ISO-Sws Studies of Wolf-Rayet Stars: Neon Abundances

Abstract

ISO-sws data of one WN star and four WC stars are presented. The ISO-sws data of WR 147 (WN8(h)+B0.5 V) show the [CaIv] 3.21 µm, [Sly] 10.5 µm and [Netts] 15.5 µm fine-structure lines. We obtain a terminal wind velocity of ~950 km s^-1 from the helium recombination lines and the observed infrared fine-structure lines. Previously derived interstellar extinction values of EB_ v ≃ 4.1 mag at Rv ≃ 2.7 are confirmed. The distance to WR 147 is revised slightly upwards to 650 pc, based on galactic and LMC WN8-9 calibrators in the K-band. The observed spectrum exhibits clear evidence for clumping in the wind of the WN8 star. Using a non-LTE, line-blanketed model that allows for clumping, we find that a volume filling factor of 10% nicely matches the observed helium line-profiles, though we cannot exclude filling factors in the range of 4% to 25%. This leads to a mass-loss rate range of (1.5-3.7) x 10^-5My⊙r ^-1, substantially lower than derived for WR 147 from optical or radio observations under assumptions of homogeneity. Additionally including the effects of line-blanketing, the wind performance factor, Mv _∞ /(L _* /c),is reduced to ~2.5. Hydrogen is severely depleted, but present at 9% by mass. Observations of the [Ca Iv] 3.21 µm, [Sly] 10.5 µm, and [Netts] 15.5 µm lines allow us to place constraints on the surface abundances of these elements. In all cases we find good agreement with cosmic abundances (adjusted to a hydrogen-poor envi-ronment) and thus with evolutionary predictions for WN stars, which should not yet exhibit the products of core-burning stages beyond the CNO cycle before reaching the WC phase.