192 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
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
Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample
The chemical compositions of seven Carbon-Enhanced Metal-Poor (CEMP) turn-off
stars are determined from high-resolution spectroscopy. Five of them are
selected from the SDSS/SEGUE sample of metal-poor stars. The effective
temperatures of these objects are all higher than 6000 K, while their
metallicities, parametrized by [Fe/H], are all below -2. Six of our program
objects exhibit high abundance ratios of barium ([Ba/H]> +1), suggesting large
contributions of the products of former AGB companions via mass transfer across
binary systems. Combining our results with previous studies provides a total of
20 CEMP main-sequence turn-off stars for which the abundances of carbon and at
least some neutron-capture elements are determined. Inspection of the [C/H]
ratios for this sample of CEMP turn-off stars show that they are generally
higher than those of CEMP giants; their dispersion in this ratio is also
smaller. We take these results to indicate that the carbon-enhanced material
provided from the companion AGB star is preserved at the surface of turn-off
stars with no significant dilution. In contrast, a large dispersion in the
observed [Ba/H] is found for the sample of CEMP turn-off stars, suggesting that
the efficiency of the s-process in very metal-poor AGB stars may differ greatly
from star to star. Four of the six stars from the SDSS/SEGUE sample exhibit
kinematics that are associated with membership in the outer-halo population, a
remarkably high fraction.Comment: 45 pages, 10 figures, 10 tables, Astrophysical Journal, in pres
Evidence for a companion to BM Gem, a silicate carbon star
Balmer and Paschen continuum emission as well as Balmer series lines of P
Cygni-type profile from H_gamma through H_23 are revealed in the violet spectra
of BM Gem, a carbon star associated with an oxygen-rich circumstellar shell
(`silicate carbon star') observed with the high dispersion spectrograph (HDS)
on the Subaru telescope. The blue-shifted absorption in the Balmer lines
indicates the presence of an outflow, the line of sight velocity of which is at
least 400 km s^-1, which is the highest outflow velocity observed to date in a
carbon star. We argue that the observed unusual features in BM Gem are strong
evidence for the presence of a companion, which should form an accretion disk
that gives rise to both an ionized gas region and a high velocity, variable
outflow. The estimated luminosity of ~0.2 (0.03-0.6) L_sun for the ionized gas
can be maintained by a mass accretion rate to a dwarf companion of ~10^-8 M_sun
yr^-1, while ~10^-10 M_sun yr^-1 is sufficient for accretion to a white dwarf
companion. These accretion rates are feasible for some detached binary
configurations on the basis of the Bond-Hoyle type accretion process. We
concluded that the carbon star BM Gem is in a detached binary system with a
companion of low mass and low luminosity. However, we are unable to determine
whether this companion object is a dwarf or a white dwarf. The upper limits for
binary separation are 210 AU and 930 AU for a dwarf and a white dwarf,
respectively. We also note that the observed features of BM Gem mimic those of
Mira (omi Cet), which may suggest actual similarities in their binary
configurations and circumstellar structures.Comment: 11 pages, 2 figures, 1 table, accepted for publication in Ap
The relationship between anemia at admission and outcome in patients older than 60 years with hip fracture
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
The Origin of Carbon-Enhancement and Initial Mass Function of Extremely Metal-Poor Stars in the Galactic Halo
It is known that the carbon-enhanced, extremely metal-poor (CEMP) stars
constitute a substantial proportion in the extremely metal-poor (EMP) stars of
the Galactic Halo, by far larger than CH stars in Population II stars. We
investigate their origin with taking into account an additional evolutionary
path to the surface carbon-enrichment, triggered by hydrogen engulfment by the
helium flash convection, in EMP stars of . This process
is distinct from the third dredge-up operating in more metal-rich stars and
also in EMP stars. In binary systems of EMP stars, the secondary stars become
CEMP stars through mass transfer from the primary stars of low and intermediate
masses, which have developed the surface carbon-enhancement. Our binary
scenario can predict the variations in the abundances not only for carbon but
also for nitrogen and s-process elements and reasonably explain the observed
properties such as the stellar distributions with respect to the carbon
abundances, the binary periods, and the evolutionary stages. Furthermore, from
the observed frequencies of CEMP stars with and without s-process element
enhancement, we demonstrate that the initial mass function of EMP stars need to
give the mean mass ~10\msun under the reasonable assumptions on the
distributions of orbital separations and mass ratio of binary components. This
also indicates that the currently observed EMP stars were exclusively born as
the secondary members of binaries, making up remnants of EMP binary
systems of mass ~10^8\msun in total; in addition to CEMP stars with white
dwarf companions, a significant fraction of them have experienced supernova
explosions of their companions. We discuss the implications of the present
results in relation to the formation of Galactic halo.Comment: 66 pages, 12 figures, 2 tables Accepted for publication in Ap
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