2024MNRAS.528.2026C

We present Herschel PACS spectroscopy of the [O III] 88.4 μm fine-structure line in the nearby WC8+O binary system γ Vel to determine its oxygen abundance. The critical density of this line corresponds to several 10⁵R_* such that it is spatially extended in PACS observations at the 336 pc distance to γ Vel. Two approaches are used, the first involving a detailed stellar atmosphere analysis of γ Vel using CMFGEN, extending to N_e ∼ 10⁰ cm^–3 in order to fully sample the line formation region of [O III] 88.4 $\mu$m. The second approach involves the analytical model introduced by Barlow et al. and revised by Dessart et al., additionally exploiting ISO LWS spectroscopy of [O III] 51.8 $\mu$m. We obtain higher luminosities for the WR and O components of γ Vel with respect to De Marco et al., log L/L_⊙ = 5.31 and 5.56, respectively, primarily due to the revised (higher) interferometric distance. We obtain an oxygen mass fraction of X_O = 1.0 ± 0.3 per cent for an outer wind volume filling factor of f = 0.5 ± 0.25, favouring either standard or slightly reduced Kunz et al. rates for the ¹²C(α, γ)¹⁶O reaction from comparison with BPASS binary population synthesis models. We also revisit neon and sulphur abundances in the outer wind of γ Vel from ISO SWS spectroscopy of [S IV] 10.5 $\mu$m, [Ne II] 12.8 $\mu$m, and [Ne III] 15.5 $\mu$m. The inferred neon abundance X_Ne = 2.0$_{-0.6}^{+0.4}$ per cent is in excellent agreement with BPASS predictions, while the sulphur abundance of X_S = 0.04 ± 0.01 per cent agrees with the solar abundance, as expected for unprocessed elements.