M4-18: The planetary nebula and its WC10 central star

We present a detailed analysis of the planetary nebula M4-18 (G146.7+07.6) and its WC10-type Wolf-Rayet 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 Teff=31kK, log(Mdot/(Msun yr))=-6.05, v_inf=160 km/s and abundance number ratios of H/He<0.5, C/He=0.60 and O/He=0.10. These parameters are remarkably similar to He2-113 ([WC10]). Assuming an identical stellar mass to that determined by De Marco et al. for He2-113, 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 3100 years, in contrast to >270 years for He2-113. This is supported by the much higher electron density of the latter. These observations may only be reconciled with evolutionary predictions if [WC]-type stars exhibit a range in stellar masses. Photo-ionization 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 Wolf-Rayet model provides the best consistency with the observed nebular properties, although discrepancies remain.