32 research outputs found
The Abundances of Light Neutron-Capture Elements in Planetary Nebulae
We present preliminary results from a large-scale survey of the
neutron(n)-capture elements Se and Kr in Galactic planetary nebulae (PNe).
These elements may be produced in PN progenitors by s-process nucleosynthesis,
and brought to the stellar envelope by third dredge-up (TDU). We have searched
for [Kr III] 2.199 and [Se IV] 2.287 m in 120 PNe, and detected one or
both lines in 79 objects, for a detection rate of 66%. In order to determine
abundances of Se and Kr, we have added these elements to the atomic database of
the photoionization code CLOUDY, and constructed a large grid of models to
derive corrections for unobserved ionization stages. Se and Kr are enriched in
73% of the PNe in which they have been detected, and exhibit a wide range of
abundances, from roughly solar to enriched by a factor of 10 or more. These
enrichments are interpreted as evidence for the operation of the s-process and
TDU in the progenitor stars. In line with theoretical expectations, Kr is more
strongly enhanced than Se, and the abundances of both elements are correlated
with the carbon abundance. Kr and Se are strongly enhanced in Type I PNe, which
may be evidence for the operation of the Ne neutron source in
intermediate-mass AGB stars. These results constitute the first broad
characterization of s-process enrichments in PNe as a population, and reveal
the impact of low- and intermediate-mass stars on the chemical evolution of
trans-iron elements in the Galaxy.Comment: 4 pages, 1 figure, to appear in IAU Symp. 234, "Planetary Nebulae in
our Galaxy and Beyond", eds. M. J. Barlow and R. H. Mende
The Abundances of Light Neutron-Capture Elements in Planetary Nebulae. II. S-Process Enrichments and Interpretation
We present the results of a large-scale survey of neutron(n)-capture elements in Galactic planetary nebulae (PNe), undertaken to study enrichments from s-process nucleosynthesis in their progenitor stars. From new K-band observations of over 100 PNe supplemented by data from the literature, we have detected the emission lines [Kr III] 2.199 mu m and/or [Se IV] 2.287 mu m in 81 of 120 objects. We determine Se and Kr elemental abundances, employing ionization correction formulae derived in the first paper of this series. We find a significant range in Se and Kr abundances, from near solar (no enrichment) to enhanced by > 1.0 dex relative to solar, which we interpret as self-enrichment due to in situ s-process nucleosynthesis. Kr tends to be more strongly enriched than Se; in 18 objects exhibiting both Se and Kr emission, we find that [Kr/Se] = 0.5 +/- 0.2. Our survey has increased the number of PNe with n-capture element abundance determinations by a factor of 10, enabling us for the first time to search for correlations with other nebular properties. As expected, we find a positive correlation between s-process enrichments and the C/O ratio. Type I and bipolar PNe, which arise from intermediate-mass progenitors (> 3-4 M-circle dot), exhibit little to no s-process enrichments. Finally, PNe with H-deficient Wolf-Rayet central stars do not exhibit systematically larger s-process enrichments than objects with H-rich nuclei. Overall, 44% of the PNe in our sample display significant s-process enrichments (> 0.3 dex). Using an empirical PN luminosity function to correct for incompleteness, we estimate that the true fraction of s-process enriched Galactic PNe is at least 20%.NSF AST 97-31156, AST 04-06809Astronom
Far-infrared emission line spectroscopy of planetary nebulae from the KAO
The main focus of the overall project was to study the properties of planetary nebulae using far-infrared emission lines. The observations were conducted with the 'cooled grating spectrometer' or CGS, a moderate-resolution echelle spectrometer designed and built at the Ames Research Center. During the first few years of the program, the emphasis was on emission lines of doubly-ionized oxygen and nitrogen ((O III) 52 and 88 microns, (N III) 57 microns), which arise in the ionized regions of the nebulae. Starting around 1989, our emphasis shifted to observing fine-structure lines of neutral oxygen and singly-ionized carbon ((O I) 63 and 145 microns, (C II) 157 microns), which arise from predominantly neutral material outside the ionized regions. This program was typically assigned one or two observing flights per year. Because these studies required obtaining a substantial sample of objects in order to reach meaningful conclusions, publication of comprehensive papers summarizing all of the results is still pending. However, numerous interim reports based on the airborne results as well as on closely-related supporting observations have been published during the grant period. The bibliographic information for these reports is given in the publications section. An overall summary of the planetary nebula results was presented at the Airborne Astronomy Symposium (20th Anniversary of the KAO) on 8 July 1994; reprints will be provided when available. In parallel with the planetary nebula study, we also observed the (O III) and (N III) lines in several H II regions, and attempted (unsuccessfully) to detect these lines in several old nova remnants and the supernova remnant Cassiopeia A
Zinc abundances of planetary nebulae
Zinc is a useful surrogate element for measuring Fe/H as, unlike iron, it is
not depleted in the gas phase media. Zn/H and O/Zn ratios have been derived
using the [Zn IV] emission line at 3.625um for a sample of nine Galactic
planetary nebulae, seven of which are based upon new observations using the
VLT. Based on photoionization models, O/O++ is the most reliable ionisation
correction factor for zinc that can readily be determined from optical emission
lines, with an estimated accuracy of 10% or better for all targets in our
sample. The majority of the sample is found to be sub-solar in [Zn/H]. [O/Zn]
in half of the sample is found to be consistent with Solar within
uncertainties, whereas the remaining half are enhanced in [O/Zn]. [Zn/H] and
[O/Zn] as functions of Galactocentric distance have been investigated and there
is little evidence to support a trend in either case.Comment: Accepted MNRAS, 11 pages, 8 figure
Nucleosynthesis Predictions for Intermediate-Mass AGB Stars: Comparison to Observations of Type I Planetary Nebulae
Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought
to be descendants of stars with initial masses of ~3-8Msun. These
characteristics indicate that the progenitor stars experienced proton-capture
nucleosynthesis at the base of the convective envelope, in addition to the slow
neutron capture process operating in the He-shell (the s-process). We compare
the predicted abundances of elements up to Sr from models of intermediate-mass
asymptotic giant branch (AGB) stars to measured abundances in Type I PNe. In
particular, we compare predictions and observations for the light trans-iron
elements Se and Kr, in order to constrain convective mixing and the s-process
in these stars. A partial mixing zone is included in selected models to explore
the effect of a 13C pocket on the s-process yields. The solar-metallicity
models produce enrichments of [(Se, Kr)/Fe] < 0.6, consistent with Galactic
Type I PNe where the observed enhancements are typically < 0.3 dex, while lower
metallicity models predict larger enrichments of C, N, Se, and Kr. O
destruction occurs in the most massive models but it is not efficient enough to
account for the > 0.3 dex O depletions observed in some Type I PNe. It is not
possible to reach firm conclusions regarding the neutron source operating in
massive AGB stars from Se and Kr abundances in Type I PNe; abundances for more
s-process elements may help to distinguish between the two neutron sources. We
predict that only the most massive models would evolve into Type I PNe,
indicating that extra-mixing processes are active in lower-mass stars
(3-4Msun), if these stars are to evolve into Type I PNe.Comment: 39 pages, accepted for publication in Ap
Neutron-Capture elements in planetary nebulae: first detections of near-Infrared [Te III] and [Br V] emission lines
We have identified two new near-infrared emission lines in the spectra of
planetary nebulae (PNe) arising from heavy elements produced by neutron capture
reactions: [Te III] 2.1019 m and [Br V] 1.6429 m. [Te III] was
detected in both NGC 7027 and IC 418, while [Br V] was seen in NGC 7027. The
observations were obtained with the medium-resolution spectrograph EMIR on the
10.4m Gran Telescopio Canarias at La Palma, and with the high-resolution
spectrograph IGRINS on the 2.7m Harlan J. Smith telescope at McDonald
Observatory. New calculations of atomic data for these ions, specifically
A-values and collision strengths, are presented and used to derive ionic
abundances of Te and Br. We also derive ionic abundances of other
neutron-capture elements detected in the near-infrared spectra, and estimate
total elemental abundances of Se, Br, Kr, Rb, and Te after correcting for
unobserved ions. Comparison of our derived enrichments to theoretical
predictions from AGB evolutionary models shows reasonable agreement for solar
metallicity progenitor stars of 2 - 4 M. The
spectrally-isolated [Br V] 1.6429 m line has advantages for determining
nebular Br abundances over optical [Br III] emission lines that can be blended
with other features. Finally, measurements of Te are of special interest
because this element lies beyond the first peak of the s-process, and thus
provides new leverage on the abundance pattern of trans-iron species produced
by AGB stars.Comment: 9 pages, 1 figure, 4 tables. Accepted for publication in ApJ Letter
Discovery of Enhanced Germanium Abundances in Planetary Nebulae with FUSE
We report the discovery of Ge III 1088.46 in the planetary nebulae
(PNe) SwSt 1, BD+303639, NGC 3132, and IC 4593, observed with the Far
Ultraviolet Spectroscopic Explorer. This is the first astronomical detection of
this line and the first measurement of Ge (Z = 32) in PNe. We estimate Ge
abundances using S and Fe as reference elements, for a range of assumptions
about gas-phase depletions. The results indicate that Ge, which is synthesized
in the initial steps of the s-process and therefore can be self-enriched in
PNe, is enhanced by factors of > 3-10. The strongest evidence for enrichment is
seen for PNe with Wolf-Rayet central stars, which are likely to contain heavily
processed material.Comment: 11 pages, 1 figure, accepted for publication in ApJ Letter
ISO SWS Observations of H II Regions in NGC 6822 and I ZW 36: Sulfur Abundances and Temperature Fluctuations
We report ISO SWS infrared spectroscopy of the H II region Hubble V in NGC
6822 and the blue compact dwarf galaxy I Zw 36. Observations of Br alpha, [S
III] at 18.7 and 33.5 microns, and [S IV] at 10.5 microns are used to determine
ionic sulfur abundances in these H II regions. There is relatively good
agreement between our observations and predictions of S^+3 abundances based on
photoionization calculations, although there is an offset in the sense that the
models overpredict the S^+3 abundances. We emphasize a need for more
observations of this type in order to place nebular sulfur abundance
determinations on firmer ground. The S/O ratios derived using the ISO
observations in combination with optical data are consistent with values of
S/O, derived from optical measurements of other metal-poor galaxies.
We present a new formalism for the simultaneous determination of the
temperature, temperature fluctuations, and abundances in a nebula, given a mix
of optical and infrared observed line ratios. The uncertainties in our ISO
measurements and the lack of observations of [S III] lambda 9532 or lambda 9069
do not allow an accurate determination of the amplitude of temperature
fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we
illustrate the diagnostic power and limitations of our new method.Comment: 32 Pages total, including 6 encapsulated postscript figures (one with
two parts). Accepted for Publication in the 20 Dec 2002 Ap