11,369 research outputs found
Uranus evolution models with simple thermal boundary layers
The strikingly low luminosity of Uranus (Teff ~ Teq) constitutes a
long-standing challenge to our understanding of Ice Giant planets. Here we
present the first Uranus structure and evolution models that are constructed to
agree with both the observed low luminosity and the gravity field data. Our
models make use of modern ab initio equations of state at high pressures for
the icy components water, methane, and ammonia. Proceeding step by step, we
confirm that adiabatic models yield cooling times that are too long, even when
uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue
that the transition between the ice/rock-rich interior and the H/He-rich outer
envelope should be stably stratified. Therefore, we introduce a simple thermal
boundary and adjust it to reproduce the low luminosity. Due to this thermal
boundary, the deep interior of the Uranus models are up to 2--3 warmer than
adiabatic models, necessitating the presence of rocks in the deep interior with
a possible I:R of solar. Finally, we allow for an equilibrium
evolution (Teff ~ Teq) that begun prior to the present day, which would
therefore no longer require the current era to be a "special time" in Uranus'
evolution. In this scenario, the thermal boundary leads to more rapid cooling
of the outer envelope. When Teff ~ Teq is reached, a shallow, subadiabatic zone
in the atmosphere begins to develop. Its depth is adjusted to meet the
luminosity constraint. This work provides a simple foundation for future Ice
Giant structure and evolution models, that can be improved by properly treating
the heat and particle fluxes in the diffusive zones.Comment: 13 pages, Accepted to Icaru
Toxicité aiguë comparée du phénol et d'une série de chlorophénols sur des organismes aquatiques et terrestres
Les chlorophĂ©nols sont des composĂ©s chimiques couramment utilisĂ©s dans de nombreuses applications industrielles et donc retrouvĂ©s dans l'environnement.L'objectif de ce travail est d'abord de comparer la toxicitĂ© aiguĂ« du phĂ©nol et de cinq dĂ©rivĂ©s chlorĂ©s vis Ă vis de trois organismes : un protozoaire d'eau douce Colpidium campylum, un vĂ©gĂ©tal terrestre Vicia lens et un mammifĂšre, la souris. Afin d'avoir une comparaison plus large, nos rĂ©sultats expĂ©rimentaux ont Ă©tĂ© confrontĂ©s avec des donnĂ©es relevĂ©es dans la littĂ©rature sur neuf autres espĂšces de l'environnement.Ă l'exception des mammifĂšres pour lesquels le 2,4 dichlorophĂ©nol et le 2,4,6 trichlorophĂ©nol sont les moins dangereux, il apparaĂźt que la toxicitĂ© est Ă©troitement liĂ©e avec le degrĂ© de substitution en chlore du noyau phĂ©nolique. Plus le nombre d'atomes de chlore est Ă©levĂ©, plus la toxicitĂ© est importante.Cette Ă©tude montre d'autre part que dix organismes, parmi les douze Ă©tudiĂ©s, rĂ©pondent de façon semblable vis Ă vis des chlorophĂ©nols. Seuls le rat et la souris prĂ©sentent une sensibilitĂ© diffĂ©rente. Cependant, le pentachlorophĂ©nol s'avĂšre ĂȘtre le composĂ© le plus dangereux dans tous les cas.Chlorophenols are chemical compounds frequently used for many industrial applications and are subsequently often found in the environment. The aim of this study was firstly to compare the acute toxicities of phenol and five chlorinated compounds towards three organisms : a freshwater protozoan, Colpidium campylum ; a terrestrial plant species, Vicia lens ; and a mammalian mouse. Our experimental results have been compared with literature data for nine other environmental species.With the exception of mammals, for which 2,4-dichlorophenol and 2,4,6-trichlorophenol are less dangerous, it appears that toxicity is strictly related to the chlorine substitution level of the phenolic nucleus: the higher the number of chlorine atoms, the more toxic effect is the compound. On the other hand, this study shows that ten species, from the twelve observed, have a similar toxicological response to chlorophenols; only the rat and the mouse have a different sensitivity. However, pentachlorophenol is always the most dangerous compound
Polarization States in B -> rho K* and New Physics
The standard-model explanations of the anomalously-large transverse
polarization fraction fT in B -> phi K* can be tested by measuring the
polarizations of the two decays B+ -> rho+ K*0 and B+ -> rho0 K*+. For the
scenario in which the transverse polarizations of both B -> rho K* decays are
predicted to be large, we derive a simple relation between the fT's of these
decays. If this relation is not confirmed experimentally, this would yield an
unambiguous signal for new physics. The new-physics operators which can account
for the discrepancy in B -> pi K decays will also contribute to the
polarization states of B -> rho K*. We compute these contributions and show
that there are only two operators which can simultaneously account for the
present B -> pi K and B -> rho K* data. If the new physics obeys an approximate
U-spin symmetry, the B -> phi K* measurements can also be explained.Comment: 20 pages, latex, no figures. Minor changes to references and Table 1.
Minor modification of terms; more complete description of triple-product
asymmetry. Analysis and conclusions unchange
Parallel Temperatures in Supersonic Beams: Ultra Cooling of Light Atoms seeded in a Heavier Carrier Gas
We have found recently that, in a supersonic expansion of a mixture of two
monoatomic gases, the parallel temperatures of the two gases can be very
different. This effect is large if the seeded gas is highly diluted and if its
atomic mass is considerably smaller than the one of the carrier gas. In the
present paper, we present a complete derivation of our theoretical analysis of
this effect. Our calculation is a natural extension of the existing theory of
supersonic cooling to the case of a gas mixture, in the high dilution limit.
Finally, we describe a set of temperature measurements made on a beam of
lithium seeded in argon. Our experimental results are in very good agreement
with the results of our calculation.Comment: 24 novembre 200
Multiple case-study analysis of quality management practices within UK Six Sigma and non-Six Sigma manufacturing small- and medium-sized enterprises
This paper examines multiple case-study analysis of quality management practices within UK Six Sigma and non-Six Sigma manufacturing small- and medium-sized enterprises
Structural and chemical requirements for histidine phosphorylation by the chemotaxis kinase CheA
The CheA histidine kinase initiates the signal transduction pathway of bacterial chemotaxis by autophosphorylating a conserved histidine on its phosphotransferase domain (P1). Site-directed mutations of neighboring conserved P1 residues (Glu-67, Lys-48, and His-64) show that a hydrogen-bonding network controls the reactivity of the phospho-accepting His (His-45) in Thermotoga maritima CheA. In particular, the conservative mutation E67Q dramatically reduces phospho-transfer to P1 without significantly affecting the affinity of P1 for the CheA ATP-binding domain. High resolution crystallographic studies revealed that although all mutants disrupt the hydrogen-bonding network to varying degrees, none affect the conformation of His-45. N-15-NMR chemical shift studies instead showed that Glu-67 functions to stabilize the unfavored (NH)-H-delta 1 tautomer of His-45, thereby rendering the N-epsilon 2 imidazole unprotonated and well positioned for accepting the ATP phosphoryl group
Symmetry of Traveling Wave Solutions to the Allen-Cahn Equation in \Er^2
In this paper, we prove even symmetry of monotone traveling wave solutions to
the balanced Allen-Cahn equation in the entire plane. Related results for the
unbalanced Allen-Cahn equation are also discussed
- âŠ