24,153 research outputs found
Position of First Doppler Peak
This talk describes an analytic calculation of the position of the first
accoustic peak as a function of the matter density and the cosmological
constant (work with Y.J. Ng and R. Rohm), agreeing with independent numerical
studies. There is also discussion of supernovae observations and how together
these approaches combine to suggest non-zero cosmological constant. An
alternative to a constant vacuum density - quintessence - is mentioned.Comment: 9 pages LaTeX. Talk at COSMO-98. Asilomar, Monterey, California.
November 199
Binary String Dynamics
In this paper we investigate the dynamical properties of binary cosmic
strings. We find extrinsic curvature dependence of the string action and show
that kinks on binary strings are eroded while cusps can play a major role in
their evolution.Comment: 10 pages, plain TeX, uses harvmac and epsf, 1 postscript figur
Modeling the magnetic field in the protostellar source NGC 1333 IRAS 4A
Magnetic fields are believed to play a crucial role in the process of star
formation. We compare high-angular resolution observations of the submillimeter
polarized emission of NGC 1333 IRAS 4A, tracing the magnetic field around a
low-mass protostar, with models of the collapse of magnetized molecular cloud
cores. Assuming a uniform dust alignment efficiency, we computed the Stokes
parameters and synthetic polarization maps from the model density and magnetic
field distribution by integrations along the line-of-sight and convolution with
the interferometric response. The synthetic maps are in good agreement with the
data. The best-fitting models were obtained for a protostellar mass of 0.8
solar masses, of age 9e4 yr, formed in a cloud with an initial mass-to-flux
ratio ~2 times the critical value. The magnetic field morphology in NGC 1333
IRAS 4A is consistent with the standard theoretical scenario for the formation
of solar-type stars, where well-ordered, large-scale, rather than turbulent,
magnetic fields control the evolution and collapse of the molecular cloud cores
from which stars form.Comment: 4 pages, 5 figures. Accepted by Astronomy and Astrophysic
The ACS Survey of Galactic Globular Clusters. XII. Photometric Binaries along the Main-Sequence
The fraction of binary stars is an important ingredient to interpret globular
cluster dynamical evolution and their stellar population. We investigate the
properties of main-sequence binaries measured in a uniform photometric sample
of 59 Galactic globular clusters that were observed by HST WFC/ACS as a part of
the Globular Cluster Treasury project. We measured the fraction of binaries and
the distribution of mass-ratio as a function of radial location within the
cluster, from the central core to beyond the half-mass radius. We studied the
radial distribution of binary stars, and the distribution of stellar mass
ratios. We investigated monovariate relations between the fraction of binaries
and the main parameters of their host clusters. We found that in nearly all the
clusters, the total fraction of binaries is significantly smaller than the
fraction of binaries in the field, with a few exceptions only. Binary stars are
significantly more centrally concentrated than single MS stars in most of the
clusters studied in this paper. The distribution of the mass ratio is generally
flat (for mass-ratio parameter q>0.5). We found a significant anti-correlation
between the binary fraction in a cluster and its absolute luminosity (mass).
Some, less significant correlation with the collisional parameter, the central
stellar density, and the central velocity dispersion are present. There is no
statistically significant relation between the binary fraction and other
cluster parameters. We confirm the correlation between the binary fraction and
the fraction of blue stragglers in the cluster.Comment: 43 Pages, 52 figures, accepted for publication in A&
Polarization analysis and microstructural characterization of SOFC anode and electrolyte supported cells
A nonsupersymmetric matrix orbifold
We construct the matrix description for a twisted version of the IIA string
theory on S^1 with fermions antiperiodic around a spatial circle. The result is
a 2+1-dimensional U(N) x U(N) nonsupersymmetric Yang-Mills theory with
fermionic matter transforming in the (N,Nbar). The two U(N)'s are exchanged if
one goes around a twisted circle of the worldvolume. Relations with Type 0
theories are explored and we find Type 0 matrix string limits of our gauge
theory. We argue however that most of these results are falsified by the
absence of SUSY nonrenormalization theorems and that the models do not in fact
have a sensible Lorentz invariant space time interpretation.Comment: JHEP LaTeX, 22 page
Gravitational collapse of magnetized clouds II. The role of Ohmic dissipation
We formulate the problem of magnetic field dissipation during the accretion
phase of low-mass star formation, and we carry out the first step of an
iterative solution procedure by assuming that the gas is in free-fall along
radial field lines. This so-called ``kinematic approximation'' ignores the back
reaction of the Lorentz force on the accretion flow. In quasi steady-state, and
assuming the resistivity coefficient to be spatially uniform, the problem is
analytically soluble in terms of Legendre's polynomials and confluent
hypergeometric functions. The dissipation of the magnetic field occurs inside a
region of radius inversely proportional to the mass of the central star (the
``Ohm radius''), where the magnetic field becomes asymptotically straight and
uniform. In our solution, the magnetic flux problem of star formation is
avoided because the magnetic flux dragged in the accreting protostar is always
zero. Our results imply that the effective resistivity of the infalling gas
must be higher by several orders of magnitude than the microscopic electric
resistivity, to avoid conflict with measurements of paleomagnetism in
meteorites and with the observed luminosity of regions of low-mass star
formation.Comment: 20 pages, 4 figures, The Astrophysical Journal, in pres
Temperature-Dependence of the Solid-Electrolyte Interphase Overpotential: Part I. Two Parallel Mechanisms, One Phase Transition
It has been shown recently that the overpotential originating from ionic
conduction of alkali-ions through the inner dense solid-electrolyte interphase
(SEI) is strongly non-linear. An empirical equation was proposed to merge the
measured resistances from both galvanostatic cycling (GS) and electrochemical
impedance spectroscopy (EIS) at 25C. Here, this analysis is extended
to the full temperature range of batteries from -40C to
+80C for Li, Na, K and Rb-metal electrodes in carbonate electrolytes.
Two different transport mechanisms are found. The first one conducts
alkali-ions at all measured temperatures. The second transport mechanism
conducts ions for all seven measured Li-ion electrolytes and one out of four
Na-ion electrolytes, however, only above a certain critical temperature .
At a phase transition is observed switching-off the more efficient
transport mechanism and leaving only the general ion conduction mechanism. The
associated overpotentials increase rapidly below depending on alkali-ion,
salt and solvent and become a limiting factor during galvanostatic operation of
all Li-ion electrolytes at low temperature. In general, the current analysis
merges the SEI resistances measured by EIS ranging from 26 cm for
the best Li up to 292 Mcm for Rb electrodes to its galvanostatic
response over seven orders of magnitude. The determined critical temperatures
are between 0-25C for the tested Li and above 50C for Na
electrolytes.Comment: 10 pages, 7 figures, file includes Suppl Info,
http://jes.ecsdl.org/content/165/2/A32
Supersymmetry breaking in M-theory and quantization rules
We analyze in detail supersymmetry breaking by compactification of the fifth
dimension in M-theory in the compactification pattern and
find that a superpotential is generated for the complex fields coming from
hypermultiplets, namely the dilaton and the complex structure moduli. Using
general arguments it is shown that these fields are always stabilized such that
they don't contribute to supersymmetry breaking, which is completely saturated
by the K\"ahler moduli coming from vector multiplets.
It is shown that this mechanism is the strong-coupling analog of the
Rohm-Witten quantization of the antisymmetric tensor field strength of string
theories. The effect of a gaugino condensate on one of the boundaries is also
considered.Comment: 16 pages, LaTex, no figure
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