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  • 1
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    Organizational participation : myth and reality (2000)
    Oxford ; New York : Oxford University Press
    Details
    Keywords: Industrial relations. ; Organizational behavior. ; Organizational sociology.
    Pages: x, 294 p.
    ISBN: 0-585-36603-9
    URL: http://www.netLibrary.com/urlapi.asp?action=summary&v=1&bookid=56038
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    E-Book (German National Licenses)
  • 2
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    Three-dimensional hydrodynamic simulations of L2 Puppis (2016)
    Oxford University Press
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    Publication Date: 2016-06-26
    Description: Recent observations of the L 2 Puppis system suggest that this Mira-like variable may be in the early stages of forming a bipolar planetary nebula. As one of nearest and brightest asymptotic giant branch (AGB) stars, thought be a binary, L 2 Puppis serves as a benchmark object for studying the late-stages of stellar evolution. We perform global, three-dimensional, adaptive-mesh-refinement hydrodynamic simulations of the L 2 Puppis system with astrobear . We use the radiative transfer code radmc- $\scriptstyle 3$ d to construct the broad-band spectral energy distribution and synthetic observational images from our simulations. Given the reported binary parameters, we are able to reproduce the current observational data if a short pulse of dense material is released from the AGB star with a velocity sufficient to escape the primary but not the binary. Such a situation could result from a thermal pulse, be induced by a periastron passage of the secondary, or could be launched if the primary ingests a planet.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Subalpine conifers in different geographical locations host highly similar foliar bacterial endophyte communities (2016)
    Oxford University Press
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    Publication Date: 2016-06-30
    Description: Pines in the subalpine environment at Niwot Ridge, CO, have been found to host communities of acetic acid bacteria (AAB) within their needles . The significance and ubiquity of this pattern is not known, but recent evidence of nitrogen (N)-fixing activity in Pinus flexilis (limber pine) foliage calls for a better understanding of the processes that regulate endophytic communities in forest tree canopies. Here, to test if AAB dominate the foliar bacterial microbiota in other subalpine locations, we compared the 16S rRNA community in needles from P. flexilis and P. contorta (lodgepole pine) growing in the Eastern Sierra Nevada, CA, and Niwot Ridge, CO. AAB made up the majority of the bacterial community in both species at both sites. Multiple distinct AAB taxa, resolved at the single nucleotide level, were shared across host species and sites, with dominant OTUs identical or highly similar to database sequences from cold environments, including high altitude air sampled in Colorado, and the endosphere of Arctic plants. Our results suggest strong selection for community composition, potentially amplified by the long lifespan of individual Pinus needles, along with low dispersal constraints on canopy bacteria.
    Print ISSN: 0168-6496
    Electronic ISSN: 1574-6941
    Topics: Biology
    Published by Oxford University Press
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  • 4
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    Molecular cloud formation in high-shear, magnetized colliding flows (2016)
    Oxford University Press
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    Publication Date: 2016-06-10
    Description: The colliding flows (CF) model is a well-supported mechanism for generating molecular clouds. However, to-date most CF simulations have focused on the formation of clouds in the normal-shock layer between head-on colliding flows. We performed simulations of magnetized colliding flows that instead meet at an oblique-shock layer. Oblique shocks generate shear in the post-shock environment, and this shear creates inhospitable environments for star formation. As the degree of shear increases (i.e. the obliquity of the shock increases), we find that it takes longer for sink particles to form, they form in lower numbers, and they tend to be less massive. With regard to magnetic fields, we find that even a weak field stalls gravitational collapse within forming clouds. Additionally, an initially oblique collision interface tends to reorient over time in the presence of a magnetic field, so that it becomes normal to the oncoming flows. This was demonstrated by our most oblique shock interface, which became fully normal by the end of the simulation.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    The creation of AGB fallback shells (2016)
    Oxford University Press
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    Publication Date: 2016-02-24
    Description: The possibility that mass ejected during Asymptotic Giant Branch (AGB) stellar evolution phases falls back towards the star has been suggested in applications ranging from the formation of accretion discs to the powering of late-thermal pulses. In this paper, we seek to explicate the properties of fallback flow trajectories from mass-loss events. We focus on a transient phase of mass ejection with sub-escape speeds, followed by a phase of a typical AGB wind. We solve the problem using both hydrodynamic simulations and a simplified one-dimensional analytic model that matches the simulations. For a given set of initial wind characteristics, we find a critical shell velocity that distinguishes between ‘shell fallback’ and ‘shell escape’. We discuss the relevance of our results for both single and binary AGB stars. In particular, we discuss how our results help to frame further studies of fallback as a mechanism for forming the substantial population of observed post-AGB stars with dusty discs.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Triggered star formation and its consequences (2014)
    Oxford University Press
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    Publication Date: 2014-09-17
    Description: Star formation can be triggered by compression from wind or supernova-driven shock waves that sweep over molecular clouds. Because these shocks will likely contain processed elements, triggered star formation has been proposed as an explanation for short-lived radioactive isotopes (SLRIs) in the Solar system. Previous studies have tracked the triggering event to the earliest phases of collapse and have focused on the shock properties required for both successful star formation and mixing of SLRIs. In this paper, we use adaptive mesh refinement simulation methods, including sink particles, to simulate the full collapse and subsequent evolution of a stable Bonnor– Ebert sphere subjected to a shock and post-shock wind. We track the flow of the cloud material after a star (a sink particle) has formed. For non-rotating clouds, we find robust triggered collapse and little bound circumstellar material remaining around the post-shock collapsed core. When we add initial cloud rotation, we observe the formation of discs around the collapsed core which then interact with the post-shock flow. Our results indicate that these circumstellar discs are massive enough to form planets and are long lived, in spite of the ablation driven by post-shock-flow ram pressure. As a function of the initial conditions, we also track the time evolution of the accretion rates and particle mixing between the ambient wind and cloud material. The latter is maximized for cases of highest Mach number.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    The inner cavity of the circumnuclear disc (2016)
    Oxford University Press
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    Publication Date: 2016-04-29
    Description: The circumnuclear disc (CND) orbiting the Galaxy's central black hole is a reservoir of material that can ultimately provide energy through accretion, or form stars in the presence of the black hole, as evidenced by the stellar cluster that is presently located at the CND's centre. In this paper, we report the results of a computational study of the dynamics of the CND. The results lead us to question two paradigms that are prevalent in previous research on the Galactic Centre. The first is that the disc's inner cavity is maintained by the interaction of the central stellar cluster's strong winds with the disc's inner rim, and secondly, that the presence of unstable clumps in the disc implies that the CND is a transient feature. Our simulations show that, in the absence of a magnetic field, the interaction of the wind with the inner disc rim actually leads to a filling of the inner cavity within a few orbital time-scales, contrary to previous expectations. However, including the effects of magnetic fields stabilizes the inner disc rim against rapid inward migration. Furthermore, this interaction causes instabilities that continuously create clumps that are individually unstable against tidal shearing. Thus the occurrence of such unstable clumps does not necessarily mean that the disc is itself a transient phenomenon. The next steps in this investigation are to explore the effect of the magnetorotational instability on the disc evolution and to test whether the results presented here persist for longer time-scales than those considered here.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    The formation and evolution of wind-capture discs in binary systems (2013)
    Oxford University Press
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    Publication Date: 2013-06-30
    Description: We study the formation, evolution and physical properties of accretion discs formed via wind capture in binary systems. Using the adaptive mesh refinement (AMR) code a stro bear , we have carried out high-resolution 3D simulations that follow a stellar mass secondary in the corotating frame as it orbits a wind producing asymptotic giant branch (AGB) primary. We first derive a resolution criteria, based on considerations of Bondi–Hoyle flows, that must be met in order to properly resolve the formation of accretion discs around the secondary. We then compare simulations of binaries with three different orbital radii ( R o  = 10, 15, 20 au). Discs are formed in all three cases, however, the size of the disc and, most importantly, its accretion rate decreases with orbital radii. In addition, the shape of the orbital motions of material within the disc becomes increasingly elliptical with increasing binary separation. The flow is mildly unsteady with ‘fluttering’ around the bow shock observed. The discs are generally well aligned with the orbital plane after a few binary orbits. We do not observe the presence of any large-scale, violent instabilities (such as the flip-flop mode). For the first time, moreover, it is observed that the wind component that is accreted towards the secondary has a vortex tube-like structure, rather than a column-like one as it was previously thought. In the context of AGB binary systems that might be precursors to pre-planetary nebula (PPN) and planetary nebula (PN), we find that the wind accretion rates at the chosen orbital separations are generally too small to produce the most powerful outflows observed in these systems if the companions are main-sequence stars but marginally capable if the companions are white dwarfs. It is likely that many of the more powerful PPN and PN involve closer binaries than the ones considered here. The results also demonstrate principles of broad relevance to all wind-capture binary systems.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Size of discs formed by wind accretion in binaries can be underestimated if the role of wind-driving force is ignored (2013)
    Oxford University Press
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    Publication Date: 2013-10-29
    Description: Binary systems consisting of a secondary accreting form a wind-emitting primary are ubiquitous in astrophysics. The phenomenology of such Bondi–Hoyle–Lyttleton (BHL) accretors is particularly rich when an accretion disc forms around the secondary. The outer radius of such discs is commonly estimated from the net angular momentum produced by a density variation of material across the BHL or Bondi accretion cylinder, as the latter is tilted with respect to the direction to the primary due to orbital motion. But this approach has ignored the fact that the wind experiences an outward driving force that the secondary does not. In actuality, the accretion stream falls towards a retarded point in the secondary's orbit as the secondary is pulled towards the primary relative to the stream. The result is a finite separation or ‘accretion stream impact parameter’ (ASIP) separating the secondary and stream. When the orbital radius a o exceeds the BHL radius r b , the ratio of outer disc radius estimated as the ASIP to the conventional estimate $a_o^{1/2}/r_b^{1/2} 〉 1$ . We therefore predict that discs will form at larger radii from the secondary than traditional estimates. This agrees with the importance of the ASIP emphasized by Huarte-Espinosa et al. and the practical consequence that resolving the initial outer radius of such an accretion disc in numerical simulations can be less demanding than what earlier estimates would suggest.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Detection of platinum-DNA adducts by32P-postlabelling (1995)
    Oxford University Press
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    Publication Date: 1995-01-01
    Print ISSN: 0305-1048
    Electronic ISSN: 1362-4962
    Topics: Biology
    Published by Oxford University Press
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