29,794 research outputs found
The big Dehn surgery graph and the link of S^3
In a talk at the Cornell Topology Festival in 2005, W. Thurston discussed a
graph which we call "The Big Dehn Surgery Graph", B. Here we explore this
graph, particularly the link of S^3, and prove facts about the geometry and
topology of B. We also investigate some interesting subgraphs and pose what we
believe are important questions about B.Comment: 15 pages, 4 figures, 4 ancillary files. Reorganized and shortened
from previous versions, while correcting one error in the proof of Theorem
5.4. Also, ancillary files detailing our computations with the computer
program ORB have been provide
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Data assimilation insights on selecting the most valuable atmospheric measurements
We discuss how objective guidance on selecting the most valuable atmospheric measurements on future Mars spacecraft missions can be provided through already developed Martian atmospheric data assimilation systems, and in particular through Observing System Simulation Experiments (OSSEs) which are widely used to design instruments for the Earth’s atmosphere
Nuclear Aspects of Nucleosynthesis in Massive Stars
Preliminary results of a new set of stellar evolution and nucleosynthesis
calculations for massive stars are presented. These results were obtained with
an extended reaction network up to Bi. The discussion focuses on the importance
of nuclear rates in pre- and post-explosive nucleosynthesis. The need for
further experiments to study specific reactions and nuclear properties (optical
alpha+nucleus potentials) is emphasized.Comment: 6 pages, 2 figures; invited talk, to appear in the Proceedings of the
Int. Conf. "Structure of the Nucleus at the Dawn of the Century", May 2000,
Bologna, Ital
Nucleosynthesis in massive stars revisited
We have performed the first calculations to follow the evolution of all
stable nuclei and their radioactive progenitors in a finely-zoned stellar model
computed from the onset of central hydrogen burning through explosion as a Type
II supernova. Calculations were done for 15, 20, and 25 solar masses Pop I
stars using the most recently available set of experimental and theoretical
nuclear data, revised opacity tables, and taking into account mass loss due to
stellar winds. Here results are presented for one 15 solar masses model.Comment: 4 pages, 1 figure; needs espcrc1.sty; talk at "Nuclei in the Cosmos
2000", Aarhus, Denmark, June 2000; will appear in Nucl. Phys.
Proceedings of workshops to define engineering requirements for a space vacuum research facility
The construction of a molecular wake shield for the shuttle orbiter is presented as well as a collision model with a program depicting emitted molecular density around the spacecraft giving estimates of backscattered flux and other collisional processes
The Production of Ti44 and Co60 in Supernova
The production of the radioactive isotopes Ti and Co in all
types of supernovae is examined and compared to observational constraints
including Galactic --ray surveys, measurements of the diffuse 511 keV
radiation, --ray observations of Cas A, the late time light curve of SN
1987A, and isotopic anomalies found in silicon carbide grains in meteorites.
The (revised) line flux from Ti decay in the Cas A supernova remnant
reported by COMPTEL on the Compton Gamma-Ray Observatory is near the upper
bound expected from our models. The necessary concurrent ejection of Ni
would also imply that Cas A was a brighter supernova than previously thought
unless extinction in the intervening matter was very large. Thus, if confirmed,
the reported amount of Ti in Cas A provides very interesting constraints
on both the supernova environment and its mechanism. The abundances of
Ti and Co ejected by Type II supernovae are such that
gamma-radiation from Ti decay SN 1987A could be detected by a future
generation of gamma-ray telescopes and that the decay of Co might
provide an interesting contribution to the late time light curve of SN 1987A
and other Type II supernovae. To produce the solar Ca abundance and
satisfy all the observational constraints, nature may prefer at least the
occasional explosion of sub-Chandrasekhar mass white dwarfs as Type Ia
supernovae. Depending on the escape fraction of positrons due to Co made
in all kinds of Type Ia supernovae, a significant fraction of the steady state
diffuse 511 keV emission may arise from the annihilation of positrons produced
during the decay of Ti to Ca. The Ca and Ti isotopic anomalies in
pre-solar grains confirm the production of Ti in supernovae and thatComment: 27 pages including 7 figures. uuencoded, compressed, postscript. in
press Ap
An analytical and experimental investigation of resistojet plumes
As a part of the electrothermal propulsion plume research program at the NASA Lewis Research Center, efforts have been initiated to analytically and experimentally investigate the plumes of resistojet thrusters. The method of G.A. Simons for the prediction of rocket exhaust plumes is developed for the resistojet. Modifications are made to the source flow equations to account for the increased effects of the relatively large nozzle boundary layer. Additionally, preliminary mass flux measurements of a laboratory resistojet using CO2 propellant at 298 K have been obtained with a cryogenically cooled quartz crystal microbalance (QCM). There is qualitative agreement between analysis and experiment, at least in terms of the overall number density shape functions in the forward flux region
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