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 _ { \odot } , 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 _ { \mathrm { exp } } = 30 km s ^ { -1 } ) and a highly ionised spectrum with an approximately Solar chemical abundance pattern .
The [ WN3 ] star is hot enough to show Ne VII emission ( T _ { \ast } = 140 \pm 20 kK ) and exhibits a fast wind ( v _ { \infty } =1900 km s ^ { -1 } ) , which at d = 3.5 kpc would yield a clumped mass loss rate of \dot { M } = 1.8 \times 10 ^ { -8 } M _ { \odot } yr ^ { -1 } with a small stellar radius ( R _ { \ast } = 0.11 R _ { \odot } ) .
Its atmosphere consists of helium ( 95 % ) , hydrogen ( < 2 % ) , nitrogen ( 0.8 % ) , neon ( 0.2 % ) and oxygen ( 0.05 % ) by mass .
Such an unusual helium-dominated composition can not 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 ] \to O ( He ) .
This suggests 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 .