Boron Abundances in Diffuse Interstellar Clouds

We present a comprehensive survey of B abundances in diffuse interstellar clouds from HST/STIS observations along 56 Galactic sight lines. Our sample is the result of a complete search of archival STIS data for the B II resonance line at 1362 angstroms, with each detection confirmed by the presence of absorption from other dominant ions at the same velocity. The data probe a range of astrophysical environments including both high-density regions of massive star formation as well as low-density paths through the Galactic halo, allowing us to clearly define the trend of B depletion onto interstellar grains as a function of gas density. Many extended sight lines exhibit complex absorption profiles that trace both local gas and gas associated with either the Sagittarius-Carina or Perseus spiral arm. Our analysis indicates a higher B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of the Sun, which may suggest that B production in the current epoch is dominated by a secondary process. The average gas-phase B abundance in the warm diffuse ISM is consistent with the abundances determined for a variety of Galactic disk stars, but is depleted by 60 percent relative to the solar system value. Our survey also reveals sight lines with enhanced B abundances that potentially trace recent production of B-11 either by cosmic-ray or neutrino-induced spallation. Such sight lines will be key to discerning the relative importance of the two production routes for B-11 synthesis.Comment: To be published in the proceedings of the IAU Symposium 268, Light Elements in the Universe, C. Charbonnel, M. Tosi, F. Primas & C. Chiappini, ed

Rubidium in the Interstellar Medium

We present observations of interstellar rubidium toward o Per, zeta Per, AE Aur, HD 147889, chi Oph, zeta Oph, and 20 Aql. Theory suggests that stable 85Rb and long-lived 87Rb are produced predominantly by high-mass stars, through a combination of the weak s- and r-processes. The 85Rb/87Rb ratio was determined from measurements of the Rb I line at 7800 angstroms and was compared to the solar system meteoritic ratio of 2.59. Within 1-sigma uncertainties all directions except HD 147889 have Rb isotope ratios consistent with the solar system value. The ratio toward HD 147889 is much lower than the meteoritic value and similar to that toward rho Oph A (Federman et al. 2004); both lines of sight probe the Rho Ophiuchus Molecular Cloud. The earlier result was attributed to a deficit of r-processed 85Rb. Our larger sample suggests instead that 87Rb is enhanced in these two lines of sight. When the total elemental abundance of Rb is compared to the K elemental abundance, the interstellar Rb/K ratio is significantly lower than the meteoritic ratio for all the sight lines in this study. Available interstellar samples for other s- and r- process elements are used to help interpret these results.Comment: 24 pages, 3 figures. Accepted for publication in Ap

Slow, Steady-State Transport with "Loading" and Bulk Reactions: the Mixed Ionic Conductor La2_2CuO4+δ_{4+\delta}

We consider slow, steady transport for the normal state of the superconductor La$_2$CuO$_{4+\delta}$ in a one-dimensional geometry, with surface fluxes sufficiently general to permit oxygen to be driven into the sample (``loaded'') either by electrochemical means or by high oxygen partial pressure. We include the bulk reaction O$\to$O$^{2-}+2h$, where neutral atoms ($a$) go into ions ($i$) and holes ($h$). For slow, steady transport, the transport equations simplify because the bulk reaction rate density $r$ and the bulk loading rates $\partial_t n$ then are uniform in space and time. All three fluxes $j$ must be specified at each surface, which for a uniform current density $J$ corresponds to five independent fluxes. These fluxes generate two types of static modes at each surface and a bulk response with a voltage profile that varies quadratically in space, characterized by $J$ and the total oxygen flux $j_O$ (neutral plus ion) at each surface. One type of surface mode is associated with electrical screening; the other type is associated both with diffusion and drift, and with chemical reaction (the {\it diffusion-reaction mode}). The diffusion-reaction mode is accompanied by changes in the chemical potentials $\mu$, and by reactions and fluxes, but it neither carries current (J=0) nor loads the system chemically ($j_O=0$). Generation of the diffusion-reaction mode may explain the phenomenon of ``turbulence in the voltage'' often observed near the electrodes of other mixed ionic electronic conductors (MIECs).Comment: 11 pages, 1 figur

Oscillator Strengths for B-X, C-X, and E-X Transitions in Carbon Monoxide

Band oscillator strengths for electronic transitions in CO were obtained at the Synchrotron Radiation Center of the University of Wisconsin-Madison. Our focus was on transitions that are observed in interstellar spectra with the Far Ultraviolet Spectroscopic Explorer; these transitions are also important in studies of selective isotope photodissociation where fractionation among isotopomers can occur. Absorption from the ground state (X ^1Sigma^+ v'' = 0) to A ^1Pi (v'= 5), B ^1Sigma^+ (v' = 0, 1), C ^1Sigma^+ (v' = 0, 1), and E ^1Pi (v' = 0) was measured. Fits to the A - X (5, 0) band, whose oscillator strength is well known, yielded the necessary column density and excitation temperature. These parameters were used in a least-squares fit of the observed profiles for the transitions of interest to extract their band oscillator strengths. Our oscillator strengths are in excellent agreement with results from recent experiments using a variety of techniques. This agreement provides the basis for a self-consistent set of f-values at far ultraviolet wavelengths for studies of interstellar (and stellar) CO.Comment: 22 pages, 3 figures, ApJS (in press

The Interstellar Rubidium Isotope Ratio toward Rho Ophiuchi A

The isotope ratio, 85Rb/87Rb, places constraints on models of the nucleosynthesis of heavy elements, but there is no precise determination of the ratio for material beyond the Solar System. We report the first measurement of the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 A for the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma) that differs significantly from the meteoritic value of 2.59. The Rb/K elemental abundance ratio for the cloud also is lower than that seen in meteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic values indicates that the interstellar 85Rb abundance in this direction is lower than the Solar System abundance. We attribute the lower abundance to a reduced contribution from the r-process. Interstellar abundances for Kr, Cd, and Sn are consistent with much less r-process synthesis for the solar neighborhood compared to the amount inferred for the Solar System.Comment: 12 pages with 2 figures and 1 table; will appear in ApJ Letter

Deposition of earth-abundant p-type CuBr films with high hole conductivity and realization of p-CuBr/n-Si heterojunction solar cell

We present details of the deposition of transparent and earth-abundant p-type CuBr films with high hole conductivity and the fabrication and characterization of a prototype solar cell based on p-CuBr/n-Si heterojunctions. p-type CuBr films with typical resistivities and hole concentrations of 7×10-1 Ωcm and 7.5×1019 cm-3, respectively, are deposited by thermal evaporation followed by oxygen plasma treatment. The transparent p-type films show strong room temperature photoluminescence at ~2.97 eV. The current voltage (I-V) characteristics of the heterojunctions show good diode behaviour. Power conversion efficiency of ~ 2 % was achieved for the heterojunction device without any optimization of the cell structure under AM 1.5 illumination condition with a short circuit current (Jsc) and open circuit voltage (Voc) of 13.2 mA/cm2 and 0.44 V, respectively

Dynamik von mikrobiellen Gemeinschaften in zeitweise überfluteten Böden mit und ohne Zugabe von Biokohle

Böden in Auen werden zeitweise überflutet, was eine Änderung des Redoxpotentials bewirkt. Diese variierenden Umweltbedingungen können Bodenmikroorganismen erheblichen Stress aussetzen, vor allem, wenn diese Schwankungen den physiologischen Toleranzbereich der Organismen überschreiten. Mikroorganismen setzen verschiedenste Anpassungs- und Vermeidungsstrategien ein, um in solchen Situationen überleben zu können. Hierdurch verändert sich die Zusammensetzung der bodenmikrobiellen Gemeinschaft, was wiederum Rückkopplungseffekte auf die biogeochemischen Eigenschaften des Bodens ausübt. Um Böden und deren Eigenschaften zu verbessern, werden seit einigen Jahren verschiedene Biokohlen eingesetzt. Welche bodenmikrobiellen Prozesse im Boden von der Biokohle auf welche Art beeinflusst werden, ist jedoch weitestgehend ungeklärt. Ziel dieser Studie war es, den Einfluss verschiedener Biokohlen und wechselnder Redoxverhältnisse auf die bodenmikrobielle Gemeinschaft in einem zeitweise überfluteten Boden zu untersuchen. Dazu wurden Bodenproben der Wupper mit unterschiedlichen Biokohlen im Laborversuch vermischt und vorinkubiert. Die eingesetzten Biokohlen BC200 und BC500 unterschieden sich in der Pyrolysetemperatur (200°C und 500°C). Die so vorbehandelten Böden wurden anschließend in biogeochemischen Mikrokosmen bei vordefinierten Redoxpotentialen inkubiert. Anschließend wurde die Dynamik der bodenmikrobiellen und der bakteriellen Gemeinschaft (PLFA und DGGE) bestimmt. Die Ergebnisse zeigen, dass sowohl die Änderung des Redoxpotentials, als auch die Zugabe von Biokohle die Zusammensetzung der mikrobiellen und bakteriellen Gemeinschaft in Bezug auf Gesamtzahl, aber auch nach taxonomischer Gruppe, verändern. Bestimmte mikrobielle Gruppen und Spezies werden gefördert bzw. unterdrückt. Möglicherweise lassen sich durch die Aufklärung dieser Zusammenhänge Vorhersagen zu Folgen bei Überflutungsereignissen treffen