Compositional Differences between Felsic Volcanic rocks from the Margin and Center of the Northern Main Ethiopian Rift

Pliocene felsic rift margin and Quaternary rift center volcanic rocks from the northern Main Ethiopian Rift (MER) exhibit contrasts in major and trace element contents and Sr-Nd isotopic ratios. Quaternary rift center felsic volcanic rocks are mainly peralkaline trachytes and rhyolites, whereas Pliocene felsic rift margin volcanic rocks are represented by benmoreites, weakly peralkaline trachytes and rare rhyolites. Most of the felsic rift margin volcanic rocks have greater Al2O3, K2O, Nb, Zr, Rb, and Sr, and lesser CaO, Zr/Nb, and CaO/Al2O3 than rift center volcanic rocks. These contrasts may have been inherited from differences in the compositions of their parental basic magmas, which were produced by variable degrees of partial melting. In both series, the felsic volcanic rocks generally have higher initial Sr- isotopic (0.7038-0.7073) ratios than their basic equivalents (0.7035-0.7046). Nd- isotopic ratios of most felsic rift center samples (0.5129-0.5126) are similar to their associated basic volcanic rocks. In contrast, the Nd-isotopic ratios (0.5128-0.5124) of felsic rift margin volcanic rocks are commonly lower than their companion basic volcanic rocks (0.512806-0.512893), and are relatively lower than rift center equivalents. The elemental and Sr-Nd isotopic compositions of the volcanic rocks suggest that fractional crystallization from differing basic parents accompanied by a limited assimilation (AFC) was the dominant process controlling the genesis of the MER felsic volcanic rocks. Keywords: Ethiopia; Northern Main Ethiopian Rift; Bimodal Volcanism; parental difference; Sr-Nd Isotopes, Fractional Crystallisatio

Synthetic Observations of Simulated Radio Galaxies I: Radio and X-ray Analysis

We present an extensive synthetic observational analysis of numerically- simulated radio galaxies designed to explore the effectiveness of conventional observational analyses at recovering physical source properties. These are the first numerical simulations with sufficient physical detail to allow such a study. The present paper focuses on extraction of magnetic field properties from nonthermal intensity information. Synchrotron and inverse-Compton intensities provided meaningful information about distributions and strengths of magnetic fields, although considerable care was called for. Correlations between radio and X-ray surface brightness correctly revealed useful dynamical relationships between particles and fields. Magnetic field strength estimates derived from the ratio of X-ray to radio intensity were mostly within about a factor of two of the RMS field strength along a given line of sight. When emissions along a given line of sight were dominated by regions close to the minimum energy/equipartition condition, the field strengths derived from the standard power-law-spectrum minimum energy calculation were also reasonably close to actual field strengths, except when spectral aging was evident. Otherwise, biases in the minimum- energy magnetic field estimation mirrored actual differences from equipartition. The ratio of the inverse-Compton magnetic field to the minimum-energy magnetic field provided a rough measure of the actual total energy in particles and fields in most instances, within an order of magnitude. This may provide a practical limit to the accuracy with which one may be able to establish the internal energy density or pressure of optically thin synchrotron sources.Comment: 43 pages, 14 figures; accepted for publication in ApJ, v601 n2 February 1, 200

Molecular Hydrogen Formation on Ice Under Interstellar Conditions

The results of experiments on the formation of molecular hydrogen on low density and high density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low density ice gives rise to a broader spectrum of energy barriers compared to the high density ice. Inserting these parameters into the rate equation model under steady state conditions we evaluate the production rate of molecular hydrogen on ice-coated interstellar dust grains

Continuous density measurement of atomic hydrogen by means of a bolometer

We developed a device which allows continuous measurement of the density of low‐temperature stabilized atomic hydrogen by means of a bolometer. This density monitor was tested in a large open‐storage cell during microwave‐induced extraction of polarized atoms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70391/2/RSINAK-62-1-251-1.pd

Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments

The study of the formation of molecular hydrogen on low temperature surfaces is of interest both because it allows to explore elementary steps in the heterogeneous catalysis of a simple molecule and because of the applications in astrochemistry. Here we report results of experiments of molecular hydrogen formation on amorphous silicate surfaces using temperature-programmed desorption (TPD). In these experiments beams of H and D atoms are irradiated on the surface of an amorphous silicate sample. The desorption rate of HD molecules is monitored using a mass spectrometer during a subsequent TPD run. The results are analyzed using rate equations and the activation energies of the processes leading to molecular hydrogen formation are obtained from the TPD data. We show that a model based on a single isotope provides the correct results for the activation energies for diffusion and desorption of H atoms. These results can thus be used to evaluate the formation rate of H_2 on dust grains under the actual conditions present in interstellar clouds.Comment: 30 pages, 1 table, 6 figures. Published versio

Dramatic Changes in the Electronic Structure Upon Transition to the Collapsed Tetragonal Phase in CaFe2As2

We use angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic structure of CaFe$_2$As$_2$ in previously unexplored collapsed tetragonal (CT) phase. This unusual phase of the iron arsenic high temperature superconductors was hard to measure as it exists only under pressure. By inducing internal strain, via the post growth, thermal treatment of the single crystals, we were able to stabilize the CT phase at ambient-pressure. We find significant differences in the Fermi surface topology and band dispersion data from the more common orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent with electronic structure calculations. The top of the hole bands sinks below the Fermi level, which destroys the nesting present in parent phases. The absence of nesting in this phase along with apparent loss of Fe magnetic moment, are now clearly experimentally correlated with the lack of superconductivity in this phase.Comment: 5 pages, 4 figures, accepted in PRB(RC

Molecular Hydrogen Formation on Ice Under Interstellar Conditions

The results of experiments on the formation of molecular hydrogen on low density and high density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low density ice gives rise to a broader spectrum of energy barriers compared to the high density ice. Inserting these parameters into the rate equation model under steady state conditions we evaluate the production rate of molecular hydrogen on ice-coated interstellar dust grains.Comment: 20 pages, 3 tables and 10 figures. Accepted to ApJ. Minor changes made and adittional references adde