20,262 research outputs found
The effect of interplanetary trajectory options on a manned Mars aerobrake configuration
Manned Mars missions originating in low Earth orbit (LEO) in the time frame 2010 to 2025 were analyzed to identify preferred mission opportunities and their associated vehicle and trajectory characteristics. Interplanetary and Mars atmospheric trajectory options were examined under the constraints of an initial manned exploration scenario. Two chemically propelled vehicle options were considered: (1) an all propulsive configuration, and (2) a configuration which employs aerobraking at Earth and Mars with low lift/drag (L/D) shapes. Both the interplanetary trajectory options as well as the Mars atmospheric passage are addressed to provide a coupled trajectory simulation. Direct and Venus swingby interplanetary transfers with a 60 day Mars stopover are considered. The range and variation in both Earth and Mars entry velocity are also defined. Two promising mission strategies emerged from the study: (1) a 1.0 to 2.0 year Venus swingby mission, and (2) a 2.0 to 2.5 year direct mission. Through careful trajectory selection, 11 mission opportunities are identified in which the Mars entry velocity is between 6 and 10 km/sec and Earth entry velocity ranges from 11.5 to 12.5 km/sec. Simulation of the Earth return aerobraking maneuver is not performed. It is shown that a low L/D configuration is not feasible for Mars aerobraking without substantial improvements in the interplanetary navigation system. However, even with an advanced navigation system, entry corridor and aerothermal requirements restrict the number of potential mission opportunities. It is also shown that for a large blunt Mars aerobrake configuration, the effects of radiative heating can be significant at entry velocities as low as 6.2 km/sec and will grow to dominate the aerothermal environment at entry velocities above 8.5 km/sec. Despite the additional system complexity associated with an aerobraking vehicle, the use of aerobraking was shown to significantly lower the required initial LEO weight. In comparison with an all propulsive mission, savings between 19 and 59 percent were obtained depending upon launch date
Possible production of exotic baryonia in relativistic heavy-ion collisions
Properties of a hypothetical baryonium with the quark content
(uds\ov{u}\ov{d}\ov{s}) are discussed. The MIT bag model predicts its mass to
be unexpectedly low, approximately 1210 MeV. Possible hadronic decay modes of
this state are analyzed. Ultrarelativistic heavy-ion collisions provide
favorable conditions for the formation of such particles from the baryon-free
quark-gluon plasma. We estimate multiplicities of such exotic baryonia on the
basis of a simple thermal model.Comment: 8 pages, 1 figur
Pinning a Domain Wall in (Ga,Mn)As with Focused Ion Beam Lithography
We utilize a focused beam of Ga+ ions to define magnetization pinning sites
in a ferromagnetic epilayer of (Ga,Mn)As. The nonmagnetic defects locally
increase the magneto-crystalline anisotropy energies, by which a domain wall is
pinned at a given position. We demonstrate techniques for manipulating domain
walls at these pinning sites as probed with the giant planar Hall effect
(GPHE). By varying the magnetic field angle relative to the crystal axes, an
upper limit is placed on the local effective anisotropy energy.Comment: 13 pages, 3 figure
Width of the QCD transition in a Polyakov-loop DSE model
We consider the pseudocritical temperatures for the chiral and deconfinement
transitions within a Polyakov-loop Dyson-Schwinger equation approach which
employs a nonlocal rank-2 separable model for the effective gluon propagator.
These pseudocritical temperatures differ by a factor of two when the quark and
gluon sectors are considered separately, but get synchronized and become
coincident when their coupling is switched on. The coupling of the
Polyakov-loop to the chiral quark dynamics narrows the temperature region of
the QCD transition in which chiral symmetry and deconfinement is established.
We investigate the effect of rescaling the parameter T_0 in the Polyakov-loop
potential on the QCD transition for both the logarithmic and polynomial forms
of the potential. While the critical temperatures vary in a similar way, the
width of the transition is stronger affected for the logarithmic potential. For
this potential the character of the transition changes from crossover to a
first order one when T_0 < 210 MeV, but it remains crossover in the whole range
of relevant T_0 values for the polynomial form.Comment: 10 pages, 6 figures, results for polynomial form of Polyakov-loop
potential included, references added, final version to appear in Phys. Rev.
Supernova Remnants in the Fossil Starburst in M82
We report the discovery of ten compact H-alpha-bright sources in the
post-starburst region northeast of the center of M82, ``M82 B.'' These objects
have H alpha luminosities and sizes consistent with Type II supernova remnants
(SNRs). They fall on the same H alpha surface brightness-diameter (Sigma-D)
relation defined by SNRs in other nearby star-forming galaxies, with the M82
candidates lying preferentially at the small diameter end. These are the first
candidates for optically-visible SNRs in M82 outside the heavily obscured
central starburst within ~250 pc from the galactic center. If these sources are
SNRs, they set an upper limit to the end of the starburst in region ``B2,''
about 500 pc from the galaxy's core, of ~50 Myr. Region ``B1,'' about 1000 pc
from the core, lacks good SNR candidates and is evidently somewhat older. This
suggests star formation in the galaxy has propagated inward toward the
present-day intense starburst core.Comment: Re-submitted to AJ, referee's comments taken into account, 15 pages
LaTeX preprint style, 4 postscript figures; full-resolution figures available
from http://www.astro.virginia.edu/~rd7a/snrs/ Changes: minor textual changes
and orientation/axes of Fig.
Phase space contraction and quantum operations
We give a criterion to differentiate between dissipative and diffusive
quantum operations. It is based on the classical idea that dissipative
processes contract volumes in phase space. We define a quantity that can be
regarded as ``quantum phase space contraction rate'' and which is related to a
fundamental property of quantum channels: non-unitality. We relate it to other
properties of the channel and also show a simple example of dissipative noise
composed with a chaotic map. The emergence of attaractor-like structures is
displayed.Comment: 8 pages, 6 figures. Changes added according to refferee sugestions.
(To appear in PRA
Distribution of Interference in the Presence of Decoherence
We study the statistics of quantum interference for completely positive maps.
We calculate analytically the mean interference and its second moment for
finite dimensional quantum systems interacting with a simple environment
consisting of one or several spins (qudits). The joint propagation of the
entire system is taken as unitary with an evolution operator drawn from the
Circular Unitary Ensemble (CUE). We show that the mean interference decays with
a power law as function of the dimension of the Hilbert space of the
environment, with a power that depends on the temperature of the environment.Comment: 28 pages of pd
Exclusive processes in position space and the pion distribution amplitude
We suggest to carry out lattice calculations of current correlators in
position space, sandwiched between the vacuum and a hadron state (e.g. pion),
in order to access hadronic light-cone distribution amplitudes (DAs). In this
way the renormalization problem for composite lattice operators is avoided
altogether, and the connection to the DA is done using perturbation theory in
the continuum. As an example, the correlation function of two electromagnetic
currents is calculated to the next-to-next-to-leading order accuracy in
perturbation theory and including the twist-4 corrections. We argue that this
strategy is fully competitive with direct lattice measurements of the moments
of the DA, defined as matrix elements of local operators, and offers new
insight in the space-time picture of hard exclusive reactions.Comment: 15 pages, 10 figure
Fixed-N Superconductivity: The Crossover from the Bulk to the Few-Electron Limit
We present a truly canonical theory of superconductivity in ultrasmall
metallic grains by variationally optimizing fixed-N projected BCS
wave-functions, which yields the first full description of the entire crossover
from the bulk BCS regime (mean level spacing bulk gap )
to the ``fluctuation-dominated'' few-electron regime (). A
wave-function analysis shows in detail how the BCS limit is recovered for , and how for pairing correlations become
delocalized in energy space. An earlier grand-canonical prediction for an
observable parity effect in the spectral gaps is found to survive the fixed-N
projection.Comment: 4 pages, 3 figures, RevTeX, V2: minor charges to mach final printed
versio
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