Discovery of the Fomalhaut C debris disc

Fomalhaut is one of the most interesting and well studied nearby stars, hosting at least one planet, a spectacular debris ring, and two distant low-mass stellar companions (TW PsA and LP 876-10, a.k.a. Fomalhaut B & C). We observed both companions with Herschel, and while no disc was detected around the secondary, TW PsA, we have discovered the second debris disc in the Fomalhaut system, around LP 876-10. This detection is only the second case of two debris discs seen in a multiple system, both of which are relatively wide ($\gtrsim$3000 AU for HD 223352/40 and 158 kAU [0.77 pc] for Fomalhaut/LP 876-10). The disc is cool (24K) and relatively bright, with a fractional luminosity $L_{\rm disc}/L_\star = 1.2 \times 10^{-4}$, and represents the rare observation of a debris disc around an M dwarf. Further work should attempt to find if the presence of two discs in the Fomalhaut system is coincidental, perhaps simply due to the relatively young system age of 440 Myr, or if the stellar components have dynamically interacted and the system is even more complex than it currently appears.Comment: Published in MNRAS Letters. Merry Xma

Bifurcation scenario to Nikolaevskii turbulence in small systems

We show that the chaos in Kuramoto-Sivashinsky equation occurs through period-doubling cascade (Feigenbaum scenario), in contrast, the chaos in Nikolaevskii equation occurs through torus-doubling bifurcation (Ruelle-Takens-Newhouse scenario).Comment: 8pages, 9figure

Interpreting the extended emission around three nearby debris disc host stars

Cool debris discs are a relic of the planetesimal formation process around their host star, analogous to the solar system's Edgeworth-Kuiper belt. As such, they can be used as a proxy to probe the origin and formation of planetary systems like our own. The Herschel Open Time Key Programmes "DUst around NEarby Stars" (DUNES) and "Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre" (DEBRIS) observed many nearby, sun-like stars at far-infrared wavelengths seeking to detect and characterize the emission from their circumstellar dust. Excess emission attributable to the presence of dust was identified from around $\sim$ 20% of stars. Herschel's high angular resolution ($\sim$ 7" FWHM at 100 $\mu$m) provided the capacity for resolving debris belts around nearby stars with radial extents comparable to the solar system (50 to 100 au). As part of the DUNES and DEBRIS surveys, we obtained observations of three debris disc stars, HIP 22263 (HD 30495), HIP 62207 (HD 110897), and HIP 72848 (HD 131511), at far-infrared wavelengths with the Herschel PACS instrument. Combining these new images and photometry with ancilliary data from the literature, we undertook simultaneous multi-wavelength modelling of the discs' radial profiles and spectral energy distributions using three different methodologies: single annulus, modified black body, and a radiative transfer code. We present the first far-infrared spatially resolved images of these discs and new single-component debris disc models. We characterize the capacity of the models to reproduce the disc parameters based on marginally resolved emission through analysis of two sets of simulated systems (based on the HIP 22263 and HIP 62207 data) with the noise levels typical of the Herschel images. We find that the input parameter values are recovered well at noise levels attained in the observations presented here.Comment: 13 pages, 5 figures, 5 tables, accepted for publication in A&

The Central Laser Facility at the Pierre Auger Observatory

The Central Laser Facility is located near the middle of the Pierre Auger Observatory in Argentina. It features a UV laser and optics that direct a beam of calibrated pulsed light into the sky. Light scattered from this beam produces tracks in the Auger optical detectors which normally record nitrogen fluorescence tracks from cosmic ray air showers. The Central Laser Facility provides a "test beam" to investigate properties of the atmosphere and the fluorescence detectors. The laser can send light via optical fiber simultaneously to the nearest surface detector tank for hybrid timing analyses. We describe the facility and show some examples of its many uses.Comment: 4 pages, 5 figures, submitted to 29th ICRC Pune Indi

Jet Collimation by Small-Scale Magnetic Fields

A popular model for jet collimation is associated with the presence of a large-scale and predominantly toroidal magnetic field originating from the central engine (a star, a black hole, or an accretion disk). Besides the problem of how such a large-scale magnetic field is generated, in this model the jet suffers from the fatal long-wave mode kink magnetohydrodynamic instability. In this paper we explore an alternative model: jet collimation by small-scale magnetic fields. These magnetic fields are assumed to be local, chaotic, tangled, but are dominated by toroidal components. Just as in the case of a large-scale toroidal magnetic field, we show that the ``hoop stress'' of the tangled toroidal magnetic fields exerts an inward force which confines and collimates the jet. The magnetic ``hoop stress'' is balanced either by the gas pressure of the jet, or by the centrifugal force if the jet is spinning. Since the length-scale of the magnetic field is small (< the cross-sectional radius of the jet << the length of the jet), in this model the jet does not suffer from the long-wave mode kink instability. Many other problems associated with the large-scale magnetic field are also eliminated or alleviated for small-scale magnetic fields. Though it remains an open question how to generate and maintain the required small-scale magnetic fields in a jet, the scenario of jet collimation by small-scale magnetic fields is favored by the current study on disk dynamo which indicates that small-scale magnetic fields are much easier to generate than large-scale magnetic fields.Comment: 14 pages, no figur

The impact of population growth and climate change on food security in Africa:looking ahead to 2050

This work was funded by a PhD studentship for CH from the Scottish Food Security Alliance-Crops (Universities of Aberdeen and Dundee and the James Hutton Institute), and contributes to the Belmont Forum funded DEVIL project (NERC fund UK contribution: NE/M021327/1). JIM and RBM acknowledge funding from the Rural and Environment Science and Analytical Services, Scottish Government.Peer reviewedPostprin

Swift-Hohenberg model for magnetoconvection

A model system of partial differential equations in two dimensions is derived from the three-dimensional equations for thermal convection in a horizontal fluid layer in a vertical magnetic field. The model consists of an equation of Swift-Hohenberg type for the amplitude of convection, coupled to an equation for a large-scale mode representing the local strength of the magnetic field. The model facilitates both analytical and numerical studies of magnetoconvection in large domains. In particular, we investigate the phenomenon of flux separation, where the domain divides into regions of strong convection with a weak magnetic field and regions of weak convection with a strong field. Analytical predictions of flux separation based on weakly nonlinear analysis are extended into the fully nonlinear regime through numerical simulations. The results of the model are compared with simulations of the full three-dimensional magnetoconvection problem.S. M. Cox, P. C. Matthews, and S. L. Pollicot

Optical excitations in a non-ideal Bose gas

Optical excitations in a Bose gas are demonstrated to be very sensitive to many-body effects. At low temperature the momentum relaxation is provided by momentum exchange collisions, rather than by elastic collisions. A collective excitation mode forms, which in a Boltzmann gas is manifest in a collision shift and dramatic narrowing of spectral lines. In the BEC state, each spectral line splits into two components. The doubling of the optical excitations results from the physics analogous to that of the second sound. We present a theory of the line doubling, and calculate the oscillator strengths and linewidth.Comment: 5 pages, 3 eps figure

Yrast line for weakly interacting trapped bosons

We compute numerically the yrast line for harmonically trapped boson systems with a weak repulsive contact interaction, studying the transition to a vortex state as the angular momentum L increases and approaches N, the number of bosons. The L=N eigenstate is indeed dominated by particles with unit angular momentum, but the state has other significant components beyond the pure vortex configuration. There is a smooth crossover between low and high L with no indication of a quantum phase transition. Most strikingly, the energy and wave function appear to be analytical functions of L over the entire range 2 < L < N. We confirm the structure of low-L states proposed by Mottelson, as mainly single-particle excitations with two or three units of angular momentum.Comment: 9 pages, 3 EPS-figures, uses psfig.st