14,817 research outputs found
Dynamical variables in Gauge-Translational Gravity
Assuming that the natural gauge group of gravity is given by the group of
isometries of a given space, for a maximally symmetric space we derive a model
in which gravity is essentially a gauge theory of translations. Starting from
first principles we verify that a nonlinear realization of the symmetry
provides the general structure of this gauge theory, leading to a simple choice
of dynamical variables of the gravity field corresponding, at first order, to a
diagonal matrix, whereas the non-diagonal elements contribute only to higher
orders.Comment: 15 page
A Study of the Dynamics of Dust from the Kuiper Belt: Spatial Distribution and Spectral Energy Distribution
The dust produced in the Kuiper Belt (KB) spreads throughout the Solar System
forming a dust disk. We numerically model the orbital evolution of KB dust and
estimate its equilibrium spatial distribution and its brightness and spectral
energy distributions (SED), assuming greybody absorption and emission by the
dust grains. We show that the planets modify the KB disk SED, so potentially we
can infer the presence of planets in spatially unresolved debris disks by
studying the shape of their SEDs. We point out that there are inherent
uncertainties in the prediction of structure in the dust disk, owing to the
chaotic dynamics of dust orbital evolution imposed by resonant gravitational
perturbations of the planets.Comment: 19 pages, 14 figures in jpg, accepted to A
Organic Molecules in the Galactic Center. Hot Core Chemistry without Hot Cores
We study the origin of large abundances of complex organic molecules in the
Galactic center (GC). We carried out a systematic study of the complex organic
molecules CH3OH, C2H5OH, (CH3)2O, HCOOCH3, HCOOH, CH3COOH, H2CO, and CS toward
40 GC molecular clouds. Using the LTE approximation, we derived the physical
properties of GC molecular clouds and the abundances of the complex
molecules.The CH3OH abundance between clouds varies by nearly two orders of
magnitude from 2.4x10^{-8} to 1.1x10^{-6}. The abundance of the other complex
organic molecules relative to that of CH3OH is basically independent of the
CH3OH abundance, with variations of only a factor 4-8. The abundances of
complex organic molecules in the GC are compared with those measured in hot
cores and hot corinos, in which these complex molecules are also abundant. We
find that both the abundance and the abundance ratios of the complex molecules
relative to CH3OH in hot cores are similar to those found in the GC clouds.
However, hot corinos show different abundance ratios than observed in hot cores
and in GC clouds. The rather constant abundance of all the complex molecules
relative to CH3OH suggests that all complex molecules are ejected from grain
mantles by shocks. Frequent (similar 10^{5}years) shocks with velocities >6km/s
are required to explain the high abundances in gas phase of complex organic
molecules in the GC molecular clouds. The rather uniform abundance ratios in
the GC clouds and in Galactic hot cores indicate a similar average composition
of grain mantles in both kinds of regions. The Sickle and the Thermal Radio
Arches, affected by UV radiation, show different relative abundances in the
complex organic molecules due to the differentially photodissociation of these
molecules.Comment: 18 pages, 10 Postscript figures, uses aa.cls, aa.bst, 10pt.rtx,
natbib.sty, revsymb.sty revtex4.cls, aps.rtx and aalongtabl.sty. Accepted in
A&A 2006. version 2. relocated figures and tables. Language editor
suggestions. added reference
Linear perturbations for the vacuum axisymmetric Einstein equations
In axial symmetry, there is a gauge for Einstein equations such that the
total mass of the spacetime can be written as a conserved, positive definite,
integral on the spacelike slices. This property is expected to play an
important role in the global evolution. In this gauge the equations reduce to a
coupled hyperbolic-elliptic system which is formally singular at the axis. Due
to the rather peculiar properties of the system, the local in time existence
has proved to resist analysis by standard methods. To analyze the principal
part of the equations, which may represent the main source of the difficulties,
we study linear perturbation around the flat Minkowski solution in this gauge.
In this article we solve this linearized system explicitly in terms of integral
transformations in a remarkable simple form. This representation is well suited
to obtain useful estimates to apply in the non-linear case.Comment: 13 pages. We suppressed the statements about decay at infinity. The
proofs of these statements were incomplete. The complete proofs will require
extensive technical analysis. We will studied this in a subsequent work. We
also have rewritten the introduction and slighted changed the titl
Second and higher-order perturbations of a spherical spacetime
The Gerlach and Sengupta (GS) formalism of coordinate-invariant, first-order,
spherical and nonspherical perturbations around an arbitrary spherical
spacetime is generalized to higher orders, focusing on second-order
perturbation theory. The GS harmonics are generalized to an arbitrary number of
indices on the unit sphere and a formula is given for their products. The
formalism is optimized for its implementation in a computer algebra system,
something that becomes essential in practice given the size and complexity of
the equations. All evolution equations for the second-order perturbations, as
well as the conservation equations for the energy-momentum tensor at this
perturbation order, are given in covariant form, in Regge-Wheeler gauge.Comment: Accepted for publication in Physical Review
Universal Conductance Distributions in the Crossover between Diffusive and Localization Regimes
The full distribution of the conductance in quasi-one-dimensional
wires with rough surfaces is analyzed from the diffusive to the localization
regime. In the crossover region, where the statistics is dominated by only one
or two eigenchannels, the numerically obtained P(G) is found to be independent
of the details of the system with the average conductance as the only
scaling parameter. For < e^2/h, P(G) is given by an essentially
``one-sided'' log-normal distribution. In contrast, for e^2/h <= 2e^2/h,
the shape of P(G) remarkable agrees with those predicted by random matrix
theory for two fluctuating transmission eigenchannels.Comment: Accepted for publication in Phys. Rev. Let
Abundant Z-cyanomethanimine in the interstellar medium: paving the way to the synthesis of adenine
We report the first detection in the interstellar medium of the Z-isomer of
cyanomethanimine (HNCHCN), an HCN dimer proposed as precursor of adenine. We
identified six transitions of Z-cyanomethanimine, along with five transitions
of E-cyanomethanimine, using IRAM 30m observations towards the Galactic Center
quiescent molecular cloud G+0.693. The Z-isomer has a column density of
(2.00.6)10 cm and an abundance of
1.510. The relative abundance ratio between the isomers is
[Z/E]6. This value cannot be explained by the two chemical formation
routes previously proposed (gas-phase and grain surface), which predicts
abundances ratios between 0.9 and 1.5. The observed [Z/E] ratio is in good
agreement with thermodynamic equilibrium at the gas kinetic temperature
(130210 K). Since isomerization is not possible in the ISM, the two species
may be formed at high temperature. New chemical models, including surface
chemistry on dust grains and gas-phase reactions, should be explored to explain
our findings. Whatever the formation mechanism, the high abundance of Z-HNCHCN
shows that precursors of adenine are efficiently formed in the ISM.Comment: Accepted in Monthly Notices of the Royal Astronomical Society Letter
Complex organic molecules in the Galactic Centre: the N-bearing family
We present an unbiased spectral line survey toward the Galactic Centre (GC)
quiescent giant molecular cloud (QGMC), G+0.693 using the GBT and IRAM 30
telescopes. Our study highlights an extremely rich organic inventory of
abundant amounts of nitrogen (N)-bearing species in a source without signatures
of star formation. We report the detection of 17 N-bearing species in this
source, of which 8 are complex organic molecules (COMs). A comparison of the
derived abundances relative to H is made across various galactic and
extragalactic environments. We conclude that the unique chemistry in this
source is likely to be dominated by low-velocity shocks with X-rays/cosmic rays
also playing an important role in the chemistry. Like previous findings
obtained for O-bearing molecules, our results for N-bearing species suggest a
more efficient hydrogenation of these species on dust grains in G+0.693 than in
hot cores in the Galactic disk, as a consequence of the low dust temperatures
coupled with energetic processing by X-ray/cosmic ray radiation in the GC.Comment: 24 pages, 23 figures, 7 tables, accepted for publication in MNRA
Magnetic Field Effects on the Transport Properties of One-sided Rough Wires
We present a detailed numerical analysis of the effect of a magnetic field on
the transport properties of a `small-' one-sided surface disordered wire.
When time reversal symmetry is broken due to a magnetic field , we find a
strong increase with not only of the localization length but also of
the mean free path caused by boundary states. Despite this, the
universal relationship between and does hold. We also analyze the
conductance distribution at the metal-insulator crossover, finding a very good
agreement with Random Matrix Theory with two fluctuating channels within the
Circular Orthogonal(Unitary) Ensemble in absence(presence) of Comment: 5 pages, 4 figures, to appear in Phys. Rev.
The central parsecs of M87: jet emission and an elusive accretion disc
We present the first simultaneous spectral energy distribution (SED) of M87
core at a scale of 0.4 arcsec () across the electromagnetic
spectrum. Two separate, quiescent, and active states are sampled that are
characterized by a similar featureless SED of power-law form, and that are thus
remarkably different from that of a canonical active galactic nuclei (AGN) or a
radiatively inefficient accretion source. We show that the emission from a jet
gives an excellent representation of the core of M87 core covering ten orders
of magnitude in frequency for both the active and the quiescent phases. The
inferred total jet power is, however, one to two orders of magnitude lower than
the jet mechanical power reported in the literature. The maximum luminosity of
a thin accretion disc allowed by the data yields an accretion rate of , assuming 10% efficiency. This power
suffices to explain M87 radiative luminosity at the jet-frame, it is however
two to three order of magnitude below that required to account for the jet's
kinetic power. The simplest explanation is variability, which requires the core
power of M87 to have been two to three orders of magnitude higher in the last
200 yr. Alternatively, an extra source of power may derive from black hole
spin. Based on the strict upper limit on the accretion rate, such spin power
extraction requires an efficiency an order of magnitude higher than predicted
from magnetohydrodynamic simulations, currently in the few hundred per cent
range.Comment: 18 pages, 6 figures. Accepted for publication in MNRA
- âŠ