Oxygen abundance of gamma Vel from [O III] 88um Herschel/PACS spectroscopy

We present Herschel PACS spectroscopy of the [O III] 88.4um fine-structureline in the nearby WC8+O binary system gamma Vel to determine its oxygenabundance. The critical density of this line corresponds to several 10^5 R*such that it is spatially extended in PACS observations at the 336 pc distanceto gamma Vel. Two approaches are used, the first involving a detailed stellaratmosphere analysis of gamma Vel using CMFGEN, extending to Ne ~ 10^0 cm^-3 inorder to fully sample the line formation region of [O III] 88.4um. The secondapproach involves the analytical model introduced by Barlow et al. and revisedby Dessart et al, additionally exploiting ISO LWS spectroscopy of [O III]51.8um. We obtain higher luminosities for the WR and O components of gamma Velwith respect to De Marco et al, log L/L_sun = 5.31 and 5.56, respectively, dueto the revised (higher) interferometric distance. We obtain an oxygen massfraction of X_O = 1.0+/- 0.3% for an outer wind volume filling factor of f =0.5+/-0.25, favouring either standard or slightly reduced Kunz et al. rates forthe ^12C(alpha, gamma)^16O reaction from comparison with BPASS binarypopulation synthesis models. We also revisit neon and sulphur abundances in theouter wind of gamma Vel from ISO SWS spectroscopy of [S IV] 10.5um [Ne II]12.8um and [Ne III] 15.5um. The inferred neon abundance X_Ne = 2.0-0.6+0.4%, isin excellent agreement with BPASS predictions, while the.sulphur abundance ofX_S = 0.04 +/- 0.01% agrees with the solar abundance, as expected forunprocessed elements.