THE EVOLVED CENTRAL STAR OF THE PLANETARY-NEBULA ESO-166-PN-21

Optical and UV spectrophotometric data of the nebula and the central s tar 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 1 2 + log O/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 appar ent magnitude V = 17.94 +/- 0.03 mag and a dereddened color index (B - V)(0) = -0.38 mag. It shows faint wide H and He absorption lines typi cal of a DAO star. By modeling the line profiles we derived T-eff = 69 200 +/- 8 700 K, log g = 7.14 +/- 0.39 and log He/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 similar to 0.55 M.. Th e bolometric correction derived from the model atmosphere is -5.6 mag which, combined with the mass, yields an absolute visual magnitude M(v ) = 6.95, a luminosity of 22 L. and a distance of 1 185 +/- 700 pc, in good agreement with the nebular distance. Therefore, ESO 166-PN 21 ce ntral 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 sta rs of S 216 and NGC 7293. A kinematical age of 16 100 yr is deduced fo r 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.