2,416 research outputs found
Canonical bifurcation in higher derivative, higher spin, theories
We present a non-perturbative canonical analysis of the D=3
quadratic-curvature, yet ghost-free, model to exemplify a novel, "constraint
bifurcation", effect. Consequences include a jump in excitation count: a
linearized level gauge variable is promoted to a dynamical one in the full
theory. We illustrate these results with their concrete perturbative
counterparts. They are of course mutually consistent, as are perturbative
findings in related models. A geometrical interpretation in terms of
propagating torsion reveals the model's relation to an (improved) version of
Einstein-Weyl gravity at the linearized level. Finally, we list some necessary
conditions for triggering the bifurcation phenomenon in general interacting
gauge systems.Comment: 10 pages, v2: typos corrected, v3: new title to reflect greatly
expanded version, to appear in special issue of J Phys A (eds, M Vasiliev & M
Gaberdiel
Molecular structure input on the web
A molecule editor, that is program for input and editing of molecules, is an indispensable part of every cheminformatics or molecular processing system. This review focuses on a special type of molecule editors, namely those that are used for molecule structure input on the web. Scientific computing is now moving more and more in the direction of web services and cloud computing, with servers scattered all around the Internet. Thus a web browser has become the universal scientific user interface, and a tool to edit molecules directly within the web browser is essential
Structure and stability of a high-coverage (1x1) oxygen phase on Ru(0001)
The formation of chemisorbed O-phases on Ru(0001) by exposure to O_2 at low
pressures is apparently limited to coverages Theta <= 0.5. Using low-energy
electron diffraction and density functional theory we show that this
restriction is caused by kinetic hindering and that a dense O overlayer (Theta
= 1) can be formed with a (1x1) periodicity. The structural and energetic
properties of this new adsorbate phase are analyzed and discussed in view of
attempts to bridge the so-called "pressure gap" in heterogeneous catalysis. It
is argued that the identified system actuates the unusually high rate of
oxidizing reactions at Ru surfaces under high oxygen pressure conditions.Comment: RevTeX, 6 pages, 3 figures, to appear in Phys. Rev. Let
A two-parameter criterion for classifying the explodability of massive stars by the neutrino-driven mechanism
Thus far, judging the fate of a massive star (either a neutron star (NS) or a
black hole) solely by its structure prior to core collapse has been ambiguous.
Our work and previous attempts find a non-monotonic variation of successful and
failed supernovae with zero-age main-sequence mass, for which no single
structural parameter can serve as a good predictive measure. However, we
identify two parameters computed from the pre-collapse structure of the
progenitor, which in combination allow for a clear separation of exploding and
non-exploding cases with only few exceptions (~1-2.5%) in our set of 621
investigated stellar models. One parameter is M4, defining the normalized
enclosed mass for a dimensionless entropy per nucleon of s=4, and the other is
mu4 = d(m/M_sun)/d(r/1000 km) at s=4, being the normalized mass-derivative at
this location. The two parameters mu4 and M4*mu4 can be directly linked to the
mass-infall rate, Mdot, of the collapsing star and the electron-type neutrino
luminosity of the accreting proto-NS, L_nue ~ M_ns*Mdot, which play a crucial
role in the "critical luminosity" concept for the theoretical description of
neutrino-driven explosions as runaway phenomenon of the stalled accretion
shock. All models were evolved employing the approach of Ugliano et al. for
simulating neutrino-driven explosions in spherical symmetry. The neutrino
emission of the accretion layer is approximated by a gray transport solver,
while the uncertain neutrino emission of the 1.1 M_sun proto-NS core is
parametrized by an analytic model. The free parameters connected to the
core-boundary prescription are calibrated to reproduce the observables of
Supernova 1987A for five different progenitor models.Comment: 23 pages, 12 figures; accepted by ApJ; revised version considerably
enlarged (Fig. 7 and Sect.3.6 added
Emission line models for the lowest-mass core collapse supernovae. I: Case study of a 9 one-dimensional neutrino-driven explosion
A large fraction of core-collapse supernovae (CCSNe), 30-50%, are expected to
originate from the low-mass end of progenitors with . However, degeneracy effects make stellar evolution modelling of
such stars challenging, and few predictions for their supernova light curves
and spectra have been presented. Here we calculate synthetic nebular spectra of
a 9 Fe CCSN model exploded with the neutrino mechanism. The model
predicts emission lines with FWHM1000 km/s, including signatures from
each deep layer in the metal core. We compare this model to observations of the
three subluminous IIP SNe with published nebular spectra; SN 1997D, SN 2005cs,
and SN 2008bk. The prediction of both line profiles and luminosities are in
good agreement with SN 1997D and SN 2008bk. The close fit of a model with no
tuning parameters provides strong evidence for an association of these objects
with low-mass Fe CCSNe. For SN 2005cs, the interpretation is less clear, as the
observational coverage ended before key diagnostic lines from the core had
emerged. We perform a parameterised study of the amount of explosively made
stable nickel, and find that none of these three SNe show the high
Ni/Ni ratio predicted by current models of electron capture SNe
(ECSNe) and ECSN-like explosions. Combined with clear detection of lines from O
and He shell material, these SNe rather originate from Fe core progenitors. We
argue that the outcome of self-consistent explosion simulations of low-mass
stars, which gives fits to many key observables, strongly suggests that the
class of subluminous Type IIP SNe is the observational counterpart of the
lowest mass CCSNe.Comment: Resubmitted to MNRAS after referee comment
The Adsorption of Atomic Nitrogen on Ru(0001): Geometry and Energetics
The local adsorption geometries of the (2x2)-N and the (sqrt(3)x
sqrt(3))R30^o -N phases on the Ru(0001) surface are determined by analyzing
low-energy electron diffraction (LEED) intensity data. For both phases,
nitrogen occupies the threefold hcp site. The nitrogen sinks deeply into the
top Ru layer resulting in a N-Ru interlayer distance of 1.05 AA and 1.10 AA in
the (2x2) and the (sqrt(3)x sqrt(3))R30^o unit cell, respectively. This result
is attributed to a strong N binding to the Ru surface (Ru--N bond length = 1.93
AA) in both phases as also evidenced by ab-initio calculations which revealed
binding energies of 5.82 eV and 5.59 eV, respectively.Comment: 17 pages, 5 figures. Submitted to Chem. Phys. Lett. (October 10,
1996
Estimation of synthetic accessibility score of drug-like molecules based on molecular complexity and fragment contributions
<p>Abstract</p> <p>Background</p> <p>A method to estimate ease of synthesis (synthetic accessibility) of drug-like molecules is needed in many areas of the drug discovery process. The development and validation of such a method that is able to characterize molecule synthetic accessibility as a score between 1 (easy to make) and 10 (very difficult to make) is described in this article.</p> <p>Results</p> <p>The method for estimation of the synthetic accessibility score (SAscore) described here is based on a combination of fragment contributions and a complexity penalty. Fragment contributions have been calculated based on the analysis of one million representative molecules from PubChem and therefore one can say that they capture historical synthetic knowledge stored in this database. The molecular complexity score takes into account the presence of non-standard structural features, such as large rings, non-standard ring fusions, stereocomplexity and molecule size. The method has been validated by comparing calculated SAscores with ease of synthesis as estimated by experienced medicinal chemists for a set of 40 molecules. The agreement between calculated and manually estimated synthetic accessibility is very good with <it>r</it><sup>2 </sup>= 0.89.</p> <p>Conclusion</p> <p>A novel method to estimate synthetic accessibility of molecules has been developed. This method uses historical synthetic knowledge obtained by analyzing information from millions of already synthesized chemicals and considers also molecule complexity. The method is sufficiently fast and provides results consistent with estimation of ease of synthesis by experienced medicinal chemists. The calculated SAscore may be used to support various drug discovery processes where a large number of molecules needs to be ranked based on their synthetic accessibility, for example when purchasing samples for screening, selecting hits from high-throughput screening for follow-up, or ranking molecules generated by various <it>de novo </it>design approaches.</p
Adsorbate-induced electronic modification of alkali-metal overlayers
For submonolayer coverages of Cs on Ru(0001) surfaces, the Cs-Ru vibration is observed at 7.8 meV (63 cm−1) by means of high-resolution electron-energy-loss spectroscopy. At Cs coverages near the complete monolayer the adlayer becomes metallic as indicated by screening of the Cs-Ru vibration and the occurrence of an electronic excitation (plasmon) at 580 meV. Coadsorption of CO as well as of oxygen leads to the reappearance of the Cs-Ru vibration and the disappearance of the electronic excitation which is interpreted as a demetallization through chemical interaction with the coadsorbed species
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