Issue |
A&A
Volume 430, Number 1, January IV 2005
|
|
---|---|---|
Page(s) | 187 - 212 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20041382 | |
Published online | 12 January 2005 |
Endogenous oxygen in the extremely metal-poor planetary nebula PN G135.9+55.9
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 collision strengths obtained by
Aggarwal et al. ([CITE]) for
transitions (
)
are too large, while those obtained by Anderson et al. ([CITE])
are acceptable. Empirical collision strengths are presented for
.
Photoionization models of PN G135 that fit all available optical data
can be demonstrated only for oxygen abundances
(>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
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
K, its luminosity
erg s-1, its mass 0.59
, the age of
the ionized shell 104 yrs, the ionized mass 0.05
and the abundances by number (H:He:C:O:Ne) = (106:81 500:90:30:4.5),
with C/H being rather an upper limit and O/H and Ne/H uncertain
by ±0.3 and ±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
© ESO, 2005
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