193 research outputs found
An Ising-Like model for protein mechanical unfolding
The mechanical unfolding of proteins is investigated by extending the
Wako-Saito-Munoz-Eaton model, a simplified protein model with binary degrees of
freedom, which has proved successful in describing the kinetics of protein
folding. Such a model is generalized by including the effect of an external
force, and its thermodynamics turns out to be exactly solvable. We consider two
molecules, the 27th immunoglobulin domain of titin and protein PIN1. In the
case of titin we determine equilibrium force-extension curves and study
nonequilibrium phenomena in the frameworks of dynamic loading and force clamp
protocols, verifying theoretical laws and finding the position of the kinetic
barrier which hinders the unfolding of the molecule. The PIN1 molecule is used
to check the possibility of computing the free energy landscape as a function
of the molecule length by means of an extended form of the Jarzynski equality.Comment: 4 pages + appendi
Detection of Lead in the Carbon-Rich, Very Metal-Poor Star LP625-44: A Strong Constraint on s-Process Nucleosynthesis at Low Metallicity
We report the detection of the Pb I 4057.8A line in the very metal-poor
([Fe/H]=-2.7), carbon-rich star, LP625-44. We determine the abundance of Pb
([Pb/Fe] = 2.65) and 15 other neutron-capture elements. The abundance pattern
between Ba and Pb agrees well with a scaled solar system s-process component,
while the lighter elements (Sr-Zr) are less abundant than Ba. The enhancement
of s-process elements is interpreted as a result of mass transfer in a binary
system from a previous AGB companion, an interpretation strongly supported by
radial velocity variations of this system.
The detection of Pb makes it possible, for the first time, to compare model
predictions of s-process nucleosynthesis in AGB stars with observations of
elements between Sr and Pb. The Pb abundance is significantly lower than the
prediction of recent models (e.g., Gallino et al. 1998), which succeeded in
explaining the metallicity dependence of the abundance ratios of light
s-elements (Sr-Zr) to heavy ones (Ba-Dy) found in previously observed
s-process-enhanced stars. This suggests that one should either (a) reconsider
the underlying assumptions concerning the 13C-rich s-processing site
(13C-pocket) in the present models, or (b) investigate alternative sites of
s-process nucleosynthesis in very metal-poor AGB stars.Comment: 10 pages, 3 figures, Astrophysical Journal Letters, in pres
Observation of thundercloud-related gamma rays and neutrons in Tibet
During the 2010 rainy season in Yangbajing (4300 m above sea level) in Tibet, China, a long-duration count enhancement associated with thunderclouds was detected by a solar-neutron telescope and neutron monitors installed at the Yangbajing Comic Ray Observatory. The event, lasting for ∼40  min, was observed on July 22, 2010. The solar-neutron telescope detected significant γ-ray signals with energies >40  MeV in the event. Such a prolonged high-energy event has never been observed in association with thunderclouds, clearly suggesting that electron acceleration lasts for 40 min in thunderclouds. In addition, Monte Carlo simulations showed that >10  MeV γ rays largely contribute to the neutron monitor signals, while >1  keV neutrons produced via a photonuclear reaction contribute relatively less to the signals. This result suggests that enhancements of neutron monitors during thunderstorms are not necessarily clear evidence for neutron production, as previously thought
Malignant Mesothelioma in the Thoracic Cavity of a Crj:CD(SD) Rat Characterized by Round Hyalinous Stroma
Spontaneous malignant mesothelioma was found in a 104-week-old male Crj:CD(SD)
rat. The tumor was scattered on the surface of the lung, heart, mediastinal
pleura and thoracic wall and metastasized to the alveolar septa.
Histopathologically, small flattened or cuboidal tumor cells proliferated with
stroma, formed almost normal papillary structures and reacted positively to
colloidal iron stain and immunohistochemical staining for mesothelin. Round
hyalinous stromata were pronounced, which is a characteristic feature, and the
possible reason for this is as follows; at first, a small amount of collagen
fibers was formed in the center of the clusters of several tumor cells, and then
the cell clusters expanded like balloons with an increase in the collagen
fibers
Game theory of mind
This paper introduces a model of ‘theory of mind’, namely, how we represent the intentions and goals of others to optimise our mutual interactions. We draw on ideas from optimum control and game theory to provide a ‘game theory of mind’. First, we consider the representations of goals in terms of value functions that are prescribed by utility or rewards. Critically, the joint value functions and ensuing behaviour are optimised recursively, under the assumption that I represent your value function, your representation of mine, your representation of my representation of yours, and so on ad infinitum. However, if we assume that the degree of recursion is bounded, then players need to estimate the opponent's degree of recursion (i.e., sophistication) to respond optimally. This induces a problem of inferring the opponent's sophistication, given behavioural exchanges. We show it is possible to deduce whether players make inferences about each other and quantify their sophistication on the basis of choices in sequential games. This rests on comparing generative models of choices with, and without, inference. Model comparison is demonstrated using simulated and real data from a ‘stag-hunt’. Finally, we note that exactly the same sophisticated behaviour can be achieved by optimising the utility function itself (through prosocial utility), producing unsophisticated but apparently altruistic agents. This may be relevant ethologically in hierarchal game theory and coevolution
The s Process: Nuclear Physics, Stellar Models, Observations
Nucleosynthesis in the s process takes place in the He burning layers of low
mass AGB stars and during the He and C burning phases of massive stars. The s
process contributes about half of the element abundances between Cu and Bi in
solar system material. Depending on stellar mass and metallicity the resulting
s-abundance patterns exhibit characteristic features, which provide
comprehensive information for our understanding of the stellar life cycle and
for the chemical evolution of galaxies. The rapidly growing body of detailed
abundance observations, in particular for AGB and post-AGB stars, for objects
in binary systems, and for the very faint metal-poor population represents
exciting challenges and constraints for stellar model calculations. Based on
updated and improved nuclear physics data for the s-process reaction network,
current models are aiming at ab initio solution for the stellar physics related
to convection and mixing processes. Progress in the intimately related areas of
observations, nuclear and atomic physics, and stellar modeling is reviewed and
the corresponding interplay is illustrated by the general abundance patterns of
the elements beyond iron and by the effect of sensitive branching points along
the s-process path. The strong variations of the s-process efficiency with
metallicity bear also interesting consequences for Galactic chemical evolution.Comment: 53 pages, 20 figures, 3 tables; Reviews of Modern Physics, accepte
- …