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Astron. Astrophys. 317, 911-918 (1997)

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The evolved central star of the planetary nebula ESO 166-PN 21

M. Peña *, ** 1, M. T. Ruiz 2, *,, P. Bergeron 3, S. Torres-Peimbert 1, **, and S. Heathcote 4

1 Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70 264, México D.F. 04510, México
2 Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
3 Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, Canada, H3C 3J7
4 Cerro Tololo Inter-American Observatory, NOAO, Casilla 603, La Serena, Chile

Received 14 February 1996 / Accepted 20 June 1996


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 = [FORMULA] and N/O = [FORMULA]. The oxygen abundance is 12 + log O/H = [FORMULA]. A distance of [FORMULA] kpc is derived for the nebula. The central star is very faint and blue, with an apparent magnitude [FORMULA] mag and a dereddened color index [FORMULA] = -0.38 mag. It shows faint wide H and He absorption lines typical of a DAO star. By modeling the line profiles we derived [FORMULA] K, log g = [FORMULA] and log He/H = [FORMULA] for the star. The position of the star in a HR diagram compared with evolutionary tracks indicates a stellar mass of [FORMULA] 0.55 [FORMULA]. The bolometric correction derived from the model atmosphere is -5.6 mag which, combined with the mass, yields an absolute visual magnitude [FORMULA] = 6.95, a luminosity of 22 [FORMULA] and a distance of [FORMULA] pc, 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 S 216 and NGC 7293. A kinematical age of 16 100 yr 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.


Key words: planetary nebulae: individual: ESO 166-PN 21 – stars: fundamental parameters – stars: AGB and post-AGB – white dwarfs

* Visiting astronomer at Cerro Tololo Inter-American Observatory operated by AURA under contract with the NSF.
** Guest observer with the International Ultraviolet Satellite operated by NASA.

Send offprint requests to: M. Peña

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998