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

B. Miszalski, P. A. Crowther, O. De Marco, J. Köppen, A. F. J. Moffat, A. Acker, T. C. Hillwig

ABSTRACT 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 Lsun, the V = 16.9 mag central star remains 46 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 multiple-shell morphology, a newly discovered Asymptotic Giant Branch (AGB) halo, a relatively low expansion velocity (vexp = 30 km/s) and a highly ionised 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_inf = 1900 km/s), which at d = 3.5 kpc would yield a clumped mass loss rate of Mdot = 1.8e-8 Msun/yr with a small stellar radius (R* = 0.11 Rsun). Its atmosphere consists of helium (95%), hydrogen (less than 2%), nitrogen (0.8%), neon (0.2%) and oxygen (0.05%) by mass. Such an unusual helium-dominated composition cannot be produced by any previous 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 CrB stars and AM CVn stars. The origin of the unusual composition of [WN] and O(He) central stars remains unexplained.