IC 4663: the first unambiguous [WN] Wolf-Rayet central star of a planetary nebula

We report on the serendipitous discovery of the first central star of a planetary nebula (PN) that mimics the helium- and nitrogen-rich WN sequence of massive Wolf-Rayet (WR) stars. The central star of IC 4663 (PN G346.2-08.2) is dominated by broad Heii and Nv emission lines which correspond to a [WN3] spectral type. Unlike previous [WN] candidates, the surrounding nebula is unambiguously a PN. At an assumed distance of 3.5kpc, corresponding to a stellar luminosity of 4000L _⊙, the V= 16.9mag central star remains 4-6mag fainter than the average luminosity of massive WN3 stars even out to an improbable d= 8kpc. The nebula is typical of PNe with an elliptical morphology, a newly discovered asymptotic giant branch (AGB) halo, a relatively low expansion velocity (v _exp= 30kms ^-1) and a highly ionized spectrum with an approximately solar chemical abundance pattern. The [WN3] star is hot enough to show Nevii emission (T _*= 140 ± 20kK) and exhibits a fast wind (v _∞= 1900kms ^-1), which at d= 3.5kpc would yield a clumped mass-loss rate of = 1.8 × 10 ^-8M _⊙yr ^-1 with a small stellar radius (R _*= 0.11R _⊙). Its atmosphere consists of helium (95per cent), hydrogen (<2per cent), nitrogen (0.8per cent), neon (0.2per cent) and oxygen (0.05per cent) by mass. Such an unusual helium-dominated composition cannot be produced by any extant scenario used to explain the H-deficiency of post-AGB stars. The O(He) central stars share a similar composition and the discovery of IC 4663 provides the first evidence for a second He-rich/H-deficient post-AGB evolutionary sequence [WN] →O(He). This suggests that there is an alternative mechanism responsible for producing the majority of H-deficient post-AGB stars that may possibly be expanded to include other He-rich/H-deficient stars such as R Coronae Borealis stars and AM Canum Venaticorum stars. The origin of the unusual composition of [WN] and O(He) central stars remains unexplained.