753 research outputs found
Anisotropic Thermal Conduction in Supernova Remnants: Relevance to Hot Gas Filling Factors in the Magnetized ISM
We explore the importance of anisotropic thermal conduction in the evolution
of supernova remnants via numerical simulations. The mean temperature of the
bubble of hot gas is decreased by a factor of ~3 compared to simulations
without thermal conduction, together with an increase in the mean density of
hot gas by a similar factor. Thus, thermal conduction greatly reduces the
volume of hot gas produced over the life of the remnant. This underscores the
importance of thermal conduction in estimating the hot gas filling fraction and
emissivities in high-stage ions in Galactic and proto-galactic ISMs.Comment: Submitted to Astrophysical Journal Letters. 4 pages, 3 figure
Thermal OH (1667/65 MHz) Absorption and Nonthermal OH (1720 MHz) Emission Towards the W28 Supernova Remnant
The W28 supernova remnant is an excellent prototype for observing shocked gas
resulting from the interaction of supernova remnants (SNRs) and adjacent
molecular clouds (MCs). We present two new signatures of shocked molecular gas
in this remnant. One is the detection of main-line extended OH (1667 MHz)
absorption with broad linewidths. The column density of OH estimated from the
optical depth profiles is consistent with a theoretical model in which
OH is formed behind a C-type shock front. The second is the detection of
extended, weak OH (1720 MHz) line emission with narrow linewidth distributed
throughout the shocked region of W28. These give observational support to the
idea that compact maser sources delineate the brightest component of a much
larger region of main line OH absorption and nonthermal OH (1720 MHz) emission
tracing the global structure of shocked molecular gas. Main line OH (1665/67)
absorption and extended OH (1720 MHz) emission line studies can serve as
powerful tools to detect SNR-MC interaction even when bright OH (1720 MHz)
masers are absent.Comment: 14 pages, 3 figures, one table, to appear in ApJ (Jan 10, 2003
Basic properties of three-leg Heisenberg tube
We study three-leg antiferromagnetic Heisenberg model with the periodic
boundary conditions in the rung direction. Since the rungs form regular
triangles, spin frustration is induced. We use the density-matrix
renormalization group method to investigate the ground state. We find that the
spin excitations are always gapped to remove the spin frustration as long as
the rung coupling is nonzero. We also visibly confirm spin-Peierls dimerization
order in the leg direction. Both the spin gap and the dimerization order are
basically enhanced as the rung coupling increases.Comment: 4 pages, 2 figure
Collective Antenna Effects in the Terahertz and Infrared Response of Highly Aligned Carbon Nanotube Arrays
We study macroscopically-aligned single-wall carbon nanotube arrays with
uniform lengths via polarization-dependent terahertz and infrared transmission
spectroscopy. Polarization anisotropy is extreme at frequencies less than
3 THz with no sign of attenuation when the polarization is perpendicular
to the alignment direction. The attenuation for both parallel and perpendicular
polarizations increases with increasing frequency, exhibiting a pronounced and
broad peak around 10 THz in the parallel case. We model the electromagnetic
response of the sample by taking into account both radiative scattering and
absorption losses. We show that our sample acts as an effective antenna due to
the high degree of alignment, exhibiting much larger radiative scattering than
absorption in the mid/far-infrared range. Our calculated attenuation spectrum
clearly shows a non-Drude peak at 10 THz in agreement with the
experiment.Comment: 5 pages, 5 figure
Antiholons in one-dimensional t-J models
Using a newly developed hybrid Monte Carlo algorithm for the nearest-neighbor
(n.n.) t-J model, we show that antiholons identified in the supersymmetric
inverse squared (IS) t-J model are clearly visible in the electron addition
spectrum of the n.n. t-J model at J=2t and also for J=0.5t, a value of
experimental relevance.Comment: 4 pages, 4 figure
Association of myosin with the connecting cilium of rod photoreceptors
The cilium of a vertebrate photoreceptor cell connects the phototransductive outer segment of the cell to the inner segment. Previous studies have shown that, within the connecting cilium, there is a small cluster of actin filaments, which play a critical role in the formation of new disk membranes. Here, we have detected a polypeptide in rat rod outer segments that is recognized by myosin heavy chain antibodies and was found to possess other characteristics of conventional non-muscle myosin heavy chain: it comigrates in SDS-PAGE with non-muscle myosin heavy chain; it associates with the cytoskeleton of rod outer segments in an ATP-sensitive manner; and it binds to purified actin filaments in the absence of ATP. Myosin ATPase activity was also detected in isolated rod outer segments. Electron immunomicroscopy revealed that myosin is present in the small actin-containing domain within the connecting cilium at the site of disk membrane morphogenesis. These results pose the possibility that an actin-myosin contractile mechanism functions in the formation of new photoreceptor disk membranes
A Survey of Hydroxyl Toward Supernova Remnants: Evidence for Extended 1720 MHz Maser Emission
We present the results of GBT observations of all four ground-state hydroxyl
(OH) transitions toward 15 supernova remnants (SNRs) which show OH(1720 MHz)
maser emission. This species of maser is well established as an excellent
tracer of an ongoing interaction between the SNR and dense molecular material.
For the majority of these objects we detect significantly higher flux densities
with a single dish than has been reported with interferometric observations. We
infer that spatially extended, low level maser emission is a common phenomenon
that traces the large-scale interaction in maser-emitting SNRs. Additionally we
use a collisional pumping model to fit the physical conditions under which OH
is excited behind the SNR shock front. We find the observed OH gas associated
with the SNR interaction having columns less than approximately 10^17 per
square cm, temperatures of 20 to 125 K, and densities 10^5 per cubic cm.Comment: 24 pages, 23 figures, Accepted to ApJ, March 26, 2008; v2 - added
Figure 6, minor clarifications to text in Sections 3 and
Molecular and Ionic shocks in the Supernova Remnant 3C391
New observations of the supernova remnant 3C391 are in the H2 2.12 micron and
[Fe II] 1.64 micron narrow-band filters at the Palomar 200-inch telescope, and
in the 5-15 micron CVF on ISOCAM. Shocked H2 emission was detected from the
region 3C391:BML, where broad millimeter CO and CS lines had previously been
detected. A new H2 clump was confirmed to have broad CO emission, demonstrating
that the near-infrared H2 images can trace previously undetected molecular
shocks. The [Fe II] emission has a significantly different distribution, being
brightest in the bright radio bar, at the interface between the supernova
remnant and the giant molecular cloud, and following filaments in the radio
shell. The near-infrared [Fe II] and the mid-infrared 12-18 micron filter
images are the first images to reveal the radiative shell of 3C391. The
mid-infrared spectrum is dominated by bright ionic lines and H2 S(2) through
S(7). There are no aromatic hydrocarbons associated with the shocks, nor is
their any mid-infrared continuum, suggesting that macromolecules and very small
grains are destroyed. Comparing 3C391 to the better-studied IC443, both
remnants have molecular- and ionic-dominated regions; for 3C391, the
ionic-dominated region is the interface into the giant molecular cloud, showing
that the main bodies of giant molecular clouds contain significant regions with
densities 100 to 1000/cm^3 and a small filling factor with higher-density. The
molecular shocked region resolves into 16 clumps of H2 emission, with some
fainter diffuse emission but with no associated near-infrared continuum
sources. One of the clumps is coincident with a previously-detected OH 1720 MHz
maser. These clumps are interpreted as a cluster of pre-stellar, dense
molecular cores that are presently being shocked by the supernova blast wave
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