2,104 research outputs found
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
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
Atomic data for neutron-capture elements III. Charge transfer rate coefficients for low-charge ions of Ge, Se, Br, Kr, Rb, and Xe
We present total and final-state resolved charge transfer (CT) rate
coefficients for low-charge Ge, Se, Br, Kr, Rb, and Xe ions reacting with
neutral hydrogen over the temperature range 10^2--10^6 K. Each of these
elements has been detected in ionized astrophysical nebulae, particularly
planetary nebulae. CT rate coefficients are a key ingredient for the ionization
equilibrium solutions needed to determine total elemental abundances from those
of the observed ions. A multi-channel Landau Zener approach was used to compute
rate coefficients for projectile ions with charges q=2-5, and for
singly-charged ions the Demkov approximation was utilized. Our results for
five-times ionized species are lower limits, due to the incompleteness of level
energies in the NIST database. In addition, we computed rate coefficients for
charge transfer ionization reactions between the neutral species of the above
six elements and ionized hydrogen. The resulting total and state-resolved CT
rate coefficients are tabulated and available at the CDS. In tandem with our
concurrent investigations of other important atomic processes in photoionized
nebulae, this work will enable robust investigations of neutron-capture element
abundances and nucleosynthesis via nebular spectroscopy.Comment: 11 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
The Abundances of Light Neutron-Capture Elements in Planetary Nebulae III. The Impact of New Atomic Data on Nebular Selenium and Krypton Abundance Determinations
The detection of neutron(n)-capture elements in several planetary nebulae
(PNe) has provided a new means of investigating s-process nucleosynthesis in
low-mass stars. However, a lack of atomic data has inhibited accurate
trans-iron element abundance determinations in astrophysical nebulae. Recently,
photoionization and recombination data were determined for Se and Kr, the two
most widely detected n-capture elements in nebular spectra. We have
incorporated these new data into the photoionization code Cloudy. To test the
atomic data, numerical models were computed for 15 PNe that exhibit emission
lines from multiple Kr ions. We found systematic discrepancies between the
predicted and observed emission lines that are most likely caused by inaccurate
photoionization and recombination data. These discrepancies were removed by
adjusting the Kr--Kr photoionization cross sections within their
cited uncertainties and the dielectronic recombination rate coefficients by
slightly larger amounts. From grids of models spanning the physical conditions
encountered in PNe, we derive new, broadly applicable ionization correction
factor (ICF) formulae for calculating Se and Kr elemental abundances. The ICFs
were applied to our previous survey of near-infrared [Kr III] and [Se IV]
emission lines in 120 PNe. The revised Se and Kr abundances are 0.1-0.3 dex
lower than former estimates, with average values of [Se/(O, Ar)]=0.120.27
and [Kr/(O, Ar)]=0.820.29, but correlations previously found between their
abundances and other nebular and stellar properties are unaffected. We also
find a tendency for high-velocity PNe that can be associated with the Galactic
thick disk to exhibit larger s-process enrichments than low-velocity PNe
belonging to the thin disk population.Comment: 73 pages, 6 figures, 18 tables, accepted for publication in ApJ
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