377 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
(Im)possibilities of Autonomy: Social Movements In and Beyond Capital, the State and Development
Recently, we have witnessed the emergence of what appears to be a new set of claims in contemporary social movements based around the idea of autonomy. In this paper we interrogate this demand for autonomy. In order to do this, we first engage with existing literatures, identifying three main conceptions of autonomy: 1) autonomous practices vis-à -vis capital, or, what Negri calls, the ‘self-valorization’ of labour; 2) self-determination and independence from the state; and 3) alternatives to hegemonic discourses of development. We will then problematize and point out the central potentials, weaknesses and antagonisms at the heart of the concept of autonomy. We argue that social movements’ demands for autonomy point to, what Laclau and Mouffe call, the impossibility of society, the idea that society can never be complete. That is, there will always be resistances, such as those expressed by autonomous social movements. However, this also lets us understand the conception of autonomy to be incomplete. Autonomy itself is hence an impossibility. To point to these limits of the discourses of autonomy, we discuss how demands for autonomy are tied up with contemporary re-organizations of: 1) the capitalist workplace, characterized by discourses of autonomy, creativity and self-management; 2) the state, which increasingly outsources public services to independent, autonomous providers, which often have a more radical, social movement history; and 3) regimes of development, which today often emphasize local practices, participation and self-determination. Behind these critical reflections on the conception and practice of autonomy is the idea that autonomy should always be seen as something relational. That is, autonomy can never be fixed; there is no definite ground for demands for autonomy to stand on. Instead, social movements’ demands for autonomy are embedded in specific social, economic, political and cultural contexts, giving rise to possibilities as well as impossibilities of autonomous practices
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
The limits of participatory democracy: social movements and the displacement of disagreement in South America
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
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