1999MNRAS.306..931D

We present a detailed analysis of the planetary nebula M4-18 (G146.7+07.6) and its WC10-type Wolf-Rayet (WR) central star, based on high-quality optical spectroscopy (WHT/UES, INT/IDS, WIYN/DensPak) and imaging (HST/WFPC2). From a non-LTE model atmosphere analysis of the stellar spectrum, we derive T_eff=31kK, log(M{dot}/M_☉.yr^–1)=-6.05, v_∞=160km.s^–1 and abundance number ratios of H/He<0.5, C/He=0.60 and O/He=0.10. These parameters are remarkably similar to those of He2-113 ([WC10]). Assuming an identical stellar mass to that determined by De Marco et al. for He 2-113, we obtain a distance of 6.8kpc to M4-18 [E(B-V)=0.55mag from nebular and stellar techniques]. This implies that the planetary nebula of M4-18 has a dynamical age of ∼3100yr, in contrast to ≥270yr for He 2-113. This is supported by the much higher electron density of the latter. These observations may be reconciled with evolutionary predictions only if [WC]-type stars exhibit a range in stellar masses.

Photoionization modelling of M4-18 is carried out using our stellar WR flux distribution, together with blackbody and Kurucz energy distributions obtained from Zanstra analyses. We conclude that the ionizing energy distribution from the WR model provides the best consistency with the observed nebular properties, although discrepancies remain.