The Weak Emission Line Stars

In this paper, we study the evolution of the weak emission line central stars of planetary nebula (WELS), which are similar to the H-deficient Wolf-Rayet central stars except for systematically weaker emission lines. Our attempts at finding an evolutionary sequence for the WELS similar to what was established for Wolf-Rayet central stars, were unsuccessful. No correlation was found between any of the analysed quantities: emission and absorption line fluxes or stellar and nebular parameters from the literature. It does appear, however, that WELS have intermediate stellar temperatures (30-80 kK), and do not reside in the middle of Type I planetary nebulae, possibly indicating lower mass precursors. Wolf-Rayet central stars ([WC] CSs) are classified in order of increasing ionization, which is the direction of their evolution (Crowther, De Marco & Barlow 1998). We tried to find a similar sequence for the weak emission lines stars (WELS; Tylenda, Acker & Stenholm 1993), but no clear correlations were found in the data that we looked at. Emission lines of C II (A4267), C III (AA4647,5696), C IV (AA4658,5801), N III (AA4634,40), N V (A4603) , He I (AA4471,5876) and He II (A4876) were measured for the WELS M2-31, NGC6567, NGC6572 and NGC6803, while for the WELS MI-46, Cn3-1, IRAS21282+5050 and A78 we measured mostly absorption lines (all measurements were carried out on I-A resolution spectroscopy, obtained at the 2m INT and 2.1m KPNO telescopes in 1996 and 1998, respectively, and were corrected for nebular contribution). Type I PN have a N/O ratio greater than 0.8 and are thought to derive from a more massive stellar population (Kingsburgh & Barlow 1994). As can be seen from Fig. 1, none of the PNe around WELS are Type I, with only 3 out of the 42 objects for which we found literature values, having unsure results. For the [WC] CSs, on the other hand, 9 out of 49 objects were Type I, with 4 having unsure results. A KS test on these two samples, shows that there is a 97% probability that the two populations are distinct. This may indicate that WELS are preferentially a lower mass population when compared with the [We] stars. This could be in line with the fact that WELS have weaker emission lines, 235 available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900208589 Downloaded from https:/www.cambridge.org/core. IP address: 54.191.125.211, on 16 Jan 2017 at 03:17:31, subject to the Cambridge Core terms of use, 236 Fogel, De Marco & Jacoby 10 Non-TYPE I TYPE I