2012MNRAS.423..934M

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 He II and N V 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 3.5 kpc, corresponding to a stellar luminosity of 4000 L_☉, the V= 16.9 mag central star remains 4-6 mag fainter than the average luminosity of massive WN3 stars even out to an improbable d= 8 kpc. 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= 30 km/s) and a highly ionized spectrum with an approximately solar chemical abundance pattern. The [WN3] star is hot enough to show Ne VII emission (T_*= 140±20 kK) and exhibits a fast wind (v_∞= 1900 km/s), which at d= 3.5 kpc would yield a clumped mass-loss rate of {img}= 1.8 {x} 10^–8 M_☉/yr with a small stellar radius (R_*= 0.11 R_☉). Its atmosphere consists of helium (95 per cent), hydrogen (<2 per cent), nitrogen (0.8 per cent), neon (0.2 per cent) and oxygen (0.05 per 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.

Based on observations made with Gemini South under programme GS-2011A-Q-65 (PI: B. Miszalski).