144 research outputs found
A generalized strategy for building resident database interfaces
A strategy for building resident interfaces to host heterogeneous distributed data base management systems is developed. The strategy is used to construct several interfaces. A set of guidelines is developed for users to construct their own interfaces
Design and manufacture of a chain driven motor shaft for Formula Electric applications
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (page 23).This document describes the design, manufacture, and testing of a motor shaft assembly that is a subcomponent of the drivetrain system in a Formula Electric racecar. The design covers detailed analysis of the bearing, sprocket, shaft, pins, and spacer created to meet a set of functional requirements. The process of determining these requirements is also covered, along with a set of best practices for thoughtful design in general. An initial phase of testing is covered as well, from the design of the testing apparatus to the execution of a torsional test on the shaft. With limited data, conclusions are hard to draw with certainty, but initial results seem to indicate that the shaft performs as expected. The intent of this document is to serve as an example of the level of detail and thoroughness that is expected when designing racecar components. To that end, this document should provide a number of unique examples of how specific problems were modeled and addressed in the design. It should also serve as a template for future documentation of such design efforts.by Jack J. Wanderman.S.B
Education for Democracy: A co-constructivist perspective
En el presente artÃculo describimos el papel que la educación ha de desempeñar en el proceso de mejora de la calidad de vida en el mundo moderno. El enfoque especÃfico de este artÃculo enfatiza el papel que la educación debe desempeñar para promover la evolución de las formas de vida democráticas como un medio para mejorar la calidad de la vida tanto personal como colectiva e institucional. La calidad de vida, al menos en lo concerniente a la conducta orientada a fines, supone elegir los valores y alcanzar con éxito los objetivos de la vida. Esto último, está relacionado con la utilización del pensamiento y la discusión crÃtica para realizar elecciones vitales de tipo individual y colectivo. Lo cual, a su vez, tiene que ver con la institucionalización del ideal democrático. En este sentido, el enfoque principal de nuestro trabajo aplicado trata de promover la evolución de las formas de vida democráticas como un medio para mejorar la calidad de vida en los niveles personal, interpersonal e institucional
Constraining the Emissivity of Ultrahigh Energy Cosmic Rays in the Distant Universe with the Diffuse Gamma-ray Emission
Ultra-high cosmic rays (UHECRs) with energies >10^19 eV emitted at
cosmological distances will be attenuated by cosmic microwave and infrared
background radiation through photohadronic processes. Lower energy
extra-galactic cosmic rays (~10^18-10^19 eV) can only travel a linear distance
smaller than ~Gpc in a Hubble time due to the diffusion if the extra-galactic
magnetic fields are as strong as nano Gauss. These prevent us from directly
observing most of the UHECRs in the universe, and thus the observed UHECR
intensity reflects only the emissivity in the nearby universe within hundreds
of Mpc. However, UHECRs in the distant universe, through interactions with the
cosmic background photons, produce UHE electrons and gamma-rays that in turn
initiate electromagnetic cascades on cosmic background photons. This secondary
cascade radiation forms part of the extragalactic diffuse GeV-TeV gamma-ray
radiation and, unlike the original UHECRs, is observable. Motivated by new
measurements of extragalactic diffuse gamma-ray background radiation by
Fermi/LAT, we obtained upper limits placed on the UHECR emissivity in the
distant universe by requiring that the cascade radiation they produce not
exceed the observed levels. By comparison with the gamma-ray emissivity of
candidate UHECR sources (such as GRBs and AGNs) at high-redshifts, we find that
the obtained upper limit for a flat proton spectrum is ~10^1.5 times larger
than the gamma-ray emissivity in GRBs and ~10 times smaller than the gamma-ray
emissivity in BL Lac objects. In the case of iron nuclei composition, the
derived upper limit of the UHECR emissivity is a factor of 3-5 times higher.
Robust upper limit on the cosmogenic neutrino flux is further obtained, which
is marginally reachable by the Icecube detector and the next-generation
detector JEM-EUSO.Comment: 14 pages, 8 figures, Replaced to match the published versio
GRBs on probation: testing the UHE CR paradigm with IceCube
Gamma ray burst (GRB) fireballs provide one of very few astrophysical
environments where one can contemplate the acceleration of cosmic rays to
energies that exceed 10^20 eV. The assumption that GRBs are the sources of the
observed cosmic rays generates a calculable flux of neutrinos produced when the
protons interact with fireball photons. With data taken during construction
IceCube has already reached a sensitivity to observe neutrinos produced in
temporal coincidence with individual GRBs provided that they are the sources of
the observed extragalactic cosmic rays. We here point out that the GRB origin
of cosmic rays is also challenged by the IceCube upper limit on a possible
diffuse flux of cosmic neutrinos which should not be exceeded by the flux
produced by all GRB over Hubble time. Our alternative approach has the
advantage of directly relating the diffuse flux produced by all GRBs to
measurements of the cosmic ray flux. It also generates both the neutrino flux
produced by the sources and the associated cosmogenic neutrino flux in a
synergetic way.Comment: 12 pages, 3 figures, matches version published in Astroparticle
Physic
Stellar black holes at the dawn of the universe
It is well established that between 380000 and 1 billion years after the Big
Bang the Inter Galactic Medium (IGM) underwent a "phase transformation" from
cold and fully neutral to warm (~10^4 K) and ionized. Whether this phase
transformation was fully driven and completed by photoionization by young hot
stars is a question of topical interest in cosmology. AIMS. We propose here
that besides the ultraviolet radiation from massive stars, feedback from
accreting black holes in high-mass X-ray binaries (BH-HMXBs) was an additional,
important source of heating and reionization of the IGM in regions of low gas
density at large distances from star-forming galaxies. METHODS. We use current
theoretical models on the formation and evolution of primitive massive stars of
low metallicity, and the observations of compact stellar remnants in the near
and distant universe, to infer that a significant fraction of the first
generations of massive stars end up as BH-HMXBs. The total number of energetic
ionizing photons from an accreting stellar black hole in an HMXB is comparable
to the total number of ionizing photons of its progenitor star. However, the
X-ray photons emitted by the accreting black hole are capable of producing
several secondary ionizations and the ionizing power of the resulting black
hole could be greater than that of its progenitor. Feedback by the large
populations of BH-HMXBs heats the IGM to temperatures of ~10^4 K and maintains
it ionized on large distance scales. BH-HMXBs determine the early thermal
history of the universe and mantain it as ionized over large volumes of space
in regions of low density. This has a direct impact on the properties of the
faintest galaxies at high redshifts, the smallest dwarf galaxies in the local
universe, and on the existing and future surveys at radio wavelengths of atomic
hydrogen in the early universe.Comment: 7 pages, 2 figures, accepted to be published in Astronomy and
Astrophysic
Population III stars and the Long Gamma Ray Burst rate
Because massive, low-metallicity population III (PopIII) stars may produce
very powerful long gamma-ray bursts (LGRBs), high-redshift GRB observations
could probe the properties of the first stars. We analyze the correlation
between early PopIII stars and LGRBs by using cosmological
N-body/hydrodynamical simulations, which include detailed chemical evolution,
cooling, star formation, feedback effects and the transition between PopIII and
more standard population I/II (PopII/I) stars. From the Swift observed rate of
LGRBs, we estimate the fraction of black holes that will produce a GRB from
PopII/I stars to be in the range 0.028<f_{GRB}<0.140, depending on the assumed
upper metallicity of the progenitor. Assuming that as of today no GRB event has
been associated to a PopIII star, we estimate the upper limit for the fraction
of LGRBs produced by PopIII stars to be in the range 0.006<f_{GRB}<0.022. When
we apply a detection threshold compatible with the BAT instrument, we find that
the expected fraction of PopIII GRBs (GRB3) is ~10% of the full LGRB population
at z>6, becoming as high has 40% at z>10. Finally, we study the properties of
the galaxies hosting our sample of GRB3. We find that the average metallicity
of the galaxies hosting a GRB3 is typically higher than the critical
metallicity used to select the PopIII stars, due to the efficiency in polluting
the gas above such low values. We also find that the highest probability of
finding a GRB3 is within galaxies with a stellar mass <10^7 Msun, independently
from the redshift.Comment: 8 pages,3 figures. Submitted to MNRAS, revised version after
referee's comment
Does Recombinant Human Bone Morphogenic Protein 2 Affect Perioperative Blood Loss after Lumbar and Thoracic Spinal Fusion?
Study Design Retrospective cohort design. Purpose This study aimed to determine whether recombinant human bone morphogenic protein 2 (rhBMP-2) reduces total perioperative blood loss during lumbar and thoracic fusion. Overview of Literature Previous studies on rhBMP-2 versus iliac crest bone grafting in thoracic and lumbar fusions have yielded mixed results regarding reductions in blood loss and have largely neglected the postoperative period when analyzing total blood loss. Additionally, these studies have been limited by heterogeneity and sample size. Methods We analyzed the blood loss patterns of 617 consecutive adult patients undergoing lumbar and/or thoracic fusions requiring subfascial drain placement at a single institution from January 2009 to December 2016. Patients were divided into BMP and non-BMP cohorts, and a propensity score analysis was conducted to account for the differences between cohorts. Results At a per-level fused basis, the BMP group exhibited a significant reduction in the intraoperative (66.1 mL per-level fused basis; 95% confidence interval [CI], 127.9 to 4.25 mL; p=0.036) and total perioperative blood loss (100.7 mL per-level fused basis; 95% CI, 200.9 to 0.5 mL; p=0.049). However, no significant differences were observed in an analysis when not controlling for the number of levels or when examining the postoperative drain output. Conclusion RhBMP-2 appears to reduce both intraoperative and total blood loss during lumbar and thoracic fusions on a per-level fused basis. This total reduction in blood loss was achieved via intraoperative effects because RhBMP-2 had no significant effect on the postoperative drain output
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