99 research outputs found
Evolution of the central stars of young planetary nebulae
The evolution of central stars of planetary nebulae was so far documented in
just a few cases. However, spectra collected a few decades ago may provide a
good reference for studying the evolution of central stars using the emission
line fluxes of their nebulae. We investigated evolutionary changes of the
[OIII] 5007 A line flux in the spectra of planetary nebulae. We compared
nebular fluxes collected during a decade or longer. We used literature data and
newly obtained spectra. A grid of Cloudy models was computed using existing
evolutionary models, and the models were compared with the observations. An
increase of the [OIII] 5007 A line flux is frequently observed in young
planetary nebulae hosting H-rich central stars. The increasing nebular
excitation is the response to the increasing temperature and hardening
radiation of the central stars. We did not observe any changes in the nebular
fluxes in the planetary nebulae hosting late-type Wolf-Rayet (WR) central
stars. This may indicate a slower temperature evolution (which may stem from a
different evolutionary status) of late-[WR] stars. In young planetary nebulae
with H-rich central stars, the evolution can be followed using optical spectra
collected during a decade or longer. The observed evolution of H-rich central
stars is consistent with the predictions of the evolutionary models provided in
the literature. Late-[WR] stars possibly follow a different evolutionary path.Comment: Accepted for publication in A&
A Herschel study of Planetary Nebulae
We present Herschel PACS and SPIRE images of the dust shells around the
planetary nebulae NGC 650, NGC 6853, and NGC 6720, as well as images showing
the dust temperature in their shells. The latter shows a rich structure, which
indicates that internal extinction in the UV is important despite the highly
evolved status of the nebulae.Comment: 2 pages, 1 figure, 2012, proceedings IAU Symposium 283 Planetary
Nebulae: An Eye to the Futur
Radiative cooling in collisionally and photo ionized plasmas
We discuss recent improvements in the calculation of the radiative cooling in
both collisionally and photo ionized plasmas. We are extending the spectral
simulation code Cloudy so that as much as possible of the underlying atomic
data is taken from external databases, some created by others, some developed
by the Cloudy team. This paper focuses on recent changes in the treatment of
many stages of ionization of iron, and discusses its extensions to other
elements. The H-like and He-like ions are treated in the iso-electronic
approach described previously. Fe II is a special case treated with a large
model atom. Here we focus on Fe III through Fe XXIV, ions which are important
contributors to the radiative cooling of hot, 1e5 to 1e7 K, plasmas and for
X-ray spectroscopy. We use the Chianti atomic database to greatly expand the
number of transitions in the cooling function. Chianti only includes lines that
have atomic data computed by sophisticated methods. This limits the line list
to lower excitation, longer wavelength, transitions. We had previously included
lines from the Opacity Project database, which tends to include higher energy,
shorter wavelength, transitions. These were combined with various forms of the
g-bar approximation, a highly approximate method of estimating collision rates.
For several iron ions the two databases are almost entirely complementary. We
adopt a hybrid approach in which we use Chianti where possible, supplemented by
lines from the Opacity Project for shorter wavelength transitions. The total
cooling including the lightest thirty elements differs significantly from some
previous calculations
The 23.01 release of Cloudy
We announce the C23.01 update of Cloudy. This corrects a simple coding error,
present since 1990, in one routine that required a conversion from the
line-center to the mean normalization of the Ly optical depth. This
affects the destruction of H I Ly by background opacities. Its largest
effect is upon the Ly intensity in high-ionization dusty clouds, where
the predicted intensity is now up to three times stronger. Other properties
that depend on Ly destruction, such as grain infrared emission, change
in response.Comment: 4 pages, 1 figur
The Herschel Planetary Nebula Survey (HerPlaNS) - a comprehensive dusty photoionization model of NGC6781
We perform a comprehensive analysis of the planetary nebula (PN) NGC6781 to
investigate the physical conditions of each of its ionized, atomic, and
molecular gas and dust components and the object's evolution, based on
panchromatic observational data ranging from UV to radio. Empirical nebular
elemental abundances, compared with theoretical predictions via nucleosynthesis
models of asymptotic giant branch (AGB) stars, indicate that the progenitor is
a solar-metallicity, 2.25-3.0 Msun initial-mass star. We derive the best-fit
distance of 0.46 kpc by fitting the stellar luminosity (as a function of the
distance and effective temperature of the central star) with the adopted
post-AGB evolutionary tracks. Our excitation energy diagram analysis indicate
high excitation temperatures in the photodissociation region (PDR) beyond the
ionized part of the nebula, suggesting extra heating by shock interactions
between the slow AGB wind and the fast PN wind. Through iterative fitting using
the Cloudy code with empirically-derived constraints, we find the best-fit
dusty photoionization model of the object that would inclusively reproduce all
of the adopted panchromatic observational data. The estimated total gas mass
(0.41 Msun) corresponds to the mass ejected during the last AGB thermal pulse
event predicted for a 2.5 Msun initial-mass star. A significant fraction of the
total mass (about 70 percent) is found to exist in the PDR, demonstrating the
critical importance of the PDR in PNe that are generally recognized as the
hallmark of ionized/H+ regions.Comment: 34 pages, 13 Figures and 16 Tables, accepted for publication in the
Astrophysical Journal Supplement Serie
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