42,750 research outputs found
Crystalline silicates as a probe of disk formation history
We present a new perspective on the crystallinity of dust in protoplanetary
disks. The dominant crystallization by thermal annealing happens in the very
early phases of disk formation and evolution. Both the disk properties and the
level of crystallinity are thereby directly linked to the properties of the
molecular cloud core from which the star+disk system was formed. We show that,
under the assumption of single star formation, rapidly rotating clouds produce
disks which, after the main infall phase (i.e. in the optically revealed class
II phase), are rather massive and have a high accretion rate but low
crystallinity. Slowly rotating clouds, on the other hand, produce less massive
disks with lower accretion rate, but high levels of crystallinity. Cloud
fragmentation and the formation of multiple stars complicates the problem and
necessitates further study. The underlying physics of the model is
insufficiently understood to provide the precise relationship between
crystallinity, disk mass and accretion rate. But the fact that with `standard'
input physics the model produces disks which, in comparison to observations,
appear to have either too high levels of crystallinity or too high disk masses,
demonstrates that the comparison of these models to observations can place
strong contraints on the disk physics. The question to ask is not why some
sources are so crystalline, but why some other sources have such a low level of
crystallinity.Comment: Accepted for publication in ApJ
The soft X-ray afterglow of gamma ray bursts, a stringent test for the fireball model
I consider the recent discovery of a soft X-ray source inside the error box
of the gamma ray burst GB 960720 by the SAX, ASCA and ROSAT satellites, in
terms of the fireball model. I show that the ejecta shell, which, after causing
the burst is cold and dense, but still relativistic, keeps plowing through the
interstellar medium, heating up the just-shocked matter which then emits
X-rays. I compute the radiation emitted by this matter. I show that, up to
about two months after the burst, in the cosmological scenario a soft X-ray
(0.1-10 keV) flux of at least 10^(-13) erg / s cm^2, well within current
observational capabilities, is generated, explaining the observations of the
three satellites. Instead, in the Galactic Halo scenario a flux 3 orders of
magnitude lower is expected. Detection of this non-thermal, declining flux in a
statistically significant number of objects would simultaneously establish the
fireball model and the cosmological nature of gamma ray bursts.Comment: Replaces previous version: now it does include figure. Accepted for
publication in the Astrophysical Journal Letter
A 3-component laser-Doppler velocimeter data acquisition and reduction system
A laser doppler velocimeter capable of measuring all three components of velocity simultaneously in low-speed flows is described. All the mean velocities, Reynolds stresses, and higher-order products can be evaluated. The approach followed is to split one of the two colors used in a 2-D system, thus creating a third set of beams which is then focused in the flow from an off-axis direction. The third velocity component is computed from the known geometry of the system. The laser optical hardware and the data acquisition electronics are described in detail. In addition, full operating procedures and listings of the software (written in BASIC and ASSEMBLY languages) are also included. Some typical measurements obtained with this system in a vortex/mixing layer interaction are presented and compared directly to those obtained with a cross-wire system
Two-player quantum pseudo-telepathy based on recent all-versus-nothing violations of local realism
We introduce two two-player quantum pseudo-telepathy games based on two
recently proposed all-versus-nothing (AVN) proofs of Bell's theorem [A.
Cabello, Phys. Rev. Lett. 95, 210401 (2005); Phys. Rev. A 72, 050101(R)
(2005)]. These games prove that Broadbent and Methot's claim that these AVN
proofs do not rule out local-hidden-variable theories in which it is possible
to exchange unlimited information inside the same light-cone (quant-ph/0511047)
is incorrect.Comment: REVTeX4, 5 page
Evaluating the articulation of programme theory in practice as observed in Quality Improvement initiatives
Background: The Action-Effect Method(AEM) was co-developed by NIHR CLAHRC Northwest London (CLAHRC NWL) researchers and QI practitioners, building on Driver Diagrams(DD). This study aimed to determine AEM effectiveness in terms of technical aspects (how diagrams produced in practice compared with theoretical ideals) and social aspects (how engagement with the method related to social benefits). Methods Diagrams were scored on criteria developed on theoretical ideals of programme theory. 65 programme theory diagrams were reviewed (21 published Driver Diagrams (External DDs), 22 CLAHRC NWL Driver Diagrams (Internal DDs), and 21 CLAHRC NWL Action-Effect Diagrams(AEDs)). Social functions were studied through ethnographic observation of frontline QI teams in AEM sessions facilitated by QI experts. Qualitative analysis used inductive and deductive coding. Results ANOVA indicated the AEM significantly improved the quality of programme theory diagrams over Internal and External DDs on an average of 5 criteria from an 8-point assessment. Articulated aims were more likely to be patient-focused and high-level in AEDs than DDs. The cause/effect relationships from intervention to overall aim also tended to be clearer and were more likely than DDs to contain appropriate measure concepts. Using the AEM also served several social functions such as facilitating dialogue among multidisciplinary teams, and encouraging teams to act scientifically and pragmatically about planning and measuring QI interventions. Implications: The Action-Effect Method developed by CLAHRC NWL resulted in improvements over Driver Diagrams in articulating programme theory, which has wide-ranging benefits to quality improvement, including encouraging broad multi-disciplinary buy-in to clear aims and pre-planning a rigorous evaluation strategy
Detecting the Dusty Debris of Terrestrial Planet Formation
We use a multiannulus accretion code to investigate debris disks in the
terrestrial zone, at 0.7-1.3 AU around a 1 solar mass star. Terrestrial planet
formation produces a bright dusty ring of debris with a lifetime of at least 1
Myr. The early phases of terrestrial planet formation are observable with
current facilities; the late stages require more advanced instruments with
adaptive optics.Comment: 11 pages of text, 3 figures, accepted for ApJ Letters, additional
info at http://cfa-www.harvard.edu/~kenyon/pf/terra/td
Argyrin B a non-competitive inhibitor of the human immunoproteasome exhibiting preference for β1i
Inhibitors of the proteasome have found broad therapeutic applications however, they show severe toxicity due to the abundance of proteasomes in healthy cells. In contrast, inhibitors of the immunoproteasome, which is upregulated during disease states, are less toxic and have increased therapeutic potential including against autoimmune disorders. In this project, we report argyrin B, a natural product cyclic peptide to be a reversible, non-competitive inhibitor of the immunoproteasome. Argyrin B showed selective inhibition of the β5i and β1i sites of the immunoproteasome over the β5c and β1c sites of the constitutive proteasome with nearly 20-fold selective inhibition of β1i over the homologous β1c. Molecular modelling attributes the β1i over β1c selectivity to the small hydrophobic S1 pocket of β1i and β5i over β5c to site-specific amino acid variations that enable additional bonding interactions and stabilization of the binding conformation. These findings facilitate the design of immunoproteasome selective and reversible inhibitors that may have a greater therapeutic potential and lower toxicity
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