3D Photoionisation Modelling of NGC 6302

We present a three-dimensional photoionisation and dust radiative transfer model of NGC 6302, an extreme, high-excitation planetary nebula. We use the 3D photoionisation code Mocassin} to model the emission from the gas and dust. We have produced a good fit to the optical emission-line spectrum, from which we derived a density distribution for the nebula. A fit to the infrared coronal lines places strong constraints on the properties of the unseen ionising source. We find the best fit comes from using a 220,000 K hydrogen-deficient central star model atmosphere, indicating that the central star of this PN may have undergone a late thermal pulse. We have also fitted the overall shape of the ISO spectrum of NGC 6302 using a dust model with a shallow power-law size distribution and grains up to 1.0 micron in size. To obtain a good fit to the infrared SED the dust must be sufficiently recessed within the circumstellar disk to prevent large amounts of hot dust at short wavelengths, a region where the ISO spectrum is particularly lacking. These and other discoveries are helping to unveil many properties of this extreme object and trace it's evolutionary history.Comment: 8 pages, 4 figures; for the proceedings of "Asymmetric Planetary Nebuale IV," R. L. M. Corradi, A. Manchado, N. Soker ed

Dichroism for orbital angular momentum using parametric amplification

We theoretically analyze parametric amplification as a means to produce dichroism based on the orbital angular momentum (OAM) of an incident signal field. The nonlinear interaction is shown to provide differential gain between signal states of differing OAM, the peak gain occurring at half the OAM of the pump field

Magnetic Flux Tube Reconnection: Tunneling Versus Slingshot

The discrete nature of the solar magnetic field as it emerges into the corona through the photosphere indicates that it exists as isolated flux tubes in the convection zone, and will remain as discrete flux tubes in the corona until it collides and reconnects with other coronal fields. Collisions of these flux tubes will in general be three dimensional, and will often lead to reconnection, both rearranging the magnetic field topology in fundamental ways, and releasing magnetic energy. With the goal of better understanding these dynamics, we carry out a set of numerical experiments exploring fundamental characteristics of three dimensional magnetic flux tube reconnection. We first show that reconnecting flux tubes at opposite extremes of twist behave very differently: in some configurations, low twist tubes slingshot while high twist tubes tunnel. We then discuss a theory explaining these differences: by assuming helicity conservation during the reconnection one can show that at high twist, tunneled tubes reach a lower magnetic energy state than slingshot tubes, whereas at low twist the opposite holds. We test three predictions made by this theory. 1) We find that the level of twist at which the transition from slingshot to tunnel occurs is about two to three times higher than predicted on the basis of energetics and helicity conservation alone, probably because the dynamics of the reconnection play a large role as well. 2) We find that the tunnel occurs at all flux tube collision angles predicted by the theory. 3) We find that the amount of magnetic energy a slingshot or a tunnel reconnection releases agrees reasonably well with the theory, though at the high resistivities we have to use for numerical stability, a significant amount of magnetic energy is lost to diffusion, independent of reconnection.Comment: 21 pages, 15 figures, submitted to Ap

Observations of apparent superslow wave propagation in solar prominences

Phase mixing of standing continuum Alfv\'en waves and/or continuum slow waves in atmospheric magnetic structures such as coronal arcades can create the apparent effect of a wave propagating across the magnetic field. We observe a prominence with SDO/AIA on 2015 March 15 and find the presence of oscillatory motion. We aim to demonstrate that interpreting this motion as a magneto hydrodynamic (MHD) wave is faulty. We also connect the decrease of the apparent velocity over time with the phase mixing process, which depends on the curvature of the magnetic field lines. By measuring the displacement of the prominence at different heights to calculate the apparent velocity, we show that the propagation slows down over time, in accordance with the theoretical work of Kaneko et al. We also show that this propagation speed drops below what is to be expected for even slow MHD waves for those circumstances. We use a modified Kippenhahn-Schl\"uter prominence model to calculate the curvature of the magnetic field and fit our observations accordingly. Measuring three of the apparent waves, we get apparent velocities of 14, 8, and 4 km/s. Fitting a simple model for the magnetic field configuration, we obtain that the filament is located 103 Mm below the magnetic centre. We also obtain that the scale of the magnetic field strength in the vertical direction plays no role in the concept of apparent superslow waves and that the moment of excitation of the waves happened roughly one oscillation period before the end of the eruption that excited the oscillation. Some of the observed phase velocities are lower than expected for slow modes for the circumstances, showing that they rather fit with the concept of apparent superslow propagation. A fit with our magnetic field model allows for inferring the magnetic geometry of the prominence.Comment: 10 pages, 6 figures, 1 of which consists of 3 panel

Two-body anticorrelation in a harmonically trapped ideal Bose gas

We predict the existence of a dip below unity in the second-order coherence function of a partially condensed ideal Bose gas in harmonic confinement, signaling the anticorrelation of density fluctuations in the sample. The dip in the second-order coherence function is revealed in a canonical-ensemble calculation, corresponding to a system with fixed total number of particles. In a grand-canonical ensemble description, this dip is obscured by the occupation-number fluctuation catastrophe of the ideal Bose gas. The anticorrelation is most pronounced in highly anisotropic trap geometries containing small particle numbers. We explain the fundamental physical mechanism which underlies this phenomenon, and its relevance to experiments on interacting Bose gases.Comment: 10 pages, 5 figures. v2: Minor changes and corrections to figures and text. To appear in PR