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

Abstract

Neon abundances are derived in four Galactic WC stars - γ²Vel (WR11, WC8+O7.5III), HD156385 (WR90, WC7), HD192103 (WR135, WC8) and WR146 (WC5+O8) - using mid-infrared fine-structure lines obtained with ISO-SWS. Stellar parameters for each star are derived using the non-local thermodynamic equilibrium model atmospheric code of Hillier & Miller, together with ultraviolet (IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In the case of γ²Vel, we adopt very recent results from De Marco et al., who followed an identical approach. ISO-SWS data sets reveal the [Neiii] 15.5-μm line in each of our targets, while [Neii] 12.8μm, [Siv] 10.5μm and [Siii] 18.7μm are observed solely in γ²Vel. Using a method updated from Barlow et al. to account for clumped winds, we derive Ne/He=(3-4)×10^-3 by number, plus S/He=6×10^-5 for γ²Vel. Neon is highly enriched, such that Ne/S in γ²Vel is eight times higher than cosmic values. However, observed Ne/He ratios are a factor of 2 lower than predictions of current evolutionary models of massive stars. An imprecise mass loss and distance were responsible for the much greater discrepancy in neon content identified by Barlow et al. Our sample of WC5-8 stars span a narrow range in T* (=55-71kK), with no trend towards higher temperature at earlier spectral type, supporting earlier results for a larger sample by Koesterke & Hamann. Stellar luminosities range from 100000 to 500000L_solar, while 10^-5.1 <= M/(M_solar yr^-1) <= 10^-4.5, adopting clumped winds, in which volume filling factors are 10per cent. In all cases, wind performance numbers are less than 10, significantly lower than recent estimates. Carbon abundances span 0.08<=C/He<=0.25 by number, while oxygen abundances remain poorly constrained.