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Many growing plant cells undergo rapid axial elongation with negligible radial expansion. Growth is driven by high internal turgor pressure causing viscous stretching of the cell wall, with embedded cellulose microfibrils providing the wall with strongly anisotropic properties. We present a theoretical model of a growing cell, representing the primary cell wall as a thin axisymmetric fibre-reinforced viscous sheet supported between rigid end plates. Asymptotic reduction of the governing equations, under simple sets of assumptions about the fibre and wall properties, yields variants of the traditional Lockhart equation, which relates the axial cell growth rate to the internal pressure. The model provides insights into the geometric and biomechanical parameters underlying bulk quantities such as wall extensibility, and shows how either dynamical changes in wall material properties or passive fibre reorientation may suppress cell elongation.
Starbursts, black holes and AGN have strong observational links, as discussed elsewhere in these proceedings. Perry & Dyson (1985 (PD), see also Perry 1994) studied the role of shocks around supernovae and stellar wind bubbles in the nuclei of active galaxies. Both the ejecta and the ambient ISM are initially shocked to high temperatures. PD found that while the shocked gas is maintained at high pressure by ram pressure, it cools rapidly, to then produce the observed optical and UV emission lines. The mass supply rate from the nuclear starburst, inferred from the strength of the emission lines, tallies well with that required by an accreting black hole to generate the observed luminosity. A symbiosis between a starburst stellar cluster and an accreting black hole naturally generates the observational features associated with QSOs.
We have observed at 5GHz the T Tau system with high resolution (≲0″.1) using the Multi-Element Radio Linked Interferometer (MERLIN) based at Jodrell Bank. Both the optical star (T Tau N) and its well-known infrared companion (T Tau S) were detected. The radio emission from T Tau S was found to be roughly extended in the direction of what is thought to be its outflow axis. More importantly we discovered that this radio emission split up into two spatially separated lobes of opposite helicity in the left and right-circular polarization channels. The circularly polarized lobes appear to straddle the star so that the “flow” and the “counterflow” were of opposite helicity. Such observations are the first direct evidence for the presence of magnetic fields in extended outflows that we are aware of. The radio flux appears to be due to gyrosynchrotron emission from mildly relativistic electrons (γ≈2–3). These electrons may have been accelerated in shocks close to the source. Using reasonable assumptions, the inferred magnetic fields strengths are surprisingly large (≳ several gauss) at distances of approximately 10-20 AU from their source. This is consistent with the magnetic fields being part of a collimated flow.
Structures like the clumps identified in the CO maps of the Rosette Molecular Cloud and the dense cores such as those in B5, a cluster of cores and young low-mass stars, are key to considerations of star formation. Whether star formation is a self-inducing process or one that causes itself to turn off depends greatly on whether the responses of the interclump and intercore media to young stars cause the collapse of clumps or cores to be faster than their ablation. We present a naive introduction to the lengthscales over which such responses are significant, mention ways in which the responses might induce collapse, review some of the little that is known of how flows of media around clumps and cores ablate them, and then return to the issue of the lengthscales over which such responses are significant by considering the global properties of mass-loaded flows in clumpy star forming regions.
Mass addition to flowing tenuous plasmas by the ablation of embedded clumps alters the flows and the observational characteristics of both flows and clumps. The boundary layers between the clumps and the flows are the sites of enhanced radiative losses. Flows which in smooth media would be driven by thermal pressure, are instead driven by momentum. There are many possible types of flows and we explore some of them in the context of Wolf-Rayet and planetary nebulae. Flows in which transsonic tenuous plasmas exit from mass-loading cores into smooth haloes are relevant for planetary nebulae. On intermediate scale lengths, the flow-clump interactions produce extended ‘tails’. We give a general discussion of this and describe applications to the cometary tails behind globules in the planetary nebula NGC 7293 and to the tail of the galactic centre red supergiant IRS7. We finally briefly discuss diagnostics of the boundary layers themselves.
Practically all diffuse media of astrophysical significance are clumpy media which are responding to energy sources. The most important distinction between flows initiated in clumpy as opposed to homogeneous media, is that in the former, there is mass, momentum and energy interchange at clumptenuous plasma boundaries, i.e. in boundary layers. The consequences are major (Hartquist & Dyson 1993). This interchange reacts back on the dynamical, physical and even chemical state of the global tenuous plasma flow; conversely, the state of the global flow influences the interchange process.
Evidence and Implications of Anisotropy in AGN
David J. Axon, Affiliated with the Space Science Division of ESA at the Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD, USA and Nuffield Radio Astronomy Laboratory, University of Manchester, Jodrell Bank, Macclesfield, Cheshire, England,
J. E. Dyson, Department of Astronomy, University of Manchester, Oxford road, Manchester, England.,
Alan Pedlar, Nuffield Radio Astronomy Laboratory, University of Manchester, Jodrell Bank, Macclesfield, Cheshire, England.
We review the evidence that ejected radio material plays a fundamental role in the formation and kinematics of the Narrow Line region and the extended emission line regions associated with radio jets in radio galaxies and QSO's. In Seyfert galaxies, the key observation is the existence of high-velocity (several hundred km s−1 from the systemic velocity of the galaxy) emission line components which are found systematically closer to the nucleus that the radio emission peaks. We describe how this result can be explained with a high speed bowshock model. In radio galaxies, the strong shock created by the jet results in a surrounding hot cocoon of gas expanding away from the jet axis. These expanding cocoons are visible in the form of double velocity structure in high resolution optical spectra and have now been detected in 3C120, 3C 171, 3C405 and 3C265. The velocity separation between the components can be as high as several thousand km s−1 We briefly discuss how these cocoons can be used to verify the relativisic beaming hypothesis in systems with strong one-sided jets.
Extended emission line regions (EELR) closely aligned with the radio structure have been found in Seyferts  and radio galaxies at both low  and high redshift . The physical conditions and kinematics of these EELR provide a probe of both the radiation field of the AGN and the role played by ejected material in exciting the emission .
‘Classical’ stellar wind-driven bubbles are either energy or momentum driven, and evolve in smooth media containing only radial density gradients. Real bubbles are produced from winds from moving stars and which blow into non-homogeneous media. The resulting mixing of clump material can produce a variety of thermal, dynamical and chemical effects. In this review we discuss some of the modifications to classical bubbles which ensue, using observational examples where appropriate.
Close links appear to exist between the optical narrow line region (NLR) in Seyfert galaxies and the nuclear non-thermal radio emission. Both occur on similar scales (~ 102 —103 pc), and correlations have been found between the 21 cm radio power and both the [OIII] λ5007Å luminosity and linewidth (de Bruyn & Wilson 1978; Whittle 1985). Approximate balance appears to exist between the thermal pressure of the NLR gas and the relativistic and magnetic pressure of the radio components (de Bruyn & Wilson 1978; Unger et al. 1985; Pedlar, Dyson & Unger 1985). There are also several examples of a direct spatial association between the optical and radio components, for example Mkn 78 (Pedlar et al. 1988).
A brief overview of the observational characteristics of HH objects is given. Current models for their production by the interaction of stellar winds and jets with interstellar gas are critically discussed. Models for two specific systems of HH objects, namely, the Orion HH objects and the HH46-47 system are described with reference to the general production mechanisms.
The impact of injury by larvae and adults of Sitona discoideus Gyllenhal on lucerne production was studied in two consecutive seasons at two adjacent sites near Christchurch, New Zealand. The root-feeding larvae were found to be more damaging than the adults; in the first cut in the 1982–83 season in a three-year-old stand, larvae reduced dry matter yield by 18%. In the wetter following season in a one-year-old stand, second and third cut losses of 43 and 30% dry matter production, respectively, were recorded. In both seasons, the lucerne had apparently substantially recovered from larval damage by the time of the last cuts. In the drier 1982–83 season, two cuts were taken compared with four in the 1983–84 season. Adult feeding was found to reduce second cut wet yields by 20–30% in the 1982–83 season but had no measurable effect in the 1983–84 season. In both seasons, manipulated ranges of larval densities showed that damage appeared only when larval populations were in excess of a distinct larval population density threshold. At larval densities less than this threshold, yield was unaffected; above the threshold, yield losses were independent of larval density. Such a yield response curve was found to be adequately described mathematically by a simple arctan model; this approach allowed the threshold to be further defined and overall estimates to be made of the yield losses arising from larval damage. The threshold occurred at about 1200 larvae/m2 in the dry season and about 2100 larvae/m2 in the wet season. This suggested moisture sensitivity and the possibility that the crop may enter a damage-induced dormancy related to that observed during a severe drought.
High speed winds from quasars interact with the gas and stars in a surrounding galaxy and produce a variety of effects. The winds sweep up the general interstellar matter into fast moving shells of cool gas which can produce lower velocity (≲, 0.01c) sharp absorption line systems. The impact of the wind on dense interstellar clouds may contribute to the narrow emission line region. Finally, supernovae or stellar winds near the QSO set up shock waves in the outflow. Shocked QSO wind material is responsible for the broad emission line material.