1997A&A...317..911P

Optical and UV spectrophotometric data of the nebula and the central star of the planetary nebula ESO 166-PN 21 are presented. The analysis of the nebular lines confirms that it is a He- and N-rich PN, with He/H=0.138±0.005 and N/O=0.58±0.08. The oxygen abundance is 12+logO/H=8.60±0.10. A distance of 1.2±0.2 kpc is derived for the nebula. The central star is very faint and blue, with an apparent magnitude V=17.94±0.03mag and a dereddened color index (B-V)₀=-0.38mag. It shows faint wide H and He absorption lines typical of a DAO star. By modeling the line profiles we derived T_eff=69200±8700K, logg=7.14±0.39 and logHe/H=-1.50±0.49 for the star. The position of the star in a HR diagram compared with evolutionary tracks indicates a stellar mass of ∼0.55M_☉. The bolometric correction derived from the model atmosphere is -5.6mag which, combined with the mass, yields an absolute visual magnitude M_V=6.95, a luminosity of 22L_☉ and a distance of 1185±700pc, in good agreement with the nebular distance. Therefore, ESO 166-PN 21 central star is among the hottest and most helium-rich DAO stars and it is one of the most evolved PN nuclei known, similar to the central stars of S216 and NGC7293. A kinematical age of 16100yr is deduced for the nebula which is lower by about two orders of magnitude than the age of the central star. The possibility that this object is a member of a close binary system is suggested.