1,411 research outputs found
Analysis of CMB foregrounds using a database for Planck
Within the scope of the Planck IDIS (Integrated Data Information System)
project we have started to develop the data model for time-ordered data and
full-sky maps. The data model is part of the Data Management Component (DMC), a
software system designed according to a three-tier architecture which allows
complete separation between data storage and processing. The DMC is already
being used for simulation activities and the modeling of some foreground
components. We have ingested several Galactic surveys into the database and
used the science data-access interface to process the data. The data structure
for full-sky maps utilises the HEALPix tessellation of the sphere. We have been
able to obtain consistent measures of the angular power spectrum of the
Galactic radio continuum emission between 408 MHz and 2417 MHz.Comment: 7 pages, 6 figures. To appear in the Proceedings of the MPA/ESO/MPE
Joint Astronomy Conference "Mining The Sky
Looking the void in the eyes - the kSZ effect in LTB models
As an alternative explanation of the dimming of distant supernovae it has
recently been advocated that we live in a special place in the Universe near
the centre of a large void described by a Lemaitre-Tolman-Bondi (LTB) metric.
The Universe is no longer homogeneous and isotropic and the apparent late time
acceleration is actually a consequence of spatial gradients in the metric. If
we did not live close to the centre of the void, we would have observed a
Cosmic Microwave Background (CMB) dipole much larger than that allowed by
observations. Hence, until now it has been argued, for the model to be
consistent with observations, that by coincidence we happen to live very close
to the centre of the void or we are moving towards it. However, even if we are
at the centre of the void, we can observe distant galaxy clusters, which are
off-centre. In their frame of reference there should be a large CMB dipole,
which manifests itself observationally for us as a kinematic Sunyaev-Zeldovich
(kSZ) effect. kSZ observations give far stronger constraints on the LTB model
compared to other observational probes such as Type Ia Supernovae, the CMB, and
baryon acoustic oscillations. We show that current observations of only 9
clusters with large error bars already rule out LTB models with void sizes
greater than approximately 1.5 Gpc and a significant underdensity, and that
near future kSZ surveys like the Atacama Cosmology Telescope, South Pole
Telescope, APEX telescope, or the Planck satellite will be able to strongly
rule out or confirm LTB models with giga parsec sized voids. On the other hand,
if the LTB model is confirmed by observations, a kSZ survey gives a unique
possibility of directly reconstructing the expansion rate and underdensity
profile of the void.Comment: 20 pages, 9 figures, submitted to JCA
Probing the last scattering surface through the recent and future CMB observations
We have constrained the extended (delayed and accelerated) models of hydrogen
recombination, by investigating associated changes of the position and the
width of the last scattering surface. Using the recent CMB and SDSS data, we
find that the recent data constraints favor the accelerated recombination
model, though the other models (standard, delayed recombination) are not ruled
out at 1- confidence level. If the accelerated recombination had
actually occurred in our early Universe, baryonic clustering on small-scales is
likely to be the cause of it. By comparing the ionization history of baryonic
cloud models with that of the best-fit accelerated recombination model, we find
that some portion of our early Universe has baryonic underdensity. We have made
the forecast on the PLANCK data constraint, which shows that we will be able to
rule out the standard or delayed recombination models, if the recombination in
our early Universe had proceeded with or lower, and
residual foregrounds and systematic effects are negligible.Comment: v2: matched with the accepted version (conclusions unchanged
H_2 Pure Rotational Lines in the Orion Bar
Using the Texas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission
in the H_2 v = 0-0 S(1) and S(2) lines toward the Orion Bar PDR at 2"
resolution. We also observed H_2 v = 0-0 S(4) at selected points toward the
front of the PDR. Our maps cover a 12" by 40" region of the bar where H_2
ro-vibrational lines are bright. The distributions of H_2 0-0 S(1), 0-0 S(2),
and 1-0 S(1) line emission agree in remarkable detail.
The high spatial resolution (0.002 pc) of our observations allows us to probe
the distribution of warm gas in the Orion Bar to a distance approaching the
scale length for FUV photon absorption. We use these new observational results
to set parameters for the PDR models described in a companion paper (Draine et
al. 2005, in prep). The best-fit model can account for the separation of the
H_2 emission from the ionization front and the intensities of the ground state
rotational lines as well as the 1-0 S(1) and 2-1 S(1) lines. This model
requires significant adjustments to the commonly used values for the dust UV
attenuation cross section and the photoelectric heating rate.Comment: 35 pages, 6 figures ApJ, accepte
Theorems on shear-free perfect fluids with their Newtonian analogues
In this paper we provide fully covariant proofs of some theorems on
shear-free perfect fluids. In particular, we explicitly show that any
shear-free perfect fluid with the acceleration proportional to the vorticity
vector (including the simpler case of vanishing acceleration) must be either
non-expanding or non-rotating. We also show that these results are not
necessarily true in the Newtonian case, and present an explicit comparison of
shear-free dust in Newtonian and relativistic theories in order to see where
and why the differences appear.Comment: 23 pages, LaTeX. Submitted to GR
Ammoniated electron as a solvent stabilized multimer radical anion
The excess electron in liquid ammonia ("ammoniated electron") is commonly
viewed as a cavity electron in which the s-type wave function fills the
interstitial void between 6-9 ammonia molecules. Here we examine an alternative
model in which the ammoniated electron is regarded as a solvent stabilized
multimer radical anion, as was originally suggested by Symons [Chem. Soc. Rev.
1976, 5, 337]. In this model, most of the excess electron density resides in
the frontier orbitals of N atoms in the ammonia molecules forming the solvation
cavity; a fraction of this spin density is transferred to the molecules in the
second solvation shell. The cavity is formed due to the repulsion between
negatively charged solvent molecules. Using density functional theory
calculations for small ammonia cluster anions in the gas phase, it is
demonstrated that such core anions would semi-quantitatively account for the
observed pattern of Knight shifts for 1-H and 14-N nuclei observed by NMR
spectroscopy and the downshifted stretching and bending modes observed by
infrared spectroscopy. It is speculated that the excess electrons in other
aprotic solvents (but not in water and alcohols) might be, in this respect,
analogous to the ammoniated electron, with substantial transfer of the spin
density into the frontier N and C orbitals of methyl, amino, and amide groups
forming the solvation cavity.Comment: 34 pages, 12 figures; to be submitted to J Phys Chem
Thermophysics Issues Relevant to High-Speed Earth Entry of Large Asteroids
Physics of atmospheric entry of meteoroids was an active area of research at NASA ARC up to the early 1970s (e.g., the oft-cited work of Baldwin and Sheaffer). However, research in the area seems to have ended with the Apollo program, and any ties with an active international meteor physics community seem to have significantly diminished thereafter. In the decades following the 1970s, the focus of entry physics at NASA ARC has been on improvement of the math models of shock-layer physics (especially in chemical kinetics and radiation) and thermal response of ablative materials used for capsule heatshields. With the overarching objectives of understanding energy deposition into the atmosphere and fragmentation, could these modern analysis tools and processes be applied to the problem of atmospheric entry of meteoroids as well? In the presentation we will explore: (i) the physics of atmospheric entries of meteoroids using our current state-of-the-art tools and processes, (ii) how multiple bodies interact, and (iii) the influence of wall blowing on flow dynamics
Infrared nanoscopy of Dirac plasmons at the graphene-SiO2 interface
We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac
fermions in graphene. This is achieved by confining mid-IR radiation at the
apex of a nanoscale tip: an approach yielding two orders of magnitude increase
in the value of in-plane component of incident wavevector q compared to free
space propagation. At these high wavevectors, the Dirac plasmon is found to
dramatically enhance the near-field interaction with mid-IR surface phonons of
SiO2 substrate. Our data augmented by detailed modeling establish graphene as a
new medium supporting plasmonic effects that can be controlled by gate voltage.Comment: 12 pages, 4 figure
- …