Jupiter - Friend or Foe? IV: The influence of orbital eccentricity and inclination

For many years, it was assumed that Jupiter prevented the Earth from being subject to a punishing impact regime that would greatly hinder the development of life. Here, we present the 4th in a series of studies investigating this hypothesis. Previously, we examined the effect of Jupiter's mass on the impact rate experienced by Earth. Here, we extend that approach to consider the influence of Jupiter's orbital eccentricity and inclination on the impact rate. We first consider scenarios in which Jupiter's orbital eccentricity was somewhat higher and somewhat lower than that in our Solar System. We find that Jupiter's orbital eccentricity plays a moderate role in determining the impact flux at Earth, with more eccentric orbits resulting in a higher impact rate of asteroids than for more circular orbits. This is particularly pronounced at high "Jupiter" masses. For short-period comets, the same effect is clearly apparent, albeit to a lesser degree. The flux of short-period comets impacting the Earth is slightly higher for more eccentric Jovian orbits. We also consider scenarios in which Jupiter's orbital inclination was greater than that in our Solar System. Increasing Jupiter's orbital inclination greatly increased the flux of asteroidal impactors. However, at the highest tested inclination, the disruption to the Asteroid belt was so great that the belt would be entirely depleted after an astronomically short period of time. In such a system, the impact flux from asteroid bodies would therefore be very low, after an initial period of intense bombardment. By contrast, the influence of Jovian inclination on impacts from short-period comets was very small. A slight reduction in the impact flux was noted for the moderate and high inclination scenarios considered in this work - the results for inclinations of five and twenty-five degrees were essentially identical.Comment: 5 figures, plus 12 as an appendi

The new aural actuality: an exploration of music, sound and meaning in the composed feature documentary podcast

This practice-led thesis explores the creative techniques and philosophies used in composing feature documentary podcasts and how listeners engage with the material and make meaning from it. Podcasting as a medium presents a new and so far unexplored way of interfacing with audio documentary and this study works to demonstrate crucial differences from radio practice in terms of intention and expression,how material is made, consideration for its audience, and how its programmes are distributed. Using post-structural theory, specifically Deleuze and Guattari’s ideas on interconnected networks of affective transmission, podcasting’s relationship to radio is explored, as is how listeners make meaning through their interaction with both the heard material and the devices upon which it is accessed. These theories are then applied to the characteristically open remit of the audio documentary to study how speech, music, sound and silence may be understood to generate meaning, emotion and a sense of immersion in the listener. It is suggested that modes of programme access, listening customs, and interpretational symbolism work together to impart information vital to the ability to connote and denote what is being heard, and that in this way the composed feature can be situated very closely to musical practice and engagement. Taking cues from musical and cinematic analytical practice three podcast programmes are closely scrutinised for an understanding of their constituent material, structural shape, and potential affective transmissions, beforeinterviewswith their producers are presented to discuss conceptual intentions and execution. Six programmes are presented as the practice component of the thesis, each made to experiment with and reflect upon different aspects of creative or listening practice, with conclusions drawn concerning their implications and effectiveness

Belonging and Becoming in the space[s] of Higher Education

This article explores the intimate entanglement of students Becoming and Belonging in the informal spaces of higher education. In so doing, it raises the many possible ways of belonging and becoming at a South African university. The entwined relationship’s contribution to student identity construction and the potential to exercise agency [or not] is discussed. The theme of Belonging and Becoming emanated from a PhD study that employed visual methodology, specifically photography, to capture students in informal spaces on campus that were of significance to them. The data arose from interviews with student participants and a larger body of students who viewed the photographs as part of an exhibition on campus. The article draws on the concepts of mobility and spatiality, recognising the dynamic nature of campus spaces that are constantly in a state of being socially reproduced. The article recommends that higher education’s obligations extend beyond students’ academic advancement

Origin and Dynamical Evolution of Neptune Trojans - II: Long Term Evolution

We present results examining the fate of the Trojan clouds produced in our previous work. We find that the stability of Neptunian Trojans seems to be strongly correlated to their initial post-migration orbital elements, with those objects that survive as Trojans for billions of years displaying negligible orbital evolution. The great majority of these survivors began the integrations with small eccentricities (e < 0.2) and small libration amplitudes (A < 30 - 40{\deg}). The survival rate of "pre-formed" Neptunian Trojans (which in general survived on dynamically cold orbits (e < 0.1, i < 5 - 10{\deg})) varied between ~5 and 70%. By contrast, the survival rate of "captured" Trojans (on final orbits spread across a larger region of e-i element space) were markedly lower, ranging between 1 and 10% after 4 Gyr. Taken in concert with our earlier work, we note that planetary formation scenarios which involve the slow migration (a few tens of millions of years) of Neptune from an initial planetary architecture that is both resonant and compact (aN < 18 AU) provide the most promising fit of those we considered to the observed Trojan population. In such scenarios, we find that the current day Trojan population would number ~1% of that which was present at the end of the planet's migration, with the bulk being sourced from captured, rather than pre-formed objects. We note, however, that even those scenarios still fail to reproduce the currently observed portion of the Neptune Trojan population moving on orbits with e 20{\deg}. Dynamical integrations of the currently observed Trojans show that five out of the seven are dynamically stable on 4 Gyr timescales, while 2001 QR322, exhibits significant dynamical instability. The seventh Trojan object, 2008 LC18, has such large orbital uncertainties that only future studies will be able to determine its stability.Comment: 24 pages, 6 figures, accepted for publication in MNRAS (The abstract was shortened. Original version can be found in the pdf file

Formation and Dynamical Evolution of the Neptune Trojans - the Influence of the Initial Solar System Architecture

In this work, we investigate the dynamical stability of pre-formed Neptune Trojans under the gravitational influence of the four giant planets in compact planetary architectures, over 10 Myr. In our modelling, the initial orbital locations of Uranus and Neptune (aN) were varied to produce systems in which those planets moved on non-resonant orbits, or in which they lay in their mutual 1:2, 2:3 and 3:4 mean-motion resonances (MMRs). In total, 420 simulations were carried out, examining 42 different architectures, with a total of 840000 particles across all runs. In the non-resonant cases, the Trojans suffered only moderate levels of dynamical erosion, with the most compact systems (those with aN less than or equal 18 AU) losing around 50% of their Trojans by the end of the integrations. In the 2:3 and 3:4 MMR scenarios, however, dynamical erosion was much higher with depletion rates typically greater than 66% and total depletion in the most compact systems. The 1:2 resonant scenarios featured disruption on levels intermediate between the non-resonant cases and other resonant scenarios, with depletion rates of the order of tens of percent. Overall, the great majority of plausible pre-migration planetary architectures resulted in severe levels of depletion of the Neptunian Trojan clouds. In particular, if Uranus and Neptune formed near their mutual 2:3 or 3:4 MMR and at heliocentric distances within 18 AU (as favoured by recent studies), we found that the great majority of pre-formed Trojans would have been lost prior to Neptune's migration. This strengthens the case for the great bulk of the current Neptunian Trojan population having been captured during that migration.Comment: 17 pages, 2 figures, MNRAS (in press). Abstract slightly reduced in size, but in original form in the PDF fil

Pinning Induced Fluctuations on Driven Vortices

We use a simple model to study the long time fluctuations induced by random pinning on the motion of driven non--interacting vortices. We find that vortex motion seen from the co--moving frame is diffusive and anisotropic, with velocity dependent diffusion constants. Longitudinal and transverse diffusion constants cross at a characteristic velocity where diffusion is isotropic. The diffusion front is elongated in the direction of the drive at low velocities and elongated in the transverse direction at large velocities. We find that the mobility in the driven direction is always larger than the transverse mobility, and becomes isotropic only in the large velocity limit.Comment: 4 pages, 3 figs, Vortex IV Proceedings, Sep. 3-9, 2005, Crete, Greec

Discovery of a very X-ray luminous galaxy cluster at z=0.89 in the WARPS survey

We report the discovery of the galaxy cluster ClJ1226.9+3332 in the Wide Angle ROSAT Pointed Survey (WARPS). At z=0.888 and L_X=1.1e45 erg/s (0.5-2.0 keV, h_0=0.5) ClJ1226.9+3332 is the most distant X-ray luminous cluster currently known. The mere existence of this system represents a huge problem for Omega_0=1 world models. At the modest (off-axis) resolution of the ROSAT PSPC observation in which the system was detected, ClJ1226.9+3332 appears relaxed; an off-axis HRI observation confirms this impression and rules out significant contamination from point sources. However, in moderately deep optical images (R and I band) the cluster exhibits signs of substructure in its apparent galaxy distribution. A first crude estimate of the velocity dispersion of the cluster galaxies based on six redshifts yields a high value of 1650 km/s, indicative of a very massive cluster and/or the presence of substructure along the line of sight. While a more accurate assessment of the dynamical state of this system requires much better data at both optical and X-ray wavelengths, the high mass of the cluster has already been unambiguously confirmed by a very strong detection of the Sunyaev-Zel'dovich effect in its direction (Joy et al. 2001). Using ClJ1226.9+3332 and ClJ0152.7-1357 (z=0.835), the second-most distant X-ray luminous cluster currently known and also a WARPS discovery, we obtain a first estimate of the cluster X-ray luminosity function at 0.8<z<1.4 and L_X>5e44 erg/s. Using the best currently available data, we find the comoving space density of very distant, massive clusters to be in excellent agreement with the value measured locally (z<0.3), and conclude that negative evolution is not required at these luminosities out to z~1. (truncated)Comment: accepted for publication in ApJ Letters, 6 pages, 2 figures, uses emulateapj.st

Play it Again: Evolved Audio Effects and Synthesizer Programming

Automatic programming of sound synthesizers and audio devices to match a given, desired sound is examined and a Genetic Algorithm (GA) that functions independent of specific synthesis techniques is proposed. Most work in this area has focused on one synthesis model or synthesizer, designing the GA and tuning the operator parameters to obtain optimal results. The scope of such inquiries has been limited by available computing power, however current software (Ableton Live, herein) and commercially available hardware is shown to quickly find accurate solutions, promising a practical application for music creators. Both software synthesizers and audio effects processors are examined, showing a wide range of performance times (from seconds to hours) and solution accuracy, based on particularities of the target devices. Random oscillators, phase synchronizing, and filters over empty frequency ranges are identified as primary challenges for GA based optimization

Quantifying Jupiter's influence on the Earth's impact flux: Implications for planetary habitability

It has long been thought that the presence of a giant planet is a pre-requisite for the development of life on potentially habitable planets. Without Jupiter, it was argued, the Earth would have been subject to a punishing impact regime, which would have significantly retarded or outright prevented the development of life on our planet. Although this idea is widely embraced, little research has previously been carried out to support it. Here, we present the results of several suites of dynamical integrations used to model the influence of Jupiter's mass and orbit on the impact rate that would be experienced by the Earth. We find that, far from being a simple shield, Jupiter's role in determining the terrestrial impact flux is significantly more complicated than previously thought. Far from being a simple friend, such giant planets are perhaps more likely to imperil the development of life on otherwise habitable planets.Comment: 16 pages, 4 figures, 1 tabl