14,116 research outputs found
Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--I: Structure and Evolution
We have performed a detailed analysis of the Ring Nebula (NGC 6720) using HST
WFC3 images and derived a new 3-D model. Existing high spectral resolution
spectra played an important supplementary role in our modeling. It is shown
that the Main Ring of the nebula is an ionization-bounded irregular
non-symmetric disk with a central cavity and perpendicular extended lobes
pointed almost towards the observer. The faint outer halos are determined to be
fossil radiation, i.e. radiation from gas ionized in an earlier stage of the
nebula when it was not ionization bounded.
The narrow-band WFC3 filters that isolate some of the emission-lines are
affected by broadening on their short wavelength side and all the filters were
calibrated using ground-based spectra. The filter calibration results are
presented in an appendix.Comment: Accepted for publication by the Astronomical Journa
Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--II:Physical Conditions
We have performed a detailed analysis of the electron temperature and density
in the the Ring Nebula using the calibrated HST WFC3 images described in the
preceding paper. The electron temperature (Te) determined from [N II] and [O
III] rises slightly and monotonically towards the central star. The observed
equivalent width (EW) in the central region indicates that Te rises as high as
13000 K. In contrast, the low EW's in the outer regions are largely due to
scattered diffuse Galactic radiation by dust. The images allowed determination
of unprecedented small scale variations in Te. These variations indicate that
the mean square area temperature fluctuations are significantly higher than
expected from simple photoionization. The power producing these fluctuations
occurs at scales of less than 3.5E15 cm. This scale length provides a strong
restriction on the mechanism causing the large t^2 values observed.Comment: Accepted for publication in the Astronomical Journa
Measurements and Information in Spin Foam Models
We present a problem relating measurements and information theory in spin
foam models. In the three dimensional case of quantum gravity we can compute
probabilities of spin network graphs and study the behaviour of the Shannon
entropy associated to the corresponding information. We present a general
definition, compute the Shannon entropy of some examples, and find some
interesting inequalities.Comment: 15 pages, 3 figures. Improved versio
Isotopic replacement in ionic systems: the 4He2+ + 3He -> 3He4He+ + 4He reaction
Full quantum dynamics calculations have been carried out for the ionic
reaction 4He2+ + 3He and state-to-state reactive probabilities have been
obtained using both a time-dependent (TD) and a time-independent (TI) approach.
An accurate ab-initio potential energy surface has been employed for the
present quantum dynamics and the two sets of results are shown to be in
agreement with each other. The results for zero total angular momentum suggest
a marked presence of atom exchange (isotopic replacement) reaction with
probabilities as high as 60%. The reaction probabilities are only weakly
dependent on the initial vibrational state of the reactants while they are
slightly more sensitive to the degree of rotational excitation. A brief
discussion of the results for selected higher total angular momentum values is
also presented, while the l-shifting approximation [1] has been used to provide
estimates of the total reaction rates for the title process. Such rates are
found to be large enough to possibly become experimentally accessible
Multimetric extension of the PPN formalism: experimental consistency of repulsive gravity
Recently we discussed a multimetric gravity theory containing several copies
of standard model matter each of which couples to its own metric tensor. This
construction contained dark matter sectors interacting repulsively with the
visible matter sector, and was shown to lead to cosmological late-time
acceleration. In order to test the theory with high-precision experiments
within the solar system we here construct a simple extension of the
parametrized post-Newtonian (PPN) formalism for multimetric gravitational
backgrounds. We show that a simplified version of this extended formalism
allows the computation of a subset of the PPN parameters from the linearized
field equations. Applying the simplified formalism we find that the PPN
parameters of our theory do not agree with the observed values, but we are able
to improve the theory so that it becomes consistent with experiments of
post-Newtonian gravity and still features its promising cosmological
properties.Comment: 19 pages, no figures, journal versio
No-go theorem for bimetric gravity with positive and negative mass
We argue that the most conservative geometric extension of Einstein gravity
describing both positive and negative mass sources and observers is bimetric
gravity and contains two copies of standard model matter which interact only
gravitationally. Matter fields related to one of the metrics then appear dark
from the point of view of an observer defined by the other metric, and so may
provide a potential explanation for the dark universe. In this framework we
consider the most general form of linearized field equations compatible with
physically and mathematically well-motivated assumptions. Using gauge-invariant
linear perturbation theory, we prove a no-go theorem ruling out all bimetric
gravity theories that, in the Newtonian limit, lead to precisely opposite
forces on positive and negative test masses.Comment: 19 pages, no figures, journal versio
Thermodynamics of phantom black holes in Einstein-Maxwell-Dilaton theory
A thermodynamic analysis of the black hole solutions coming from the
Einstein-Maxwell-Dilaton theory (EMD) in 4D is done. By consider the canonical
and grand-canonical ensemble, we apply standard method as well as a recent
method known as Geometrothermodynamics (GTD). We are particularly interested in
the characteristics of the so called phantom black hole solutions. We will
analyze the thermodynamics of these solutions, the points of phase transition
and their extremal limit. Also the thermodynamic stability is analyzed. We
obtain a mismatch of the between the results of the GTD method when compared
with the ones obtained by the specific heat, revealing a weakness of the
method, as well as possible limitations of its applicability to very
pathological thermodynamic systems. We also found that normal and phantom
solutions are locally and globally unstable, unless for certain values of the
coupled constant of the EMD action. We also shown that the
anti-Reissner-Nordstrom solution does not posses extremal limit nor phase
transition points, contrary to the Reissner-Nordstrom case.Comment: 23 pages, version accepted for publication in Physical Review
Higgs Production from Gluon Fusion in Warped Extra Dimensions
We present an analysis of the loop-induced couplings of the Higgs boson to
the massless gauge fields (gluons and photons) in the warped extra dimension
models where all Standard Model fields propagate in the bulk. We show that in
such models corrections to the hgg and couplings are
potentially very large. These corrections can lead to generically sizable
deviations in the production and decay rates of the Higgs boson, even when the
new physics states lie beyond the direct reach of the LHC.Comment: 24 pages, 5 figures. Added Appendix C, minor changes in the text,
replaced figures 2-
Propagation of gravitational waves in multimetric gravity
We discuss the propagation of gravitational waves in a recently discussed
class of theories containing N >= 2 metric tensors and a corresponding number
of standard model copies. Using the formalism of gauge-invariant linear
perturbation theory we show that all gravitational waves propagate at the speed
of light. We then employ the Newman-Penrose formalism to show that two to six
polarizations of gravitational waves may exist, depending on the parameters
entering the equations of motion. This corresponds to E(2) representations N_2,
N_3, III_5 and II_6. We finally apply our general discussion to a recently
presented concrete multimetric gravity model and show that it is of class N_2,
i.e., it allows only two tensor polarizations, as it is the case for general
relativity. Our results provide the theoretical background for tests of
multimetric gravity theories using the upcoming gravitational wave experiments.Comment: 21 pages, no figures, journal versio
Probing vortices in 4He nanodroplets
We present static and dynamical properties of linear vortices in 4He droplets
obtained from Density Functional calculations. By comparing the adsorption
properties of different atomic impurities embedded in pure droplets and in
droplets where a quantized vortex has been created, we suggest that Ca atoms
should be the dopant of choice to detect vortices by means of spectroscopic
experiments.Comment: Typeset using Revtex4, 4 pages and 2 Postscript file
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