Quantitative field constraints on the dynamics of silicic magma chamber rejuvenation and overturn

The catastrophic eruption of large-volume, crystal-rich silicic magmas is often proposed to be a consequence of reheating, melting and overturn of partially molten, buoyant silicic material following repeated injection of dense, hot mafic magma. To test this “rejuvenation hypothesis”, we analyze at high spatial resolution 33 examples of deformed interfaces between intrusive mafic and silicic layers in two plutons of the Coastal Maine Magmatic Province, USA. These deformed interfaces are thought to record the buoyant overturn of silicic crystal mush layers, apparently in response to the injection and cooling of hot, dense mafic magmas. We use spectral analysis and scaling theory along with petrologic and textural data to identify, characterize, and understand periodic deformations from the scale of individual crystals (≈1 cm) to the thicknesses of mafic and silicic layers. Deformations at the largest scale lengths (>100 m) are at wavelengths comparable to, or greater than, silicic layer thicknesses and support a conjecture that mafic recharge can cause large-scale Rayleigh–Taylor-type overturning of silicic mushy layers. By contrast, the smallest scales of individual crystals probably record effects related to production and buoyancy-driven rise of melt from the tops of silicic mushes in contact with overlying hot basalt, whereas intermediate scales are explained by compaction. Our results constrain the evolution of a thermal rejuvenation event and potentially identify a condition for a large-scale overturn of the magma chamber that may lead to eruption. This work provides the first quantitative field-based constraints on some of the key physical processes inherent to the rejuvenation hypothesis.     

Mechanisms to limit higher mode effects in a controlled rocking steel frame. 1: Concept,modelling, and low‐amplitude shake table testing

Controlled rocking steel frames have been proposed as an efficient way to avoid the structural damage and residual deformations that are expected in conventional seismic force resisting systems. Although the base rocking response is intended to limit the force demands, higher mode effects can amplify member design forces, reducing the viability of the system. This paper suggests that seismic forces may be limited more effectively by providing multiple force‐limiting mechanisms. Two techniques are proposed: detailing one or more rocking joints above the base rocking joint and providing a self‐centring energy dissipative (SCED) brace at one or more levels. These concepts are applied to the design of an eight‐storey prototype structure and a shake table model at 30% scale. A simple numerical model that was used as a design tool is in good agreement with frequency characterization and low‐amplitude seismic tests of the shake table model, particularly when multiple force‐limiting mechanisms are active. These results suggest that the proposed mechanisms can enable better capacity design by reducing the variability of peak seismic force demands without causing excessive displacements. Similar results are expected for other systems that rely on a single location of concentrated nonlinearity to limit peak seismic loads. Copyright © 2012 John Wiley & Sons, Ltd.     

Star formation around the H ii region Sh2-235

We present a picture of star formation around the H  ii region Sh2-235 (S235) based upon data on the spatial distribution of young stellar clusters and the distribution and kinematics of molecular gas around S235. We observed ¹³CO (1–0) and CS (2–1) emission toward S235 with the Onsala Space Observatory 20-m telescope and analysed the star density distribution with archival data from the Two Micron All-Sky Survey (2MASS). Dense molecular gas forms a shell-like structure at the southeastern part of S235. The young clusters found with 2MASS data are embedded in this shell. The positional relationship of the clusters, the molecular shell and the H  ii region indicates that expansion of S235 is responsible for the formation of the clusters. The gas distribution in the S235 molecular complex is clumpy, which hampers interpretation exclusively on the basis of the morphology of the star-forming region. We use data on kinematics of molecular gas to support the hypothesis of induced star formation, and distinguish three basic types of molecular gas components. The first type is primordial undisturbed gas of the giant molecular cloud, the second type is gas entrained in motion by expansion of the H  ii region (this is where the embedded clusters were formed) and the third type is a fast-moving gas, which might have been accelerated by winds from the newly formed clusters. The clumpy distribution of molecular gas and its kinematics around the H  ii region implies that the picture of triggered star formation around S235 can be a mixture of at least two possibilities: the 'collect-and-collapse' scenario and the compression of pre-existing dense clumps by the shock wave.     

Infrared Morphology of Regions of Ionized Hydrogen

A search for infrared ring nebulae associated with regions of ionized hydrogen has been carried out. The New GPS Very Large Array survey at 20 cm forms the basis of the search, together with observations obtained with the Spitzer Space Telescope at 8 and 24 μm and the Herschel Space Telescope at 70 μm. Objects having ring-like morphologies at 8 μm and displaying extended emission at 20 cm were selected visually. Emission at 24 μm having the form of an inner ring or central peak is also observed in the selected objects. A catalog of 99 ring nebulae whose shapes at 8 and 70 μm are well approximated by ellipses has been compiled. The catalog contains 32 objects whose shapes are close to circular (eccentricities of the fitted ellipses at 8 μm no greater than 0.6, angular radius exceeding 20″). These objects are promising for comparisons with the results of one-dimensional hydrodynamical simulations of expanding regions of ionized hydrogen.     

A possible origin of the high metallicity in the nuclei of massive galaxies and quasars

According to current observations, the relative abundance of gas-phase metals in distant quasars with ages of only ～10⁹ yr (z～5) can be appreciably higher than the solar abundance. We show that there are two main ways to explain the high metallicity of these galactic nuclei: a high gas density in the central regions, or an increase in the minimum masses of forming stars to several solar masses. The results of numerical modeling confirm this conclusion.     

Molecular structure of brown-dwarf disks

We describe typical features of the chemical composition of proto-planetary disks around brown dwarfs. We model the chemical evolution in the disks around a low-mass T Tauri star and a cooler brown dwarf over a time span of 1 Myr using a model for the physical structure of an accretion disk with a vertical temperature gradient and an extensive set of gas-phase chemical reactions. We find that the disks of T Tauri stars are, in general, hotter and denser than the disks of lower-luminosity substellar objects. In addition, they have more pronounced vertical temperature gradients. The atmospheres of the disks around low-mass stars are more strongly ionized by UV and X-ray radiation, while less dense brown-dwarf disks have higher fractional ionizations in their midplanes. Nevertheless, in both cases, most molecules are concentrated in the so-called warm molecular layer between the ionized atmosphere and cold midplane, where grains with ice mantles are abundant.     

Restructuring and destruction of hydrocarbon dust in the interstellar medium

A model describing the main processes determining the evolution of hydrocarbon dust grains of arbitrary size under astrophysical conditions corresponding to regions of ionized hydrogen (HII regions) and supernova remnants is presented. The processes considered include aromatization and photodestruction, sputtering by electrons and ions, and shattering during collisions between grains. The model can be used to calculate the size distribution of the grains and the degree of aromatization during the evolution of HII regions and supernova remnants for a specified radiation field, relative velocity between the gas and dust, etc. The contribution of various processes to the evolution of hydrocarbon dust grains for parameters typical for the interstellar medium of our Galaxy is considered. Small grains (with fewer than 50 carbon atoms) should be fully aromatized in the interstellar medium. If larger grains initially have an aliphatic structure, this is preserved to a substantial extent. Variation in the size distribution of the grains due to collisions between grains depend appreciably on the adopted initial size distribution. With an initial distribution corresponding to that of Mathis et al. (1977), the mass fraction contributed by smaller grains tends to increase with time, while, with an initial distribution corresponding to that of Jones et al. (2013), in which the fraction of small grains is initially high, there is a general decrease in the number of grains of various sizes with time.     

Evaluation of ocean and climate models using present‐day observations and forcing

Abstract

The most common method used to evaluate climate models involves spinning them up under perpetual present‐day forcing and comparing the model results with present‐day observations. This approach clearly ignores any potential long‐term memory of the model ocean to past climatic conditions. Here we examine the validity of this approach through the 6000‐year integration of a coupled atmosphere–ocean–sea‐ice model. The coupled model is initially spun‐up with atmospheric CO₂ concentrations and orbital parameters applicable for 6KBP. The model is then integrated forward in time to 2100. Results from this transient coupled model simulation are compared with the results from two additional simulations, in which the model is spun up with perpetual 1850 (preindustrial) and 1998 (present‐day) atmospheric CO₂ concentrations and orbital parameters. This comparison leads to substantial differences between the equilibrium climatologies and the transient simulation, even at 1850 (in weakly ventilated regions), prior to any significant changes in atmospheric CO₂. When compared to the present‐day equilibrium climatology, differences are very large: the global mean surface air and sea surface temperatures are ,0.5°C and ,0.4°C colder, respectively, deep ocean temperatures are substantially cooler, Southern Hemisphere sea‐ice cover is 38% larger, and the North Atlantic conveyor 16% weaker in the transient case. These differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene, as well as to its large thermal inertia. It is also demonstrated that a ‘cold start’ global warming simulation (one that starts from a 1998 equilibrium climatology) underestimates the global temperature increase at 2100 by ,10%. Our results question the accuracy of current techniques for climate model evaluation and underline the importance of using paleoclimatic simulations in parallel with present‐day simulations in this evaluation process.