The Thermal Evolution of Ices in the Environments of Newly Formed Stars: The CO_2 Diagnostic

Archival data from the Infrared Spectrometer of the Spitzer Space Telescope are used to study the 15 μm absorption feature of solid CO_2 toward 28 young stellar objects (YSOs) of approximately solar mass. Fits to the absorption profile using laboratory spectra enable categorization according to the degree of thermal processing of the ice matrix that contains the CO_2. The majority of YSOs in our sample (20 out of 28) are found to be consistent with a combination of polar (H_2O-rich) and nonpolar (CO-rich) ices at low temperature; the remainder exhibit profile structure consistent with partial crystallization as the result of significant heating. Ice-phase column densities of CO_2 are determined and compared with those of other species. Lines of sight with crystallization signatures in their spectra are found to be systematically deficient in solid-phase CO, as expected if CO is being sublimated in regions where the ices are heated to crystallization temperatures. Significant variation is found in the CO2 abundance with respect to both H_2O (the dominant ice constituent) and total dust column (quantified by the extinction, AV ). YSOs in our sample display typically higher CO_2 concentrations (independent of evidence for thermal processing) in comparison to quiescent regions of the prototypical cold molecular cloud. This suggests that enhanced CO_2 production is driven by photochemical reactions in proximity to some YSOs, and that photoprocessing and thermal processing may occur independently

Growth inhibition of cytosolic Salmonella by caspase-1 and caspase-11 precedes host cell death

Sensing bacterial products in the cytosol of mammalian cells by NOD-like receptors leads to the activation of caspase-1 inflammasomes, and the production of the pro-inflammatory cytokines interleukin (IL)-18 and IL-1β. In addition, mouse caspase-11 (represented in humans by its orthologs, caspase-4 and caspase-5) detects cytosolic bacterial LPS directly. Activation of caspase-1 and caspase-11 initiates pyroptotic host cell death that releases potentially harmful bacteria from the nutrient-rich host cell cytosol into the extracellular environment. Here we use single cell analysis and time-lapse microscopy to identify a subpopulation of host cells, in which growth of cytosolic Salmonella Typhimurium is inhibited independently or prior to the onset of cell death. The enzymatic activities of caspase-1 and caspase-11 are required for growth inhibition in different cell types. Our results reveal that these proteases have important functions beyond the direct induction of pyroptosis and proinflammatory cytokine secretion in the control of growth and elimination of cytosolic bacteria

Phase Diagram of the Attractive Hubbard Model with Inhomogeneous Interactions

The phase diagram of the attractive Hubbard model with spatially inhomogeneous interactions is obtained using a single site dynamical mean field theory like approach. The model is characterized by three parameters: the interaction strength, the active fraction (fraction of sites with the attractive interaction), and electron filling. The calculations indicate that in a parameter regime with intermediate values of interaction strength (compared to the bare bandwidth of the electrons), and intermediate values of the active fraction, "non-BCS" superconductivity is obtained. The results of this work are likely to be relevant to many systems with spatially inhomogeneous superconductivity such as strongly correlated oxides, systems with negative U centers, and, in future, cold atom optical lattices.Comment: 9 pages, 7 figures, to appear in Physical Review

A Catalog of Background Stars Reddened by Dust in the Taurus Dark Clouds

Normal field stars located behind dense clouds are a valuable resource in interstellar astrophysics, as they provide continua in which to study phenomena such as gas-phase and solid-state absorption features, interstellar extinction and polarization. This paper reports the results of a search for highly reddened stars behind the Taurus Dark Cloud complex. We use the Two Micron All Sky Survey (2MASS) Point Source Catalog to survey a 50 sq deg area of the cloud to a limiting magnitude of K = 10.0. Photometry in the 1.2-2.2 micron passbands from 2MASS is combined with photometry at longer infrared wavelengths (3.6-12 micron) from the Spitzer Space Telescope and the Infrared Astronomical Satellite to provide effective discrimination between reddened field stars and young stellar objects (YSOs) embedded in the cloud. Our final catalog contains 248 confirmed or probable background field stars, together with estimates of their total visual extinctions, which span the range 2-29 mag. We also identify the 2MASS source J04292083+2742074 (IRAS 04262+2735) as a previously unrecognized candidate YSO, based on the presence of infrared emission greatly in excess of that predicted for a normal reddened photosphere at wavelengths >5 microns

Training-induced criticality in martensites

We propose an explanation for the self-organization towards criticality observed in martensites during the cyclic process known as `training'. The scale-free behavior originates from the interplay between the reversible phase transformation and the concurrent activity of lattice defects. The basis of the model is a continuous dynamical system on a rugged energy landscape, which in the quasi-static limit reduces to a sandpile automaton. We reproduce all the principal observations in thermally driven martensites, including power-law statistics, hysteresis shakedown, asymmetric signal shapes, and correlated disorder.Comment: 5 pages, 4 figure

Rheograms for asphalt from single viscosity measurement

Asphalt materials are used in a variety of applications such as road paving, waterproofing, roofing membranes, adhesive binders, rust proofing and water resistant coatings. There are available in a number of grades distinguished in terms of their softening point and flow resistance. The selection of the proper grade of asphalt for a particular application is governed by the desired flow behaviour. A knowledge of the complete flow curve depicting the variation of melt viscosity with shear rate at the relevant temperatures is necessary not only for proper grade selection, but also for specifying processing conditions for aggregate mixing and spraying. The rheological data are also useful in assessing end use performance. The scientific techniques for generating the rheological data involve the use of expensive, sophisticated instruments. Generation of the necessary flow data using these instruments is beyond the financial and technical means of most processors of asphalt materials. The engineering techniques involving the use of inexpensive vacuum viscometers are relatively easy, but provide a single point viscosity measurement at low shear rate. In the present work, a method is proposed for unifying the viscosity versus shear rate a data at various temperatures for a number of asphalt grades. A master curve has been generated that is independent of the grade of asphalt and the temperature of viscosity measurement. The master curve can be used to generate rheograms at desired temperatures for the asphalt grade of interest, knowing its zero-shear viscosity at that temperature

Melt rheology of polymer blends from melt flow index

Polymer blends have received a lot of attention in recent years due to the possibility of getting compounds with novel and/or different properties through proper marriage of the properties of the respective mono-components. The flow behaviour of blends when subjected to stresses is complex and is often found not to vary monotonically with composition. Mixing rules and mixture theories have been used for estimating melt viscosity of a blend at zero shear rate. However a knowledge of the entire rheogram is desirable for process optimization, process design and trouble shooting. In the present paper a method proposed earlier to estimate the rheograms of polymer melts through the use of the melt flow index has been extended to polyblends. A method for obtaining the melt flow index of the polymer blends at various compositions from the melt flow index of the individual components and the blend ratio has been suggested based on the altered free volume state model. Curves that are coalesced using the melt flow index of the blend at different blend ratios have been presented for a polypropylene-high density polyethylene blend, a high density polyethylene-polymethyl methacrylate blend, a polystyrene-polymethyl-methacrylate blend, a polystyrene-polyacetal blend and a polymethylmethacrylate-polyacetal blend

Rheology of nylon 6 containing metal halides

Addition of metal halides to nylons has been shown to be advantageous in a number of ways. The decrease in melting temperature, increase in glass transition temperature and melt viscosity by such additions have allowed more convenient processing of low molecular weight polymers and thermally unstable polymers. Rheological data depicting the variation of melt viscosity with shear rate at temperatures relevant to processing are necessary in optimizing and trouble-shooting plastics processing operations. In the present paper, a method has been proposed to estimate, complete flow curves or rheograms of nylon-metal halide systems with the use of a master curve knowing the melt flow index and glass transition temperature of the system. The validity of the approach has been verified for the nylon 6-lithium chloride system and shown to hold good for any nylon-metal halide combination

Rheograms for engineering thermoplastics from melt flow index

A method proposed earlier has been extended to estimate complete flow curves or rheograms of engineering plastics. Master curves that are independent of the grade and temperature have been generated and presented for acrylics, polyacetal, nylons, polyethylene terephthalate, polycarbonate and polysulfone. The influence of the various molecular parameters on the viscosity behaviour of polymer melts have been explained rationally. More specifically, the effects of chain branching and of chain rigidity on the master curve of a resin type have been elucidated with reference to polyacetal and polysulfone, respectively. The method presented here can be used effectively by processors of engineering plastics