A Multiwavelength View at the Heart of the Superwind in NGC 253

Here we present new optical data from the Hubble Space Telescope of the NGC 253 central region, which reveal numerous discrete sources in a ring--like structure. This is combined with data at infrared, millimeter, radio and X-ray wavelengths to examine the nature of these discrete sources and the nucleus itself. We find that the majority of optical/IR/mm sources are young star clusters which trace out a ~50 pc ring, that defines the inner edge of a cold gas torus. This reservoir of cold gas has probably been created by gas inflow from a larger scale bar and deposited at the inner Lindblad resonance. The family of compact radio sources lie interior to the starburst ring, and in general do not have optical or IR counterparts. They are mostly SNRs. The radio nucleus, which is probably an AGN, lies near the centre of the ring. The X-ray emission from the nuclear source is extended in the ROSAT HRI detector indicating that not all of the X-ray emission can be associated with the AGN. The lack of X-ray variability and the flat radio spectrum of the nucleus, argues against an ultraluminous SN as the dominant energetic source at the galaxy core. The diffuse emission associated with the outflowing superwind is present in the central region on a size scale consistent with the idea of collimation by the gas torus.Comment: 26 pages, Latex, 6 figures, 4 tables, submitted to MNRA

Laser-induced nucleation promotes crystal growth of anhydrous sodium bromide

We report on a study of crystal hydrate formation in supersaturated aqueous sodium bromide using different methods to induce nucleation: mechanical shock-induced nucleation (MSIN), nucleation by ultrasound (sonocrystallization), non-photochemical laser-induced nucleation (NPLIN) and laser-trapping nucleation. The most stable crystal form at room temperature is known to be sodium bromide dihydrate (DH) and this form was favoured (>95%) through spontaneous nucleation or mechanical shock. Sonocrystallization favoured DH crystals (74%). Remarkably both laser-induced nucleation methods showed a strong preference (>90%) for anhydrous (AH) crystals. The nucleation mechanisms are discussed with reference to the solution–solid phase diagram. For laser-trapping nucleation, the results are consistent with previous studies showing that nucleation is preceded by formation of a localised volume of increased solute concentration. The common mechanistic feature linking sonocrystallization, MSIN and NPLIN is cavitation. The preference for AH sodium bromide suggests that nanosecond laser pulses produce cavitation events with more thermal energy compared to the other methods. The results demonstrate the value of laser-induced nucleation in controlling crystal hydrate growth and provide new understanding of the nucleation mechanisms

Antisolvent addition at extreme conditions

This article describes the use of antisolvent addition at high-pressure to aid precipitation and recovery of high-pressure phases to ambient pressure. Paracetamol (PCM) was used as a model system to demonstrate the principle due to the extensive literature of paracetamol at high-pressure and ambient pressure. We have observed that we are able to recover the orthorhombic form of paracetamol to ambient pressure using this technique, although solvent-mediated transformations are a hurdle. During this investigation we observed a new methanol solvate of paracetamol that is simlar in structure to the known form. The methanol solvate is stable to 0.2 GPa before transformation to the orthorhombic form that is known to be the stable form at high pressure

On the Use of Historical Bathymetric Data to Determine Changes in Bathymetry: An Analysis of Errors and Application to Great Bay Estuary, NH

The depth measurements that are incorporated into bathymetric charts have associated errors with magnitudes depending on the survey circumstances and applied techniques. For this reason, combining and comparing depth measurements collected over many years with different techniques and standards is a difficult task which must be done with great caution. In this study we have developed an approach for comparing historical bathymetric surveys. Our methodology uses Monte Carlo modelling to account for the random error components inherited in the data due to positioning and depth measurement uncertainties

The role of cavitation and gas bubbles in the non-photochemical laser-induced nucleation of sodium acetate

An experimental study of the effects of the sodium salt of poly(methacrylic acid) (Na-PMAA) on non-photochemical laser-induced nucleation (NPLIN) of sodium acetate crystals is presented. Seeding of supersaturated aqueous solutions with anhydrous (AH) seeds always produced trihydrate (TH) crystals, with or without polymer additive. Using NPLIN, with no Na-PMAA and at low Na-PMAA concentrations (0.25% w/w) AH sodium acetate was produced, firstly as plate-like form IV, but subsequently growing needles, likely to be form I. At high Na-PMAA concentrations (0.73% w/w) we observe formation mostly of stable bubbles. In all samples at low laser peak power densities (<26 MW cm−2) we show for the first time using NPLIN that both crystals and bubbles can be nucleated with a single laser pulse. Measurements of the dependence of bubble or crystal count on laser pulse power indicate a common mechanistic origin for nucleation, which is cavitation due to laser heating of impurity nanoparticles. The bubbles observed are attributed to laser heating of the nanoparticles to high temperatures, resulting in gas formed by thermochemical reactions or gas that was previously dissolved in the solution. Our results provide new insight into the particle-heating mechanism for NPLIN, but whether stable bubbles play a defining role in the nucleation of crystals remains to be resolved

Non-Abelian Tensor Multiplet Equations from Twistor Space

We establish a Penrose-Ward transform yielding a bijection between holomorphic principal 2-bundles over a twistor space and non-Abelian self-dual tensor fields on six-dimensional flat space-time. Extending the twistor space to supertwistor space, we derive sets of manifestly N=(1,0) and N=(2,0) supersymmetric non-Abelian constraint equations containing the tensor multiplet. We also demonstrate how this construction leads to constraint equations for non-Abelian supersymmetric self-dual strings.Comment: v3: 23 pages, revised version published in Commun. Math. Phy

Role of Impurity Nanoparticles in Laser-Induced Nucleation of Ammonium Chloride

Results of experiments on laser-induced nucleation (LIN) in supersaturated (120%) aqueous ammonium chloride solutions are presented. Measurement of the particle-size distribution in unfiltered solutions near saturation (95%) indicates a population of nanometer-scale species with a mean hydrodynamic diameter of 750 nm, which is almost entirely removed by single-pass filtration through a poly­(ether sulfone) membrane (0.2 μm pores). Analysis of filter residues reveals iron and phosphate as major impurities in the solute. Experiments show that the number of nuclei induced by LIN can be reduced substantially by preprocessing (filtering or long-term exposure to laser pulses) and that this reduction can be reversed by intentional doping with iron-oxide (Fe₃O₄) nanoparticles. The use of surfactant to assist dispersion of the nanoparticles was found to increase the number of laser-induced nuclei. We discuss the results with reference to mechanisms of non-photochemical laser-induced nucleation