8,005 research outputs found
Binarity as the solution to the stellar evolution enigma posed by NGC 6791
Binary evolution is investigated as the source for the extreme horizontal
branch (EHB) stars in the old and metal rich open cluster NGC~6791. Employing
an updated version of our binary stellar evolution code we demonstrate that EHB
stars naturally emerge from the common envelope phase. In sum, the binary model
reproduces the observed (, ) and temporal properties of
the EHB over-density tied to NGC 6971, without needing an ad-hoc and anomalous
mass-loss prescription.Comment: 9 pages, 5 eps figures, accepted for publication in the Astrophysical
Journal Letter
On the non-local heat kernel expansion
We propose a novel derivation of the non-local heat kernel expansion, first
studied by Barvinsky, Vilkovisky and Avramidi, based on simple diagrammatic
equations satisfied by the heat kernel. For Laplace-type differential operators
we obtain the explicit form of the non-local heat kernel form factors to second
order in the curvature. Our method can be generalized easily to the derivation
of the non-local heat kernel expansion of a wide class of differential
operators.Comment: 23 pages, 1 figure, 31 diagrams; references added; to appear in JM
Fe-C and Fe-H systems at pressures of the Earth's inner core
The solid inner core of the Earth is predominantly composed of iron alloyed
with several percent Ni and some lighter elements, Si, S, O, H, and C being the
prime candidates. There have been a growing number of papers investigating C
and H as possible light elements in the core, but the results are
contradictory. Here, using ab initio simulations, we study the Fe-C and Fe-H
systems at inner core pressures (330-364 GPa). Using the evolutionary structure
prediction algorithm USPEX, we have determined the lowest-enthalpy structures
of possible carbides (FeC, Fe2C, Fe3C, Fe4C, FeC2, FeC3, FeC4 and Fe7C3) and
hydrides (Fe4H, Fe3H, Fe2H, FeH, FeH2, FeH3, FeH4) and have found that Fe2C
(Pnma) is the most stable iron carbide at pressures of the inner core, while
FeH, FeH3 and FeH4 are stable iron hydrides at these conditions. For Fe3C, the
cementite structure (Pnma) and the Cmcm structure recently found by random
sampling are less stable than the I-4 and C2/m structures found here. We found
that FeH3 and FeH4 adopt chemically interesting thermodynamically stable
structures, in both compounds containing trivalent iron. The density of the
inner core can be matched with a reasonable concentration of carbon, 11-15
mol.percent (2.6-3.7 wt.percent) at relevant pressures and temperatures. This
concentration matches that in CI carbonaceous chondrites and corresponds to the
average atomic mass in the range 49.3-51.0, in close agreement with inferences
from the Birch's law for the inner core. Similarly made estimates for the
maximum hydrogen content are unrealistically high, 17-22 mol.percent (0.4-0.5
wt.percent), which corresponds to the average atomic mass in the range
43.8-46.5. We conclude that carbon is a better candidate light alloying element
than hydrogen.Comment: Published in Physics-Uspekhi: full text will soon appear at
http://ufn.ru/en/articles/2012/5/c/ (currently, only abstract is available
Innovative all composite multi-pultrusion truss system for stressed arch deployable shelters
Trusses are one of the successful structural forms that have been utilised, at extended scale, since the nineteen century. Fibre composite materials are relatively new to civil engineering applications. The increased interest in using composites in civil applications can be attributed to advantages when compared to other construction materials that offset their associated costs. Using conventional approaches for truss systems in composite materials can undermine their efficiency. This is mainly due to concentration of stresses at connections which usually govern the truss design.
The Military Modular Shelter System (M2S2) initiative is a research project that aims to develop a fibre composite re-deployable arched shelter system with rigid PVC or fabric cladding. The main frames are formed from modular fibre composite panels that are connected and stressed into position by prestressing cables. Different geometries can be obtained using this system by changing the number of panels per frame and the packer sizes between panels.
This paper presents the development and testing of innovative fibre composite truss modules that were investigated as part of this project. The truss system is based on using multi-pultrusion sections for the chord and vertical members. Truss bracing is provided by a double skin laminated web. This structure offers many advantages including semi-ductile failure that occurred outside the joint area and ease of manufacturing. In spite of being developed for the M2S2 system, the concept is similarly applicable as a general purpose truss system
The Unusual Super-Luminous Supernovae SN 2011kl and ASASSN-15lh
Two recently discovered very luminous supernovae (SNe) present stimulating
cases to explore the extents of the available theoretical models. SN 2011kl
represents the first detection of a supernova explosion associated with an
ultra-long duration gamma ray burst. ASASSN-15lh was even claimed as the most
luminous SN ever discovered, challenging the scenarios so far proposed for
stellar explosions. Here we use our radiation hydrodynamics code in order to
simulate magnetar powered SNe. To avoid explicitly assuming neutron star
properties we adopt the magnetar luminosity and spin-down timescale as free
parameters of the model. We find that the light curve (LC) of SN 2011kl is
consistent with a magnetar power source, as previously proposed, but we note
that some amount of 56^Ni (> 0.08 M_sun) is necessary to explain the low
contrast between the LC peak and tail. For the case of ASASSN-15lh we find
physically plausible magnetar parameters that reproduce the overall shape of
the LC provided the progenitor mass is relatively large (a mass of the ejecta
approx 6 M_sun). The ejecta hydrodynamics of this event is dominated by the
magnetar input, while the effect is more moderate for SN 2011kl. We conclude
that a magnetar model may be used for the interpretation of these events and
that the hydrodynamic modeling is necessary to derive the properties of
powerful magnetars and their progenitors.Comment: Accepted by Astrophysical Journal Letters, 5 pages, 5 figure
Heterogeneous Impact of the "Seguro Popular" Program on the Utilization of Obstetrical Services in Mexico, 2001-2006: A Multinomial Probit Model with a Discrete Endogenous Variable
Objective: We evaluated the impact of Seguro Popular (SP), a program introduced in 2001 in Mexico primarily to finance health care for the poor. We studied the effect of SP on pregnant women's access to obstetrical services. Data: We analyzed the cross-sectional 2006 National Health and Nutrition Survey (ENSANUT), focusing on the responses of 3,890 women who delivered babies during 2001-2006 and whose households lacked employer-based health care coverage. Methods: We formulated a multinomial probit model that distinguished between three mutually exclusive sites for delivering a baby: a health unit accredited by SP; a clinic run by the Department of Health (Secretaria de Salud, or SSA); and private obstetrical care. Our model accounted for the endogeneity of the household's binary decision to enroll in the SP program. Results: Women in households that participated in the SP program had a much stronger preference for having a baby in a SP-sponsored unit rather than paying out of pocket for a private delivery. At the same time, participation in SP was associated with a stronger preference for delivering in the private sector rather than at a state-run SSA clinic. On balance, the Seguro Popular program reduced pregnant women's attendance at an SSA clinic much more than it reduced the probability of delivering a baby in the private sector. The impacts of the SP program at the individual and population levels varied with the woman's education and health, as well as the assets and location (rural versus urban) of the household. Conclusions: The SP program had a robust, significantly positive impact on access to obstetrical services. Our finding that women enrolled in SP switched from non-SP state-run facilities, rather than from out-of-pocket private services, is important for public policy and requires further exploration.
The West should rethink its approach for promoting gay rights abroad and instead focus on strengthening democracy and civil society
How should Europe and the United States seek to promote gay rights across the world? Omar G. Encarnación writes that while the West has tended to adopt a model which seeks to ‘shame’ states that discriminate against homosexuals, in many cases this may be self-defeating and could even increase the prevalence of discrimination within the target country. Reasoning that strong democratic foundations are a prerequisite for gay rights, he argues that a better approach would be to focus on promoting democracy, civil society and the rule of law
Electron Fabry-Perot interferometer with two entangled magnetic impurities
We consider a one-dimensional (1D) wire along which single conduction
electrons can propagate in the presence of two spin-1/2 magnetic impurities.
The electron may be scattered by each impurity via a contact-exchange
interaction and thus a spin-flip generally occurs at each scattering event.
Adopting a quantum waveguide theory approach, we derive the stationary states
of the system at all orders in the electron-impurity exchange coupling
constant. This allows us to investigate electron transmission for arbitrary
initial states of the two impurity spins. We show that for suitable electron
wave vectors, the triplet and singlet maximally entangled spin states of the
impurities can respectively largely inhibit the electron transport or make the
wire completely transparent for any electron spin state. In the latter case, a
resonance condition can always be found, representing an anomalous behaviour
compared to typical decoherence induced by magnetic impurities. We provide an
explanation for these phenomena in terms of the Hamiltonian symmetries.
Finally, a scheme to generate maximally entangled spin states of the two
impurities via electron scattering is proposed.Comment: 19 page
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