110 research outputs found
Ionization Correction Factors for Planetary Nebulae: I- Using optical spectra
We compute a large grid of photoionization models that covers a wide range of
physical parameters and is representative of most of the observed PNe. Using
this grid, we derive new formulae for the ionization correction factors (ICFs)
of He, O, N, Ne, S, Ar, Cl, and C. Analytical expressions to estimate the
uncertainties arising from our ICFs are also provided. This should be useful
since these uncertainties are usually not considered when estimating the error
bars in element abundances. Our ICFs are valid over a variety of assumptions
such as the input metallicities, the spectral energy distribution of the
ionizing source, the gas distribution, or the presence of dust grains. Besides,
the ICFs are adequate both for large aperture observations and for pencil-beam
observations in the central zones of the nebulae. We test our ICFs on a large
sample of observed PNe that extends as far as possible in ionization, central
star temperature, and metallicity, by checking that the Ne/O, S/O, Ar/O, and
Cl/O ratios show no trend with the degree of ionization. Our ICFs lead to
significant differences in the derived abundance ratios as compared with
previous determinations, especially for N/O, Ne/O, and Ar/O.Comment: 19 pages, 22 figures. Accepted for publication in MNRA
Chlorine and Sulfur in Nearby Planetary Nebulae and H II Regions
We derive the chlorine abundances in a sample of nearby planetary nebulae
(PNe) and H II regions that have some of the best available spectra. We use a
nearly homogeneous procedure to derive the abundance in each object and find
that the Cl/H abundance ratio shows similar values in H II regions and PNe.
This supports our previous interpretation that the underabundance we found for
oxygen in the H II regions is due to the depletion of their oxygen atoms into
organic refractory dust components. For other elements, the bias introduced by
ionization correction factors in their derived abundances can be very
important, as we illustrate here for sulfur using photoionization models. Even
for low-ionization PNe, the derived sulfur abundances can be lower than the
real ones by up to 0.3 dex, and the differences found with the abundances
derived for H II regions that have similar S/H can reach 0.4 dex.Comment: 2 pages, 1 figure, proceedings of the IAU Symposium No. 283,
Planetary Nebulae: an Eye to the Futur
Oxygen enrichment in carbon-rich planetary nebulae
We study the relation between the chemical composition and the type of dust
present in a group of 20 Galactic planetary nebulae (PNe) that have high
quality optical and infrared spectra. The optical spectra are used, together
with the best available ionization correction factors, to calculate the
abundances of Ar, C, Cl, He, N, Ne, and O relative to H. The infrared spectra
are used to classify the PNe in two groups depending on whether the observed
dust features are representative of oxygen-rich or carbon-rich environments.
The sample contains one object from the halo, eight from the bulge, and eleven
from the local disc. We compare their chemical abundances with nucleosynthesis
model predictions and with the ones obtained in seven Galactic H II regions of
the solar neighbourhood.
We find evidence of O enrichment (by 0.3 dex) in all but one of the
PNe with carbon-rich dust (CRD). Our analysis shows that Ar, and especially Cl,
are the best metallicity indicators of the progenitors of PNe. There is a tight
correlation between the abundances of Ar and Cl in all the objects, in
agreement with a lockstep evolution of both elements. The range of
metallicities implied by the Cl abundances covers one order of magnitude and we
find significant differences in the initial masses and metallicities of the PNe
with CRD and oxygen-rich dust (ORD). The PNe with CRD tend to have intermediate
masses and low metallicities, whereas most of the PNe with ORD show higher
enrichments in N and He, suggesting that they had high-mass progenitors.Comment: Accepted for publication in MNRAS. 14 pages, 8 figures, 5 table
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