Quantitative analysis of WC stars: constraints on neon abundances from ISO-SWS spectroscopy

Neon abundances are derived in four Galactic WC stars - gamma(2) Vel (WR 11 , WC8+O7.5III), HD 156385 (WR 90, WC7), HD 192103 (WR 135, WC8) and WR 146 (WC5+O8) - using mid-infrared fine-structure lines obtained with ISO-SWS. S tellar parameters for each star are derived using the non-local thermodynam ic equilibrium model atmospheric code of Hillier & Miller, together with ul traviolet (IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In the case of gamma(2) Vel, we adopt very recent results from De Marco et al., who followed an identical approach. ISO-SWS data sets reveal the [Ne III] 15.5-mu m line in each of our targets , while [Ne II] 12.8 mu m, [S IV] 10.5 mu m and [S III] 18.7 mu m are obser ved solely in gamma(2) Vel. Using a method updated from Barlow et al. to ac count for clumped winds, we derive Ne/He = (3-4) x 10(-3) by number, plus S /He = 6 x 10(-5) for gamma(2) Vel. Neon is highly enriched, such that Ne/S in gamma(2) Vel is eight times higher than cosmic values. However, observed Ne/He ratios are a factor of 2 lower than predictions of current evolution ary models of massive stars. An imprecise mass loss and distance were respo nsible for the much greater discrepancy in neon content identified by Barlo w et al. Our sample of WC5-8 stars span a narrow range in T-* (=55-71 kK), with no t rend towards higher temperature at earlier spectral type, supporting earlie r results for a larger sample by Koesterke & Hamann. Stellar luminosities r ange from 100 000 to 500 000 L., while 10(-5.1) less than or equal to M/(M. yr(-1)) less than or equal to 10(-4.5) adopting clumped winds, in which vo lume filling factors are 10 per cent. In all cases, wind performance number s are less than 10, significantly lower than recent estimates. Carbon abund ances span 0.08 less than or equal to C/He less than or equal to 0.25 by nu mber, while oxygen abundances remain poorly constrained.