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 -5806 Å = 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. [CITE] (CMB98). We propose 20 selected line ratios and the FWHM of and lines as classification diagnostics, which agree well with the 7 line ratios and the FWHM proposed 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.