3,476 research outputs found
The Sizes of 1720 MHz OH Masers: VLBA and MERLIN Observations of the Supernova Remnants W44 and W28
We have used the NRAO Very Long Baseline Array (VLBA) to image OH(1720 MHz)
masers in the supernova remnants W28 and W44 at a resolution of 40 mas. We also
used MERLIN to observe the same OH(1720 MHz) masers in W44 at a resolution of
290 x 165 mas. All the masers are resolved by these VLBA and MERLIN
observations. The measured sizes range from 50 to 180 mas and yield brightness
temperature estimates from 0.3--20 x 10**8 K. We investigate whether these
measured angular sizes are intrinsic and hence originate as a result of the
physical conditions in the supernova remnant shock, or whether they are scatter
broadened sizes produced by the turbulent ionized gas along the line of sight.
While the current data on the temporal and angular broadening of pulsars,
masers and extragalactic soures toward W44 and W28 can be understood in terms
of scattering, we cannot rule out that these large sizes are intrinsic. Recent
theoretical modeling by Lockett et al. suggests that the physical parameters in
the shocked region are indicative of densities and OH abundances which lead to
estimates of sizes as large as what we measure. If the sizes and structure are
intrinsic, then the OH(1720 MHz) masrs may be more like the OH(1612 MHz) masers
in circumstellar shells than OH masers associated with HII regions. At two
locations in W28 we observe the classical S-shapes in the Stokes V profiles
caused by Zeeman splitting and use it to infer magnetic fields of order 2
milliGauss.Comment: 24 pages, 6 figures, accepted by Ap
Photoemission of a doped Mott insulator: spectral weight transfer and qualitative Mott-Hubbard description
The spectral weight evolution of the low-dimensional Mott insulator TiOCl
upon alkali-metal dosing has been studied by photoelectron spectroscopy. We
observe a spectral weight transfer between the lower Hubbard band and an
additional peak upon electron-doping, in line with quantitative expectations in
the atomic limit for changing the number of singly and doubly occupied sites.
This observation is an unconditional hallmark of correlated bands and has not
been reported before. In contrast, the absence of a metallic quasiparticle peak
can be traced back to a simple one-particle effect.Comment: 4 pages, 4 figures, related theoretical work can be found in
arXiv:0905.1276; shortene
The Radio Recovery of SN 1970G: The Continuing Radio Evolution of SN 1970G
Using the Very Large Array, we have detected radio emission from the site of
SN 1970G in the Sc galaxy M101. These observations are 31 years after the
supernova event, making SN 1970G the longest monitored radio supernova. With
flux densities of 0.12 +/- 0.020 mJy at 6 cm and 0.16 +/- 0.015 mJy at 20 cm,
the spectral index of -0.24 +/- 0.20 appears to have flattened somewhat when
compared with the previously reported value of -0.56 +/- 0.11, taken in 1990.
The radio emission at 20 cm has decayed since the 1990 observations with a
power-law index of beta_20cm = -0.28 +/- 0.13. We discuss the radio properties
of this source and compare them to those of other Type II radio supernovae.Comment: 11 pages, 1 table and 2 figures; To appear in Astrophysical Journal
Letter
Proper Motions of PSRs B1757-24 and B1951+32: Implications for Ages and Associations
Over the last decade, considerable effort has been made to measure the proper
motions of the pulsars B1757-24 and B1951+32 in order to establish or refute
associations with nearby supernova remnants and to understand better the
complicated geometries of their surrounding nebulae. We present proper motion
measurements of both pulsars with the Very Large Array, increasing the time
baselines of the measurements from 3.9 yr to 6.5 yr and from 12.0 yr to 14.5
yr, respectively, compared to previous observations. We confirm the
non-detection of proper motion of PSR B1757-24, and our measurement of (mu_a,
mu_d) = (-11 +/- 9, -1 +/- 15) mas yr^{-1} confirms that the association of PSR
B1757-24 with SNR G5.4-1.2 is unlikely for the pulsar characteristic age of
15.5 kyr, although an association can not be excluded for a significantly
larger age. For PSR B1951+32, we measure a proper motion of (mu_a, mu_d) =
(-28.8 +/- 0.9, -14.7 +/- 0.9) mas yr^{-1}, reducing the uncertainty in the
proper motion by a factor of two compared to previous results. After correcting
to the local standard of rest, the proper motion indicates a kinetic age of ~51
kyr for the pulsar, assuming it was born near the geometric center of the
supernova remnant. The radio-bright arc of emission along the pulsar proper
motion vector shows time-variable structure, but moves with the pulsar at an
approximately constant separation ~2.5", lending weight to its interpretation
as a shock structure driven by the pulsar.Comment: LaTeX file uses emulateapj.cls; 7 pages, 4 figures, to be published
ApJ February 10, 2008, v674 p271-278. Revision reflects journal formatting;
there are no substantial revision
Chromosome mapping: radiation hybrid data and stochastic spin models
This work approaches human chromosome mapping by developing algorithms for
ordering markers associated with radiation hybrid data. Motivated by recent
work of Boehnke et al. [1], we formulate the ordering problem by developing
stochastic spin models to search for minimum-break marker configurations. As a
particular application, the methods developed are applied to 14 human
chromosome-21 markers tested by Cox et al. [2]. The methods generate
configurations consistent with the best found by others. Additionally, we find
that the set of low-lying configurations is described by a Markov-like ordering
probability distribution. The distribution displays cluster correlations
reflecting closely linked loci.Comment: 26 Pages, uuencoded LaTex, Submitted to Phys. Rev. E,
[email protected], [email protected]
Cereal eelworm : a new disease of cereal crops in the Geraldton area
CEREAL eelworm (Heterodera avenae) is a potentially serious disease of crops which was seen in the Geraldton area for the first time in the 1966-67 season.
It was first identified in this State at Beverley in 1958 and later near Northam in 1963.
The Geraldton region has been the only area found to be affected since.
The disease was identified on eight properties in 1966-67, and a further ten properties in 1967- 68. There are indications that many more properties could be affected in varying degrees
The Structure of the Cold Neutral ISM on 10-100 Astronomical Unit Scales
We have used the Very Long Baseline Array (VLBA) and the Very Large Array
(VLA) to image Galactic neutral hydrogen in absorption towards four compact
extragalactic radio sources with 10 milliarcsecond resolution. Previous VLBA
data by Faison et al (1998) have shown the existence of prominent structures in
the direction of the extragalactic source 3C~138 with scale sizes of 10-20 AU
with changes in HI optical depth in excess of 0.8 0.1. In this paper we
confirm the small scale \hi optical depth variations toward 3C~147 suggested
earlier at a level up to 20 % 5% . The sources 3C~119, 2352+495 and
0831+557 show no significant change in \hi optical depth across the sources
with one sigma limits of 30%, 50%, and 100%. Of the seven sources recently
investigated with the VLBA and VLA, only 3C~138 and 3C~147 show statistically
significant variations in HI opacities.
Deshpande (2000) have attempted to explain the observed small-scale structure
as an extension of the observed power spectrum of structure on parsec size
scales. The predictions of Deshpande (2000) are consistent with the VLBA HI
data observed in the directions of a number of sources, including 3C~147, but
are not consistent with our previous observations of the HI opacity structure
toward 3C~138
PKS B1400-33: an unusual radio relic in a poor cluster
We present new arcminute resolution radio images of the low surface
brightness radio source PKS B1400-33 that is located in the poor cluster Abell
S753. The observations consist of 330 MHz VLA, 843 MHz MOST and 1398 and 2378
MHz ATCA data. These new images, with higher surface brightness sensitivity
than previous observations, reveal that the large scale structure consists of
extended filamentary emission bounded by edge-brightened rims. The source is
offset on one side of symmetrically distributed X-ray emission that is centered
on the dominant cluster galaxy NGC 5419. PKS B1400-33 is a rare example of a
relic in a poor cluster with radio properties unlike those of most relics and
halos observed in cluster environments.
The diffuse source appears to have had an unusual origin and we discuss
possible mechanisms. We examine whether the source could be re-energized relic
radio plasma or a buoyant synchrotron bubble that is a relic of activity in NGC
5419. The more exciting prospect is that the source is relic plasma preserved
in the cluster gaseous environment following the chance injection of a radio
lobe into the ICM as a result of activity in a galaxy at the periphery of the
cluster.Comment: 26 pages, 8 figures, accepted for publication in the Astronomical
Journa
Unified processing of constraints for interactive simulation
International audienceThis paper introduces a generic way of dealing with a set of different constraints (bilateral, unilateral, dry friction) in the context of interactive simulation. We show that all the mentioned constraints can be handled within a unified framework: we define the notion of generalized constraints, which can be derived into most classical constraints types. The solving method is based on an implicit treatment of constraints that provides good stability for interactive applications using deformable models and rigid bodies. Each constraint law is expressed in constraint subspace, making constraint evaluation much easier. A global solution is calculated using an iterative process that takes into account the mechanical coupling between the constraints. Various examples, from basic to more complex, show the practical advantage of using generalized constraints, as a way of creating heterogeneously constrained systems, as well as the scalability of the proposed method
One-pot access to L-5,6-dihalotryptophans and L-alknyltryptophans using tryptophan synthase
The authors thank the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013/ERC grant agreement no 614779, and the University of St Andrews for a studentship (to D. R. M. Smith).We report, for the first time, the use of tryptophan synthase in the generation of L- dihalotryptophans and L-alkynyltryptophans. These previously unpublished compounds will be useful tools in the generation of probes for chemical biology, in biosynthetic diversification and as convenient building blocks for synthesis.PostprintPeer reviewe
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