984 M. Peña et al.: WR planetary nebulae in the Galaxy. II. towards larger effective temperatures and start ionizing the matter lost previously in the winds. The physical reason for the departure from the asymptotic giant branch is not quite understood, nor is the nature of the energy source (hydrogen- or helium-burning) during the evolution of the planetary nebula nuclei (Iben 1995). About 50 planetary nebulae in our Galaxy are known to have central stars of Wolf-Rayet (hereinafter [WR]) type among 350 PNe with a stellar continuum measured and over 1000 PNe with nebular spectroscopy available (Tylenda et al. 1993; Górny & Stasińska 1995). All these [WR] central stars have been reported to be of [WC] spectral type, mostly [WC 2–4] and [WC 8–11] (Tylenda et al. 1993) and their atmospheres are almost pure helium and carbon (e.g. Hamann 1997). These planetary nebulae with [WR] nuclei, hereinafter WRPNe, have recently been the subject of much attention (e.g. Górny & Stasińska 1995; Crowther et al. 1998; Górny & Tylenda 2000; Górny et al. 2000), as they pose a problem for the theory of the evolution of PN central stars. Indeed, helium-burning models (Wood & Faulkner 1986; Vassiliadis & Wood 1994) do maintain a thin hydrogen-rich outer layer, as emphasized by Górny & Tylenda (2000). One way to obtain a hydrogen-free outer layer is through the born-again scenario (Iben et al. 1983), when a final helium-shell flash occurs while the star is in its cooling phase, and drives it back to the asymptotic giant branch (AGB). However, such a scenario cannot hold for all the WRPNe, as shown by Górny & Tylenda (2000). They suggest that the central stars of most WRPNe evolve directly from the AGB. The high carbon abundance observed in these stars implies that deep mixing has occurred (Herwig et al. 1997; Herwig et al. 1998). It has been suggested that the spectral sequence of [WC] stars corresponds to an evolutionary sequence from late to early types, ending with the PG 1159 type stars. This suggestion was based on the analysis of the stellar atmospheres of [WC] stars (Hamann 1997; Leuenhagen & Hamann 1998) and on the nebular properties of the WRPNe (Acker et al. 1996). Górny & Tylenda (2000) provided further observational arguments for the existence of such an evolutionary sequence. Tylenda et al. (1993) defined a category of “weak emission line stars” (hereinafter WELS), whose emission lines have much lower equivalent widths than the bona fide [WR] stars. Parthasarathy et al. (1998) claim that WELS are an intermediate stage between [WC] and PG 1159 stars. This, however, is not the end of the story. First, not all WRPNe belong to proposed evolutionary sequence. For example 4 WRPNe with late type central stars, namely M 4-18, He 2-459, He 2-99 and NGC 40, were considered to have too low densities (...truncated)