Constraints on gamma-ray burst and supernova progenitors through circumstellar absorption lines. (II): Post-LBV Wolf-Rayet stars

Van Marle et al. (2005) showed that circumstellar absorption lines in early Type Ib/c supernova and gamma-ray burst afterglow spectra may reveal the progenitor evolution of the exploding Wolf-Rayet star. While the quoted paper deals with Wolf-Rayet stars which evolved through a red supergiant stage, we investigate here the initially more massive Wolf-Rayet stars which are thought to evolve through a Luminous Blue Variable (LBV) stage. We perform hydrodynamic simulations of the evolution of the circumstellar medium around a 60 Msol star, from the main sequence through the LBV and Wolf-Rayet stages, up to core collapse. We then compute the column density of the circumstellar matter as a function of radial velocity, time and angle. This allows a comparison with the number and blue-shifts, of absorption components in the spectra of LBVs, Wolf-Rayet stars, Type Ib/c supernovae and gamma-ray burst afterglows. Our simulation for the post-LBV stage shows the formation of various absorption components, which are, however, rather short lived; they dissipate on time scales shorter than 50,000yr. As the LBV stage is thought to occur at the beginning of core helium burning, the remaining Wolf-Rayet life time is expected to be one order of magnitude larger. When interpreting the absorption components in the afterglow spectrum of GRB-021004 as circumstellar, it can be concluded that the progenitor of this source did most likely not evolve through an LBV stage. However, a close binary with late common-envelope phase (Case C) may produce a circumstellar medium that closely resembles the LBV to Wolf-Rayet evolution, but with a much shorter Wolf-Rayet period.Comment: accepted for publication by A&

Continuum driven winds from super-Eddington stars. A tale of two limits

Continuum driving is an effective method to drive a strong stellar wind. It is governed by two limits: the Eddington limit and the photon-tiring limit. A star must exceed the effective Eddington limit for continuum driving to overcome the stellar gravity. The photon-tiring limit places an upper limit on the mass loss rate that can be driven to infinity, given the energy available in the radiation field of the star. Since continuum driving does not require the presence of metals in the stellar atmosphere it is particularly suited to removing mass from low- and zero-metallicity stars and can play a crucial part in their evolution. Using a porosity length formalism we compute numerical simulations of super-Eddington, continuum driven winds to explore their behaviour for stars both below and above the photon-tiring limit. We find that below the photon tiring limit, continuum driving can produce a large, steady mass loss rate at velocities on the order of the escape velocity. If the star exceeds the photon-tiring limit, a steady solution is no longer possible. While the effective mass loss rate is still very large, the wind velocity is much smallerComment: to be published in the conference proceedings of: First Stars III, Santa Fe, 200

Eyes in the sky: Interactions between AGB winds and the interstellar magnetic field

We aim to examine the role of the interstellar magnetic field in shaping the extended morphologies of slow dusty winds of Asymptotic Giant-branch (AGB) stars in an effort to pin-point the origin of so-called eye shaped CSE of three carbon-rich AGB stars. In addition, we seek to understand if this pre-planetary nebula (PN) shaping can be responsible for asymmetries observed in PNe. Hydrodynamical simulations are used to study the effect of typical interstellar magnetic fields on the free-expanding spherical stellar winds as they sweep up the local interstellar medium (ISM). The simulations show that typical Galactic interstellar magnetic fields of 5 to 10 muG, are sufficient to alter the spherical expanding shells of AGB stars to appear as the characteristic eye shape revealed by far-infrared observations. The typical sizes of the simulated eyes are in accordance with the observed physical sizes. However, the eye shapes are of transient nature. Depending on the stellar and interstellar conditions they develop after 20,000 to 200,000yrs and last for about 50,000 to 500,000 yrs, assuming that the star is at rest relative to the local interstellar medium. Once formed the eye shape will develop lateral outflows parallel to the magnetic field. The "explosion" of a PN in the center of the eye-shaped dust shell gives rise to an asymmetrical nebula with prominent inward pointing Rayleigh-Taylor instabilities. Interstellar magnetic fields can clearly affect the shaping of wind-ISM interaction shells. The occurrence of the eyes is most strongly influenced by stellar space motion and ISM density. Observability of this transient phase is favoured for lines-of-sight perpendicular to the interstellar magnetic field direction. The simulations indicate that shaping of the pre-PN envelope can strongly affect the shape and size of PNe.Comment: Accepted for publication in A&A. Final version will contain animated result

Forming a constant density medium close to long gamma-ray bursts

The progenitor stars of long Gamma-Ray Bursts (GRBs) are thought to be Wolf-Rayet stars, which generate a massive and energetic wind. Nevertheless, about 25 percent of all GRB afterglows light curves indicate a constant density medium close to the exploding star. We explore various ways to produce this, by creating situations where the wind termination shock arrives very close to the star, as the shocked wind material has a nearly constant density. Typically, the distance between a Wolf-Rayet star and the wind termination shock is too large to allow afterglow formation in the shocked wind material. Here, we investigate possible causes allowing for a smaller distance: A high density or a high pressure in the surrounding interstellar medium (ISM), a weak Wolf-Rayet star wind, the presence of a binary companion, and fast motion of the Wolf-Rayet star relative to the ISM. We find that all four scenarios are possible in a limited parameter space, but that none of them is by itself likely to explain the large fraction of constant density afterglows. A low GRB progenitor metallicity, and a high GRB energy make the occurrence of a GRB afterglow in a constant density medium more likely. This may be consistent with constant densities beingpreferentially found for energetic, high redshift GRBs.Comment: 13 pages, 13 figures, new version: as accepted by Astronomy & Astrophysic

The origin of blue-shifted absorption lines in a gamma-ray burst afterglow

The afterglow spectrum of GRB 021004 shows a system of blueshifted absorption lines, indicating the presence of matter moving towards us at discrete velocities in the range from 0 to more than 2000 km/s. We propose that these lines are the result of absorption by circumstellar matter, which was ejected by gamma-ray burst progenitor (a massive star) during its evolution. We have simulated the evolution of the circumstellar medium around such a star and find that the evolutionary sequence: main-sequence, Red Supergiant, Wolf-Rayet star can qualitatively reproduce the various absorption lines systems

Poisson structures for reduced non-holonomic systems

Borisov, Mamaev and Kilin have recently found certain Poisson structures with respect to which the reduced and rescaled systems of certain non-holonomic problems, involving rolling bodies without slipping, become Hamiltonian, the Hamiltonian function being the reduced energy. We study further the algebraic origin of these Poisson structures, showing that they are of rank two and therefore the mentioned rescaling is not necessary. We show that they are determined, up to a non-vanishing factor function, by the existence of a system of first-order differential equations providing two integrals of motion. We generalize the form of that Poisson structures and extend their domain of definition. We apply the theory to the rolling disk, the Routh's sphere, the ball rolling on a surface of revolution, and its special case of a ball rolling inside a cylinder.Comment: 22 page

Characterization of changes in potato tissue during cooking in relation to texture development

Texture of cooked potatoes is an important quality aspect. The diversity in texture types was sensory evaluated. Most of the differences between texture types could be explained by differences between mealy and non-mealy characteristics. Furthermore, cultivars with similar mealy/non-mealy characteristics could be discriminated on basis of firmness of cooked potato tissue.Cryo-scanning electron microscopy showed differences in intercellular contact and appearance of cell walls between fracture planes of cooked tissue from mealy and non- mealy cooking potato cultivars. Therefore, further research was focused on structure and composition of cell walls from the mealy cooking cv. Irene and the nonmealy cooking cv. Nicola.The degradation of middle lamellae during cooking was determined by recording the release of pectic material in cooking media. It was found that a given percentage release of pectic material results in more cell sloughing for cv. Irene than for cv. Nicola.Firstly, the effect of differences in ionic conditions in cell walls and middle lamellae on the degradation of pectic material was studied by recording the transfer of calcium, potassium and citrate during cooking of potato tissue. The transfer rates of potassium and citrate for cv. Irene are lower than expected in comparison with cv. Nicola. Calcium showed a deviant behaviour, since 80% of the calium initially present remains in tissue during cooking.Furthermore, composition and structure of the pectic polysaccharides in the cell walls and middle lamellae of both cultivars were studied. Although, isolated cell wall material has a comparable molar composition for both cultivars, different types of pectic polysaccharides are solubilized during cooking.The structure of pectic polysaccharides; was further elucidated using chemical fractionation and enzymic degradation. It is proposed that the pectin matrix in the primary cell wall of cv. Irene had a thicker and/or less porous structure than the matrix in the cell wall of cv. Nicola. Additionally, the primary cell wall of cv. Irene has a more dense and/or thicker cellulose-xyloglucan network than the primary cell wall of cv. Nicola

Afriforum v Malema: The limits of law and complexity

The Afriforum v Malema 2011 6 SA 240 (EqC) case drew considerable attention in the media and in the public discourse. The purpose of this note is to reflect upon the judgment from a theoretical vantage point. More specifically, by reading the judgment through an autopoietic systems theory lens, some points of criticism on the judgment in particular and the law in general become apparent. It is contended that the judgment illustrates how law necessarily excludes the factual complexity of a case, first by deciding which are the only facts legally relevant, and then second by reducing their meaning to a simple judgment of legal or illegal. Since law recognises only legal communication, this function means that the communication and identity are removed from legal subjects and given legal meanings. An attempt is made to open law to considerations external to what it traditionally considers to be relevant to its operation. The  problem that law excludes facts it deems irrelevant is addressed through the introduction of a third value whereby to measure the legal and illegal, namely justice. Through asking if its judgments of legal or illegal are just, law becomes capable of reflexive self-observation. In this manner the very complex historical and narrative trappings of the case at hand do not need to be excluded as they are in the judgment. Rather than absolute, binary judgments, a slower, reflective engagement that makes modest claims is supported.  

Linking the hydrodynamic and kinetic description of a dissipative relativistic conformal theory

We use the entropy production variational method to associate a one particle distribution function to the assumed known energy-momentum and entropy currents describing a relativistic conformal fluid. Assuming a simple form for the collision operator we find this one particle distribution function explicitly, and show that this method of linking the hydro and kinetic description is a non trivial generalization of Grad's ansatz. The resulting constitutive relations are the same as in the conformal dissipative type theories discussed in J. Peralta-Ramos and E. Calzetta, Phys. Rev. D {\bfseries 80}, 126002 (2009). Our results may prove useful in the description of freeze-out in ultrarelativistic heavy-ion collisions.Comment: v2: 23 pages, no figures, accepted in Phys. Rev.