Quantitative classification of WR nuclei of planetary nebulae

Abstract

We analyse 42 emission-line nuclei of Planetary Nebulae (PNe), in the framework of a large spectrophotometric survey of (WC) nuclei of PNe conducted since 1994, using low/medium resolution spectra obtained at ESO and at OHP. We construct a grid of selected line-intensities (normalized to C iv-5806 A= 100) ordered by decreasing ionisation potential going from 871 to 24 eV. In this grid, the stars appear to belong clearly to prominent O (hot (WO1-4) types) or C (cooler (WC4-11) types) line-sequences, in agreement with the classification of massive WR stars applied to Central Stars of Planetary Nebulae (CSPNe) by Crowther et al. 1998 (CMB98). We propose 20 selected line ratios and the FWHM of Civ and Ciii lines as classification diagnostics, which agree well with the 7 line ratios and the FWHMproposed by CMB98. This classification based on ionisation is related to the evolution of the temperature and of the stellar wind, reflecting the mass-loss history. In particular, inside the hot (WO4)-class, we discover four stars showing very broad lines over the whole spectral range. These stars possibly mark the transition from the initial momentum-driven phase to the later energy-driven phase of the CSPNe along their evolution from the post-Asymptotic Giant Branch (post-AGB) phase through (WC) late, (WC4) and (WO)-types. The HR diagram and the diagram linking the terminal velocity and the temperature indicate highly dispersed values of the stellar mass for our sample, around a mean mass higher than for normal CSPNe. The distribution of the 42 stars along the ionisation sequence shows 24% of (WO1-3), 21% of (WO4), 17% of (WC4) hot stars, and 26% of (WC9-11) cool stars. The (WC5-8) classes remain poorly represented (12%). This distribution is confirmed on the basis of a large compilation of the 127 known emission-lines CSPNe, which represent about 5% of the known PNe.