Infrared emission lines in planetary nebulae

Infrared spectroscopy was used to detect many forbidden fine structure emission lines in planetary nebulae. Measurements of these lines offer sensitive probes of the physical conditions and ionization structure, and lead to improved abundance determinations

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 $\mu$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 $^{22}$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

(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

Optimal length and signal amplification in weakly activated signal transduction cascades

Weakly activated signaling cascades can be modeled as linear systems. The input-to-output transfer function and the internal gain of a linear system, provide natural measures for the propagation of the input signal down the cascade and for the characterization of the final outcome. The most efficient design of a cascade for generating sharp signals, is obtained by choosing all the off rates equal, and a ``universal'' finite optimal length.Comment: 27 pages, 10 figures, LaTeX fil

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