1,218 research outputs found
Asymptotically simple solutions of the vacuum Einstein equations in even dimensions
We show that a set of conformally invariant equations derived from the
Fefferman-Graham tensor can be used to construct global solutions of the vacuum
Einstein equations, in all even dimensions. This gives, in particular, a new,
simple proof of Friedrich's result on the future hyperboloidal stability of
Minkowski space-time, and extends its validity to even dimensions.Comment: 25p
Accretion of Warm Chondrules in Weakly Metamorphosed Ordinary Chondrites and Their Subsequent Reprocessing
To better understand chondrite accretion and subsequent processes, the textures, crystallography, deformation, and compositions of some chondrite constituents in ten lithologies of different cluster texture strength were studied in seven weakly metamorphosed (Type 3) and variably shocked ordinary chondrites (RaglandâLL3 S1, TieschitzâH/L3 S1, NWA 5421âLL3 S2, NWA 5205âLL3 S2, NWA 11905âLL3-5 S3, NWA 5781âLL3 S3, NWA 11351âLL3-6 S4) using optical and electron microscopy and microtomography techniques. Results support a four-stage model for chondrite formation. This includes 1) limited annealing following collisions during chondrule crystallization and rapid cooling in space prior to accretion, as evidenced by olivine microstructures consistent with dislocation recovery and diffusion; 2) initial accretion of still-warm chondrules into aggregates at an effective chondrite accretion temperature of âŒ900-950 °C with nearly in situ impingement deformation between adjacent chondrules in strongly clustered lithologies (NWA 5781, Tieschitz, NWA 5421, NWA 5205 Lithology A), as evidenced by intragranular lattice distortions in olivine consistent with high-temperature slip systems, and by evidence that some olivine-rich objects in Tieschitz accreted while partly molten; 3) syn- or post-accretion bleaching of chondrule mesostases, which transferred feldspathic chondrule mesostasis to an interchondrule glass deposit found in strongly clustered lithologies, as evidenced by chemical data and textures; and 4) post-bleaching weak or strong shocks that resulted in destruction of interchondrule glass and some combination of brecciation, foliation of metal and sulfide, and melting and shock-overprinting effects, as evidenced by poor cluster textures and presence of clastic texture, alignment of metal and sulfide grains caused by shock compression, presence of impact-generated glass, and changes in olivine slip systems. The data support the model of Metzler (2012), who suggested that chondrules in ordinary chondrites accreted while still warm to form cluster chondrite textures as a âprimary accretionary rockâ (our Stage 2), and that subsequent brecciation destroyed this texture to create chondrites with weak cluster texture (our Stage 4)
An Approximation for the rp-Process
Hot (explosive) hydrogen burning or the Rapid Proton Capture Process
(rp-process) occurs in a number of astrophysical environments. Novae and X-ray
bursts are the most prominent ones, but accretion disks around black holes and
other sites are candidates as well. The expensive and often multidimensional
hydro calculations for such events require an accurate prediction of the
thermonuclear energy generation, while avoiding full nucleosynthesis network
calculations. In the present investigation we present an approximation scheme
applicable in a temperature range which covers the whole range of all presently
known astrophysical sites. It is based on the concept of slowly varying
hydrogen and helium abundances and assumes a kind of local steady flow by
requiring that all reactions entering and leaving a nucleus add up to a zero
flux. This scheme can adapt itself automatically and covers situations at low
temperatures, characterized by a steady flow of reactions, as well as high
temperature regimes where a -equilibrium is established.
In addition to a gain of a factor of 15 in computational speed over a full
network calculation, and an energy generation accurate to more than 15 %, this
scheme also allows to predict correctly individual isotopic abundances. Thus,
it delivers all features of a full network at a highly reduced cost and can
easily be implemented in hydro calculations.Comment: 18 pages, LaTeX using astrobib and aas2pp4, includes PostScript
figures; Astrophysical Journal, in press. PostScript source also available at
http://quasar.physik.unibas.ch/preps.htm
Three-dimensional simulations of gravitationally confined detonations compared to observations of SN 1991T
The gravitationally confined detonation (GCD) model has been proposed as a
possible explosion mechanism for Type Ia supernovae in the single-degenerate
evolution channel. Driven by buoyancy, a deflagration flame rises in a narrow
cone towards the surface. For the most part, the flow of the expanding ashes
remains radial, but upon reaching the outer, low-pressure layers of the white
dwarf, an additional lateral component develops. This makes the deflagration
ashes converge again at the opposite side, where the compression heats fuel and
a detonation may be launched. To test the GCD explosion model, we perform a 3D
simulation for a model with an ignition spot offset near the upper limit of
what is still justifiable, 200 km. This simulation meets our deliberately
optimistic detonation criteria and we initiate a detonation. The detonation
burns through the white dwarf and leads to its complete disruption. We
determine nucleosynthetic yields by post-processing 10^6 tracer particles with
a 384 nuclide reaction network and we present multi-band light curves and
time-dependent optical spectra. We find that our synthetic observables show a
prominent viewing-angle sensitivity in UV and blue bands, which is in tension
with observed SNe Ia. The strong dependence on viewing-angle is caused by the
asymmetric distribution of the deflagration ashes in the outer ejecta layers.
Finally, we perform a comparison of our model to SN 1991T. The overall
flux-level of the model is slightly too low and the model predicts pre-maximum
light spectral features due to Ca, S, and Si that are too strong. Furthermore,
the model chemical abundance stratification qualitatively disagrees with recent
abundance tomography results in two key areas: our model lacks low velocity
stable Fe and instead has copious amounts of high-velocity 56Ni and stable Fe.
We therefore do not find good agreement of the model with SN 1991T.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic
Remarks on evolution of space-times in 3+1 and 4+1 dimensions
A large class of vacuum space-times is constructed in dimension 4+1 from
hyperboloidal initial data sets which are not small perturbations of empty
space data. These space-times are future geodesically complete, smooth up to
their future null infinity, and extend as vacuum space-times through their
Cauchy horizon. Dimensional reduction gives non-vacuum space-times with the
same properties in 3+1 dimensions.Comment: 10pp, exposition improved; final versio
The New Zealand Kauri (Agathis Australis) Research Project: A Radiocarbon Dating Intercomparison of Younger Dryas Wood and Implications for IntCal13
We describe here the New Zealand kauri (Agathis australis) Younger Dryas (YD) research project, which aims to undertake Î14C analysis of ~140 decadal floating wood samples spanning the time interval ~13.1â11.7 kyr cal BP. We report 14C intercomparison measurements being undertaken by the carbon dating laboratories at University of Waikato (Wk), University of California at Irvine (UCI), and University of Oxford (OxA). The Wk, UCI, and OxA laboratories show very good agreement with an interlaboratory comparison of 12 successive decadal kauri samples (average offsets from consensus values of â7 to +4 14C yr). A University of Waikato/University of Heidelberg (HD) intercomparison involving measurement of the YD-age Swiss larch tree Ollon505, shows a HD/Wk offset of ~10â20 14C yr (HD younger), and strong evidence that the positioning of the Ollon505 series is incorrect, with a recommendation that the 14C analyses be removed from the IntCal calibration database
Neutral Nickel(II)-Based Catalysts for Ethylene Polymerization
Neutral salicylaldiminato Ni(II) complexes have been synthesized, and their structure has been confirmed by an X-ray analysis of complex 4e. These compounds are active catalysts for the polymerization of ethylene under mild conditions in the presence of a phosphine scavenger such as Ni(COD)_2 or B(C_6F_5)_3
The nuclear reaction waiting points, Mg22, Si26, S30, and Ar34, and bolometrically double peaked type I X-ray bursts
Type I X-ray bursts with a double peak in the bolometric luminosity have been
observed from several sources. The separation between the two peaks are on the
order of a few seconds. We propose a nuclear waiting point impedance in the
thermonuclear reaction flow to explain these observations. Nuclear structure
information suggests the potential waiting points: Mg22, Si26, S30 and Ar34,
which arise in conditions, where a further reaction flow has to await a
beta-decay, because the (alpha,p)-reaction is too weak to overcome the target
Coulomb-barrier and the (p,gamma)-reaction is quenched by photo-disintegration
at the burst temperature. The conclusion is that the effects of the
experimentally unknown S30(alpha,p)Cl33 and Ar34(alpha,p)K37 might be directly
visible in the observation of X-ray burst light curves.Comment: 5 pages, 3 figures, submitted to Astrophys. J. Let
Hegel, Adorno and the origins of immanent criticism
âImmanent criticism' has been discussed by philosophers of quite different persuasions, working in separate areas and in different traditions of philosophy. Almost all of them agree on roughly the same story about its origins: It is that Hegel invented immanent criticism, that Marx later developed it, and that the various members of the Frankfurt School, particularly Adorno, refined it in various ways, and that they are all paradigmatic practitioners of immanent criticism. I call this the Continuity Thesis. There are four different claims that interest me. (i) Hegel is the originator of immanent criticism. (ii) Hegel's dialectical method is that of immanent criticism. (iii) Adorno practises immanent criticism and endorses the term as a description of his practice. (iv) Adorno's dialectical method is fundamentally Hegelian. In this article, I offer an account of immanent criticism, on the basis of which, I evaluate these four claims and argue that the Continuity Thesis should be rejected
The Hydrogen Atom in Combined Electric and Magnetic Fields with Arbitrary Mutual Orientations
For the hydrogen atom in combined magnetic and electric fields we investigate
the dependence of the quantum spectra, classical dynamics, and statistical
distributions of energy levels on the mutual orientation of the two external
fields. Resonance energies and oscillator strengths are obtained by exact
diagonalization of the Hamiltonian in a complete basis set, even far above the
ionization threshold. At high excitation energies around the Stark saddle point
the eigenenergies exhibit strong level repulsions when the angle between the
fields is varied. The large avoided crossings occur between states with the
same approximately conserved principal quantum number, n, and this
intramanifold mixing of states cannot be explained, not even qualitatively, by
conventional perturbation theory. However, it is well reproduced by an extended
perturbation theory which takes into account all couplings between the angular
momentum and Runge-Lenz vector. The large avoided crossings are interpreted as
a quantum manifestation of classical intramanifold chaos. This interpretation
is supported by both classical Poincar\'e surfaces of section, which reveal a
mixed regular-chaotic intramanifold dynamics, and the statistical analysis of
nearest-neighbor-spacingComment: two-column version, 10 pages, REVTeX, 10 figures, uuencoded,
submitted to Rhys. Rev.
- âŠ