Anomalous exponents at the onset of an instability

Critical exponents are calculated exactly at the onset of an instability, using asymptotic expansiontechniques. When the unstable mode is subject to multiplicative noise whose spectrum at zero frequency vanishes, we show that the critical behavior can be anomalous, i.e. the mode amplitude X scales with departure from onset \mu as $~ \mu^\beta$ with an exponent $\beta$ different from its deterministic value. This behavior is observed in a direct numerical simulation of the dynamo instability and our results provide a possible explanation to recent experimental observations

Magnetic domain wall propagation in a submicron spin-valve stripe: influence of the pinned layer

The propagation of a domain wall in a submicron ferromagnetic spin-valve stripe is investigated using giant magnetoresistance. A notch in the stripe efficiently traps an injected wall stopping the domain propagation. The authors show that the magnetic field at which the wall is depinned displays a stochastic nature. Moreover, the depinning statistics are significantly different for head to head and tail-to-tail domain walls. This is attributed to the dipolar field generated in the vicinity of the notch by the pinned layer of the spin-valve

OzDES multifibre spectroscopy for the Dark Energy Survey: 3-yr results and first data release

We present results for the first three years of OzDES, a six year programme to obtain redshifts for objects in the Dark Energy Survey (DES) supernova fields using the 2dF fibre positioner and AAOmega spectrograph on the Anglo-Australian Telescope. OzDES is a multi-object spectroscopic survey targeting multiple types of targets at multiple epochs over a multiyear baseline and is one of the first multi-object spectroscopic surveys to dynamically include transients into the target list soon after their discovery. At the end of three years, OzDES has spectroscopically confirmed almost 100 supernovae, and has measured redshifts for 17 000 objects, including the redshifts of 2566 supernova hosts. We examine how our ability to measure redshifts for targets of various types depends on signal-to-noise ratio (S/N), magnitude and exposure time, finding that our redshift success rate increases significantly at a S/N of 2–3 per 1-Å bin. We also find that the change in  S/N with exposure time closely matches the Poisson limit for stacked exposures as long as 10 h. We use these results to predict the redshift yield of the full OzDES survey, as well as the potential yields of future surveys on other facilities such as (i.e. the 4-m Multi-Object Spectroscopic Telescope, the Subaru Prime Focus Spectrograph and the Maunakea Spectroscopic Explorer). This work marks the first OzDES data release, comprising 14 693 redshifts. OzDES is on target to obtain over 30 000 redshifts over the 6-yr duration of the survey, including a yield of approximately 5700 supernova host-galaxy redshifts

Towards an experimental von Karman dynamo: numerical studies for an optimized design

Numerical studies of a kinematic dynamo based on von Karman type flows between two counterrotating disks in a finite cylinder are reported. The flow has been optimized using a water model experiment, varying the driving impellers configuration. A solution leading to dynamo action for the mean flow has been found. This solution may be achieved in VKS2, the new sodium experiment to be performed in Cadarache, France. The optimization process is described and discussed, then the effects of adding a stationary conducting layer around the flow on the threshold, on the shape of the neutral mode and on the magnetic energy balance are studied. Finally, the possible processes involved into kinematic dynamo action in a von Karman flow are reviewed and discussed. Among the possible processes we highlight the joint effect of the boundary-layer radial velocity shear and of the Ohmic dissipation localized at the flow/outer-shell boundary

OzDES multifibre spectroscopy for the Dark Energy Survey: 3-yr results and first data release

We present results for the first three years of OzDES, a six year programme to obtain redshifts for objects in the Dark Energy Survey (DES) supernova fields using the 2dF fibre positioner and AAOmega spectrograph on the Anglo-Australian Telescope. OzDES is a multi-object spectroscopic survey targeting multiple types of targets at multiple epochs over a multiyear baseline and is one of the first multi-object spectroscopic surveys to dynamically include transients into the target list soon after their discovery. At the end of three years, OzDES has spectroscopically confirmed almost 100 supernovae, and has measured redshifts for 17 000 objects, including the redshifts of 2566 supernova hosts. We examine how our ability to measure redshifts for targets of various types depends on signal-to-noise ratio (S/N), magnitude and exposure time, finding that our redshift success rate increases significantly at a S/N of 2–3 per 1-Å bin. We also find that the change in  S/N with exposure time closely matches the Poisson limit for stacked exposures as long as 10 h. We use these results to predict the redshift yield of the full OzDES survey, as well as the potential yields of future surveys on other facilities such as (i.e. the 4-m Multi-Object Spectroscopic Telescope, the Subaru Prime Focus Spectrograph and the Maunakea Spectroscopic Explorer). This work marks the first OzDES data release, comprising 14 693 redshifts. OzDES is on target to obtain over 30 000 redshifts over the 6-yr duration of the survey, including a yield of approximately 5700 supernova host-galaxy redshifts

Large scale flow effects, energy transfer, and self-similarity on turbulence

The effect of large scales on the statistics and dynamics of turbulent fluctuations is studied using data from high resolution direct numerical simulations. Three different kinds of forcing, and spatial resolutions ranging from 256^3 to 1024^3, are being used. The study is carried out by investigating the nonlinear triadic interactions in Fourier space, transfer functions, structure functions, and probability density functions. Our results show that the large scale flow plays an important role in the development and the statistical properties of the small scale turbulence. The role of helicity is also investigated. We discuss the link between these findings and intermittency, deviations from universality, and possible origins of the bottleneck effect. Finally, we briefly describe the consequences of our results for the subgrid modeling of turbulent flows

Kiruna-Type Iron Oxide-Apatite (IOA) and Iron Oxide Copper-Gold (IOCG) Deposits Form by a Combination of Igneous and Magmatic-Hydrothermal Processes: Evidence from the Chilean Iron Belt

Iron oxide copper-gold (IOCG) and Kiruna-type iron oxide-apatite (IOA) deposits are commonly spatially and temporally associated with one another, and with coeval magmatism. Here, we use trace element concentrations in magnetite and pyrite, Fe and O stable isotope abundances of magnetite and hematite, H isotopes of magnetite and actinolite, and Re-Os systematics of magnetite from the Los Colorados Kiruna-type IOA deposit in the Chilean iron belt to develop a new genetic model that explains IOCG and IOA deposits as a continuum produced by a combination of igneous and magmatic-hydrothermal processes. The concentrations of [Al + Mn] and [Ti + V] are highest in magnetite cores and decrease systematically from core to rim, consistent with growth of magnetite cores from a silicate melt, and rims from a cooling magmatic-hydrothermal fluid. Almost all bulk δ 18 O values in magnetite are within the range of 0 to 5‰, and bulk δ 56 Fe for magnetite are within the range 0 to 0.8‰ of Fe isotopes, both of which indicate a magmatic source for O and Fe. The values of δ 18 O and δD for actinolite, which is paragenetically equivalent to magnetite, are, respectively, 6.46 ± 0.56 and-59.3 ± 1.7‰, indicative of a mantle source. Pyrite grains consistently yield Co/Ni ratios that exceed unity, and imply precipitation of pyrite from an ore fluid evolved from an intermediate to mafic magma. The calculated initial 187 Os/ 188 Os ratio (Osi) for magnetite from Los Colorados is 1.2, overlapping Osi values for Chilean porphyry-Cu deposits, and consistent with an origin from juvenile magma. Together, the data are consistent with a geologic model wherein (1) magnetite microlites crystallize as a near-liquidus phase from an intermediate to mafic silicate melt; (2) magnetite microlites serve as nucleation sites for fluid bubbles and promote volatile saturation of the melt; (3) the volatile phase coalesces and encapsulates magnetite microlites to form a magnetite-fluid suspension; (4) the suspension scavenges Fe, Cu, Au, S, Cl, P, and rare earth elements (REE) from the melt; (5) the suspension ascends from the host magma during regional extension; (6) as the suspension ascends, originally igneous mag-netite microlites grow larger by sourcing Fe from the cooling magmatic-hydrothermal fluid; (7) in deep-seated crustal faults, magnetite crystals are deposited to form a Kiruna-type IOA deposit due to decompression of the magnetite-fluid suspension; and (8) the further ascending fluid transports Fe, Cu, Au, and S to shallower levels or lateral distal zones of the system where hematite, magnetite, and sulfides precipitate to form IOCG deposits. The model explains the globally observed temporal and spatial relationship between magmatism and IOA and IOCG deposits, and provides a valuable conceptual framework to define exploration strategies

Supernova 2012ec: Identification of the progenitor and early monitoring with PESSTO

We present the identification of the progenitor of the Type IIP SN 2012ec in archival pre-explosion HST WFPC2 and ACS/WFC F814W images. The properties of the progenitor are further constrained by non-detections in pre-explosion WFPC2 F450W and F606W images. We report a series of early photometric and spectroscopic observations of SN 2012ec. The r'-band light curve shows a plateau with M(r')=-17.0. The early spectrum is similar to the Type IIP SN 1999em, with the expansion velocity measured at Halpha absorption minimum of -11,700 km/s (at 1 day post-discovery). The photometric and spectroscopic evolution of SN 2012ec shows it to be a Type IIP SN, discovered only a few days post-explosion (<6d). We derive a luminosity for the progenitor, in comparison with MARCS model SEDs, of log L/Lsun = 5.15+/-0.19, from which we infer an initial mass range of 14-22Msun. This is the first SN with an identified progenitor to be followed by the Public ESO Spectroscopic Survey of Transient Objects (PESSTO).Comment: 6 pages, 3 figures, MNRAS accepte

Global bifurcations to subcritical magnetorotational dynamo action in Keplerian shear flow

Magnetorotational dynamo action in Keplerian shear flow is a three-dimensional, non-linear magnetohydrodynamic process whose study is relevant to the understanding of accretion processes and magnetic field generation in astrophysics. Transition to this form of dynamo action is subcritical and shares many characteristics of transition to turbulence in non-rotating hydrodynamic shear flows. This suggests that these different fluid systems become active through similar generic bifurcation mechanisms, which in both cases have eluded detailed understanding so far. In this paper, we build on recent work on the two problems to investigate numerically the bifurcation mechanisms at work in the incompressible Keplerian magnetorotational dynamo problem in the shearing box framework. Using numerical techniques imported from dynamical systems research, we show that the onset of chaotic dynamo action at magnetic Prandtl numbers larger than unity is primarily associated with global homoclinic and heteroclinic bifurcations of nonlinear magnetorotational dynamo cycles. These global bifurcations are found to be supplemented by local bifurcations of cycles marking the beginning of period-doubling cascades. The results suggest that nonlinear magnetorotational dynamo cycles provide the pathway to turbulent injection of both kinetic and magnetic energy in incompressible magnetohydrodynamic Keplerian shear flow in the absence of an externally imposed magnetic field. Studying the nonlinear physics and bifurcations of these cycles in different regimes and configurations may subsequently help to better understand the physical conditions of excitation of magnetohydrodynamic turbulence and instability-driven dynamos in a variety of astrophysical systems and laboratory experiments. The detailed characterization of global bifurcations provided for this three-dimensional subcritical fluid dynamics problem may also prove useful for the problem of transition to turbulence in hydrodynamic shear flows