1,664 research outputs found
Spitzer 24 um Images of Planetary Nebulae
Spitzer MIPS 24 um images were obtained for 36 Galactic planetary nebulae
(PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective
temperatures of ~100,000 K or higher. Diffuse 24 um emission is detected in 28
of these PNe. The eight non-detections are angularly large PNe with very low
H-alpha surface brightnesses. We find three types of correspondence between the
24 um emission and H-alpha line emission of these PNe: six show 24 um emission
more extended than H-alpha emission, nine have a similar extent at 24 um and
H-alpha, and 13 show diffuse 24 um emission near the center of the H-alpha
shell. The sizes and surface brightnesses of these three groups of PNe and the
non-detections suggest an evolutionary sequence, with the youngest ones being
brightest and the most evolved ones undetected. The 24 um band emission from
these PNe is attributed to [O IV] 25.9 um and [Ne V] 24.3 um line emission and
dust continuum emission, but the relative contributions of these three
components depend on the temperature of the central star and the distribution
of gas and dust in the nebula.Comment: 24 pages, 8 figures, to appear in the Astronomical Journal, September
issue. Relace previous file; two references are added and typos are correcte
Graph-theory induced gravity and strongly-degenerate fermions in a self-consistent Einstein universe
We study UV-finite theory of induced gravity. We use scalar fields, Dirac
fields and vector fields as matter fields whose one-loop effects induce the
gravitational action. To obtain the mass spectrum which satisfies the
UV-finiteness condition, we use a graph-based construction of mass matrices.
The existence of a self-consistent static solution for an Einstein universe is
shown in the presence of degenerate fermion.Comment: 16pages, 1figur
Resource Competition Among the Uinta Basin Fremont
Archaeologists describe the Uinta Fremont (A.D. 0 – 1300) as a mixed foraging-farming society that underwent a dramatic social change from A.D. 700 – 1000. Researchers observe through different architectural styles and subsistence activity a change from large, aggregated settlements to more dispersed and defensively oriented villages and hamlets. The Ideal Free Distribution (IFD) model provides an explanatory framework through which to interpret these changes. IFD predicts the order in which people or animals will occupy habitats based on a habitat’s relative suitability and suggests hypothetical behaviors that people or animals might engage in to improve or maintain the relative suitability of a habitat. One prediction of IFD is that behaviors indicating resource competition will become more frequent when population density increases. I test whether this hypothesis explains changes in storage features by considering storage behavior as a manifestation of resource competition, and I investigate whether storage feature frequency correlates with periods of Fremont population increases and paleoenvironmental degradation.
These tests explain aspects of Fremont culture change and suggest future research possibilities. Storage feature frequency, representing resource competition, remains low from A.D. 0 – 700, suggesting that the habitats could absorb growing Fremont populations. After A.D. 700, however, resource competition rose and remained high, a condition that likely spurred the defensive architecture and dispersed settlements that became increasingly common after A.D. 1000. The successes and limitations of applying IFD to the archaeological record point the way toward future uses of the model to investigate settlement spacing and reaffirm the use of radiocarbon data in archaeological science
Maxwell-Chern-Simons vortices in a CPT-odd Lorentz-violating Higgs Electrodynamics
We have studied BPS vortices in a CPT-odd and Lorentz-violating
Maxwell-Chern-Simons-Higgs (MCSH) electrodynamics attained from the dimensional
reduction of the Carroll-Field-Jackiw-Higgs model. The Lorentz-violating
parameter induces a pronounced behavior at origin (for the magnetic/electric
fields and energy density) which is absent in the MCSH vortices. For some
combination of the Lorentz-violating coefficients there always exist a
sufficiently large winding number such that for all
the magnetic field flips its signal, yielding two well defined regions with
opposite magnetic flux. However, the total magnetic flux remains quantized and
proportional to the winding number.Comment: Revtex style, 8 page
Bose Einstein condensation on inhomogeneous amenable graphs
We investigate the Bose-Einstein Condensation on nonhomogeneous amenable
networks for the model describing arrays of Josephson junctions. The resulting
topological model, whose Hamiltonian is the pure hopping one given by the
opposite of the adjacency operator, has also a mathematical interest in itself.
We show that for the nonhomogeneous networks like the comb graphs, particles
condensate in momentum and configuration space as well. In this case different
properties of the network, of geometric and probabilistic nature, such as the
volume growth, the shape of the ground state, and the transience, all play a
role in the condensation phenomena. The situation is quite different for
homogeneous networks where just one of these parameters, e.g. the volume
growth, is enough to determine the appearance of the condensation.Comment: 43 pages, 12 figures, final versio
Long beating wavelength in the Schwarz-Hora effect
Thirty years ago, H.Schwarz has attempted to modulate an electron beam with
optical frequency. When a 50-keV electron beam crossed a thin crystalline
dielectric film illuminated with laser light, electrons produced the
electron-diffraction pattern not only at a fluorescent target but also at a
nonfluorescent target. In the latter case the pattern was of the same color as
the laser light (the Schwarz-Hora effect). This effect was discussed
extensively in the early 1970s. However, since 1972 no reports on the results
of further attempts to repeat those experiments in other groups have appeared,
while the failures of the initial such attempts have been explained by Schwarz.
The analysis of the literature shows there are several unresolved up to now
contradictions between the theory and the Schwarz experiments. In this work we
consider the interpretation of the long-wavelength spatial beating of the
Schwarz-Hora radiation. A more accurate expression for the spatial period has
been obtained, taking into account the mode structure of the laser field within
the dielectric film. It is shown that the discrepancy of more than 10% between
the experimental and theoretical results for the spatial period cannot be
reduced by using the existing quantum models that consider a collimated
electron beam.Comment: 3 pages, RevTe
Infrared Observations of the Helix Planetary Nebula
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRAC instrument on the Spitzer Space Telescope. The Helix is one of the closest bright PNs and therefore provides an opportunity to resolve the small-scale structure in the nebula. The emission from this PN in the 5.8 and 8 μm IRAC bands is dominated by the pure rotational lines of molecular hydrogen, with a smaller contribution from forbidden line emission such as [Ar III] in the ionized region. The IRAC images resolve the "cometary knots," which have been previously studied in this PN. The "tails" of the knots and the radial rays extending into the outer regions of the PN are seen in emission in the IRAC bands. IRS spectra on the main ring and the emission in the IRAC bands are consistent with shock-excited H_2 models, with a small (~10%) component from photodissociation regions. In the northeast arc, the H_2 emission is located in a shell outside the Hα emission
Optical and Mid-Infrared Observations of the Planetary Nebula NGC 6781
Although the planetary nebula NGC 6781 appears to possess an elliptical
morphology, its kinematic and emission characteristics are in many ways
unusual, and it is possible that it may represent a bipolar source oriented
close to the line of sight. We shall present deep imaging of this nebula in [O
III], Ha and [N II], and using broad-band (F555W and F814W) filters. These were
taken with the 2.56-m Nordic Optical Telescope and Hubble Space Telescope. This
is combined with mid-infrared (MIR) imaging and spectroscopy acquired with the
Spitzer Space Telescope (Spitzer), and near-infrared spectroscopy deriving from
the Infrared Space Observatory (ISO). These reveal details of the complex [N
II] structure associated with extended shell emission, perhaps associated with
highly inclined bipolar lobes. We also note the presence of narrow absorbing
filaments and clumps projected against the surface of the envelope, components
which may be responsible for much of the molecular emission. We point out that
such clumps may be responsible for complex source structure in the MIR, and
give rise to asymmetries in emission along the major axis of the source.Comment: 12 pages, 10 figures, Accepted for publication in MNRAS. 32 pages in
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