Issue |
A&A
Volume 378, Number 3, November II 2001
|
|
---|---|---|
Page(s) | 958 - 985 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20011270 | |
Published online | 15 November 2001 |
Evolution of planetary nebulae
I. An improved synthetic model
1
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching bei München, Germany
2
Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
3
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85740 Garching bei München, Germany
Corresponding author: P. Marigo, marigo@pd.astro.it
Received:
31
May
2001
Accepted:
6
September
2001
We present a new synthetic model to follow the evolution of a
planetary nebula (PN) and its central star, starting from the onset of
AGB phase up to the white dwarf cooling sequence. The model suitably
combines various analytical prescriptions to account for different
(but inter-related) aspects of planetary nebulae, such as: the
dynamical evolution of the primary shell and surrounding ejecta, the
photoionisation of H and He by the central star, the nebular emission
of a few relevant optical lines (e.g. Hβ; He
ii
λ4686; [Oiii]λ5007). Particular
effort has been put into the analytical description of dynamical
effects such as the three-winds interaction and the shell thickening
due to ionisation (i.e. the thin-shell approximation
is relaxed), that are nowadays considered important aspects of
the PN evolution.
Predictions of the synthetic model are tested by comparison with both
findings of hydrodynamical calculations, and observations of Galactic
PNe. The sensitiveness of the results to the model parameters
(e.g. transition time, mass of the central star, H-/He-burning tracks,
etc.) is also discussed.
We briefly illustrate the systematic differences that are
expected in the luminosities and lifetimes of PNe with either H- or
He-burning central stars, which result in different "detection
probabilities" across the H-R diagram, in both Hβ and [O
iii] lines.
Adopting reasonable values of the model
parameters, we are able to reproduce, in a satisfactory way, many
general properties of PNe, like the ionised mass-nebular radius
relationship, the trends of a few main nebular line ratios,
and the observed ranges of
nebular shell thicknesses, electron densities, and expansion
velocities. The models naturally predict also the possible
transitions from optically-thick to optically-thin configurations
(and vice versa).
In this context, our analysis indicates that the
condition of optical thinness to the H continuum plays an
important role in producing the observed "Zanstra discrepancy"
between the temperatures determined from H or Heii lines,
as well as it affects
the mass-increasing part of the ionised mass-radius relation.
These predictions are supported by
observational indications by Méndez et al. ([CITE]).
Another interesting result is that
the change of slope in the electron density-nebular radius
relation at
pc, pointed out by Phillips
([CITE]), is also displayed by the models and
may be interpreted as the result of the progressive
convergence of the PNe to the condition of constant ionised mass.
Finally we would like to remark that, thanks to its computational
agility, our synthetic PN model is particularly suitable to
population synthesis studies, and it
represents the basic ground from which many future
applications will be developed.
Key words: stars: evolution / stars: AGB and post-AGB / stars: mass-loss / planetary nebulae: general
© ESO, 2001
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