749 research outputs found
Modeling of the Super-Eddington Phase for Classical Novae: Five IUE Novae
We present a light curve model for the super-Eddington luminosity phase of
five classical novae observed with IUE. Optical and UV light curves are
calculated based on the optically thick wind theory with a reduced effective
opacity for a porous atmosphere. Fitting a model light curve with the UV 1455
\AA light curve, we determine the white dwarf mass and distance to be (1.3
M_sun, 4.4 kpc) for V693 CrA, (1.05 M_sun, 1.8 kpc) for V1974 Cyg, (0.95 M_sun,
4.1 kpc) for V1668 Cyg, (1.0 M_sun, 2.1 kpc) for V351 Pup, and (1.0 M_sun, 4.3
kpc) for OS And.Comment: 9 pages including 8 figures, to appear in the Astrophysical Journa
A Possible Periodicity in the Radio Lightcurves of 3C454.3
During the period 1966.5 - 2006.2 the 15GHz and 8GHz lightcurves of 3C454.3
(z=0.859) show a qsasi-periodicity of ~12.8 yr (~6.9 yr in the rest frame of
the source) with a double-bump structure. This periodic behaviour is
interpreted in terms of a rotating double-jet model in which the two jets are
created from the black holes in a binary system and rotate with the period of
the orbital motion. The periodic variations in the radio fluxes of 3C454.3 are
suggested to be mainly due to the lighthouse effects (or the variation in
Doppler boosting) of the precessing jets which are caused by the orbital
motion. In addition, variations in the mass-flow rates accreting onto the black
holes may be also involved.Comment: 15 pages, 11 figure
Exploring Curved Superspace
We systematically analyze Riemannian manifolds M that admit rigid
supersymmetry, focusing on four-dimensional N=1 theories with a U(1)_R
symmetry. We find that M admits a single supercharge, if and only if it is a
Hermitian manifold. The supercharge transforms as a scalar on M. We then
consider the restrictions imposed by the presence of additional supercharges.
Two supercharges of opposite R-charge exist on certain fibrations of a
two-torus over a Riemann surface. Upon dimensional reduction, these give rise
to an interesting class of supersymmetric geometries in three dimensions. We
further show that compact manifolds admitting two supercharges of equal
R-charge must be hyperhermitian. Finally, four supercharges imply that M is
locally isometric to M_3 x R, where M_3 is a maximally symmetric space.Comment: 39 pages; minor change
Hydrogen bonding of nitroxide spin labels in membrane proteins
On the basis of experiments at 275 GHz, we reconsider the dependence of the
continuous-wave EPR spectra of nitroxide spin-labeled protein sites in
sensory- and bacteriorhodopsin on the micro-environment. The high magnetic
field provides the resolution necessary to disentangle the effects of hydrogen
bonding and polarity. In the gxx region of the 275 GHz EPR spectrum, bands are
resolved that derive from spin-label populations carrying no, one or two
hydrogen bonds. The gxx value of each population varies hardly from site to
site, significantly less than deduced previously from studies at lower
microwave frequencies. The fractions of the populations vary strongly, which
provides a consistent description of the variation of the average gxx and the
average nitrogen-hyperfine interaction Azz from site to site. These variations
reflect the difference in the proticity of the micro-environment, and
differences in polarity contribute marginally. Concomitant W-band ELDOR-
detected NMR experiments on the corresponding nitroxide in perdeuterated water
resolve population-specific nitrogen-hyperfine bands, which underlies the
interpretation for the proteins
Ferredoxin:NADP(H) Oxidoreductase Abundance and Location Influences Redox Poise and Stress Tolerance
In linear photosynthetic electron transport, ferredoxin:NADP(H) oxidoreductase (FNR) transfers electrons from ferredoxin (Fd) to NADP(+). Both NADPH and reduced Fd (Fd(red)) are required for reductive assimilation and light/dark activation/deactivation of enzymes. FNR is therefore a hub, connecting photosynthetic electron transport to chloroplast redox metabolism. A correlation between FNR content and tolerance to oxidative stress is well established, although the precise mechanism remains unclear. We investigated the impact of altered FNR content and localization on electron transport and superoxide radical evolution in isolated thylakoids, and probed resulting changes in redox homeostasis, expression of oxidative stress markers, and tolerance to high light in planta. Our data indicate that the ratio of Fd(red) to FNR is critical, with either too much or too little FNR potentially leading to increased superoxide production, and perception of oxidative stress at the level of gene transcription. In FNR overexpressing plants, which show more NADP(H) and glutathione pools, improved tolerance to high-light stress indicates that disturbance of chloroplast redox poise and increased free radical generation may help “prime” the plant and induce protective mechanisms. In fnr1 knock-outs, the NADP(H) and glutathione pools are more oxidized relative to the wild type, and the photoprotective effect is absent despite perception of oxidative stress at the level of gene transcription
All solutions of the localization equations for N=2 quantum black hole entropy
We find the most general bosonic solution to the localization equations
describing the contributions to the quantum entropy of supersymmetric black
holes in four-dimensional N=2 supergravity coupled to n_v vector multiplets.
This requires the analysis of the BPS equations of the corresponding off-shell
supergravity (including fluctuations of the auxiliary fields) with AdS2 \times
S2 attractor boundary conditions. Our work completes and extends the results of
arXiv:1012.0265 that were obtained for the vector multiplet sector, to include
the fluctuations of all the fields of the off-shell supergravity. We find that,
when the auxiliary SU(2) gauge field strength vanishes, the most general
supersymmetric configuration preserving four supercharges is labelled by n_v+1
real parameters corresponding to the excitations of the conformal mode of the
graviton and the scalars of the n_v vector multiplets. In the general case, the
localization manifold is labelled by an additional SU(2) triplet of one-forms
and a scalar function.Comment: 27 page
Understanding the co-existence of conflict and cooperation: Transboundary ecosystem management in the Virunga Massif
Potassium tert-Butoxide-Catalyzed Dehydrogenative C–H Silylation of Heteroaromatics: A Combined Experimental and Computational Mechanistic Study
We recently reported a new method for the direct dehydrogenative C–H silylation of heteroaromatics utilizing Earth-abundant potassium tert-butoxide. Herein we report a systematic experimental and computational mechanistic investigation of this transformation. Our experimental results are consistent with a radical chain mechanism. A trialkylsilyl radical may be initially generated by homolytic cleavage of a weakened Si–H bond of a hypercoordinated silicon species as detected by IR, or by traces of oxygen which can generate a reactive peroxide by reaction with (KOt-Bu)_4 as indicated by density functional theory (DFT) calculations. Radical clock and kinetic isotope experiments support a mechanism in which the C–Si bond is formed through silyl radical addition to the heterocycle followed by subsequent β-hydrogen scission. DFT calculations reveal a reasonable energy profile for a radical mechanism and support the experimentally observed regioselectivity. The silylation reaction is shown to be reversible, with an equilibrium favoring products due to the generation of H_2 gas. In situ NMR experiments with deuterated substrates show that H_2 is formed by a cross-dehydrogenative mechanism. The stereochemical course at the silicon center was investigated utilizing a ^2H-labeled silolane probe; complete scrambling at the silicon center was observed, consistent with a number of possible radical intermediates or hypercoordinate silicates
- …