EDP Sciences
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Volume 430, Number 1, January IV 2005
Page(s) 187 - 212
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20041382

A&A 430, 187-212 (2005)
DOI: 10.1051/0004-6361:20041382

Endogenous oxygen in the extremely metal-poor planetary nebula PN G135.9+55.9

D. Péquignot and Y. G. Tsamis

LUTH, Laboratoire l'Univers et ses Théories, associé au CNRS (FRE 2462) et à l'Université Paris 7, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
    e-mail: Daniel.Pequignot@obspm.fr

(Received 1 June 2004 / Accepted 7 September 2004)

It is shown that, in contrast to recent claims, oxygen (and helium) may not be extraordinarily underabundant in the new galactic halo planetary nebula (GHPN) PN G135.9+55.9 (hereafter PN $\,$G135). Determining elemental abundances in hot, highly ionized objects such as PN $\,$G135 depends critically on a proper description of the collisional excitation of the hydrogen Balmer lines, the departure from Case B recombination of hydrogen, the underlying stellar absorption lines, the shape of the primary continuum and the ionization equilibrium of highly ionized species of both oxygen and neon. Conversely, PN $\,$G135 provides unique checks of atomic data in unusual conditions: the $\ion{H}{i}$ collision strengths obtained by Aggarwal et al. (1991) for 1s-n transitions ( $3\leq n\leq 5$) are too large, while those obtained by Anderson et al. (2002) are acceptable. Empirical collision strengths are presented for n > 5. Photoionization models of PN $\,$G135 that fit all available optical data can be demonstrated only for oxygen abundances $12 + \log{\rm (O/H)} > 7.2$ ( >1/30 solar) and values 0.6 dex larger are possible, depending on the assumed C/O abundance ratio. Plausible variations in the geometry of the nebula, the primary stellar continuum and the atomic data do not alter this conclusion. The C/O ratio is less than 10 by number and Ne/O is at most solar. A satisfactory model for PN $\,$G135 can be obtained in which elemental abundances are nearly the same as those of a new detailed model for K $\,$648, the prototypical GHPN in the old globular cluster M 15 (with $12 + \log{\rm (O/H)} = 7.58 \sim 1/13$ solar), although C/O may be smaller. Nonetheless, given the paucity of argon and iron in the nebula, PN $\,$G135 is likely to be a more extreme Population II object than K $\,$648, reinforcing the idea of an endogenous origin for part of the oxygen in very metal-poor PNe. Assuming a standard H-burning post-Asymptotic Giant Branch evolution, timescale and spectroscopic considerations lead to an optimal solution, in which the distance to PN $\,$G135 is 8K $\,$648, the effective temperature of the nucleus slightly less than $1.3\times10^5$ K, its luminosity $1.4\times10^{37}$ erg s -1, its mass 0.59 $M_{\odot} $, the age of the ionized shell 10 4 yrs, the ionized mass 0.05 $M_{\odot} $ and the abundances by number (H:He:C:O:Ne) = (10 6:81 500:90:30:4.5), with C/H being rather an upper limit and O/H and Ne/H uncertain by $\pm$0.3 and $\pm$0.1 dex respectively. Line intensities that could be used as diagnostics of the nebular elemental abundances are provided. Detailed imaging together with ultraviolet and very deep far-red spectra of PN $\,$G135 will be essential to definitely narrow the range of acceptable parameters and help us decide whether this exceptional PN is so oxygen-poor as to possibly influence current views on stellar evolution.

Key words: stars: Population II -- Galaxy: halo -- ISM: planetary nebulae: general -- ISM: planetary nebulae: individual: PN G135.9+55.9 -- ISM: planetary nebulae: individual: K648 (Ps 1) -- atomic data

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