Oxygen abundance of γ Vel from [O iii] 88μm Herschel/PACS spectroscopy

Abstract

We present Herschel PACS spectroscopy of the [O iii] 88.4μm 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 Ne ∼ 100 cm−3 in order to fully sample the line formation region of [O iii] 88.4 μm. 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 μm. 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 XO = 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 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 μm, [Ne ii] 12.8 μm and [Ne iii] 15.5 μm. The inferred neon abundance XNe = 2.0$_{-0.6}^{+0.4}$% is in excellent agreement with BPASS predictions, while the sulphur abundance of XS = 0.04 ± 0.01% agrees with the solar abundance, as expected for unprocessed elements.