1,161 research outputs found
Letter from the Executive Committee of the Alumni Association
Letter concerning copies of a paper and a letter to be distributed at Utah Agricultural College
Simulation and analysis of solenoidal ion sources
We present a detailed analysis and simulation of solenoidal, magnetically confined electron bombardment ion sources, aimed at molecular beam detection. The aim is to achieve high efficiency for singly ionized species while minimizing multiple ionization. Electron space charge plays a major role and we apply combined ray tracing and finite element simulations to determine the properties of a realistic geometry. The factors controlling electron injection and ion extraction are discussed. The results from simulations are benchmarked against experimental measurements on a prototype source
On realizing diagrams of Pi-algebras
Given a diagram of Pi-algebras (graded groups equipped with an action of the
primary homotopy operations), we ask whether it can be realized as the homotopy
groups of a diagram of spaces. The answer given here is in the form of an
obstruction theory, of somewhat wider application, formulated in terms of
generalized Pi-algebras. This extends a program begun in [J. Pure Appl. Alg.
103 (1995) 167-188] and [Topology 43 (2004) 857-892] to study the realization
of a single Pi-algebra. In particular, we explicitly analyze the simple case of
a single map, and provide a detailed example, illustrating the connections to
higher homotopy operations.Comment: This is the version published by Algebraic & Geometric Topology on 21
June 200
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
table
Fine structure of alpha decay in odd nuclei
Using an alpha decay level scheme, an explanation for the fine structure in
odd nuclei is evidenced by taking into account the radial and rotational
couplings between the unpaired nucleon and the core of the decaying system. It
is stated that the experimental behavior of the alpha decay fine structure
phenomenon is directed by the dynamical characteristics of the system.Comment: 8 pages, 3 figures, REVTex, submitted to Physical Review
Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. II. Mixed Trends in VB10 and LSR1835+32 and the Possible Role of Rotation
[Abridged] As part of our on-going investigation of magnetic activity in
ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical
observations of LSR1835+32 (M8.5), and simultaneous X-ray and UV observations
of VB10 (M8), both with a duration of about 9 hours. LSR1835+32 exhibits
persistent radio emission and H-alpha variability on timescales of ~0.5-2 hr.
The detected UV flux is consistent with photospheric emission, and no X-ray
emission is detected to a deep limit of L_X/L_bol<10^-5.7. The H-alpha and
radio emission are temporally uncorrelated, and the ratio of radio to X-ray
luminosity exceeds the correlation seen in F-M6 stars by >2x10^4. Similarly,
L_Halpha/L_X>10 is at least 30 times larger than in early M dwarfs, and
eliminates coronal emission as the source of chromospheric heating. The lack of
radio variability during four rotations of LSR1835+32 requires a uniform
stellar-scale field of ~10 G, and indicates that the H-alpha variability is
dominated by much smaller scales, <10% of the chromospheric volume. VB10, on
the other hand, shows correlated flaring and quiescent X-ray and UV emission,
similar to the behavior of early M dwarfs. Delayed and densely-sampled optical
spectra exhibit a similar range of variability amplitudes and timescales to
those seen in the X-rays and UV, with L_Halpha/L_X~1. Along with our previous
observations of the M8.5 dwarf TVLM513-46546 we conclude that late M dwarfs
exhibit a mix of activity patterns, which points to a transition in the
structure and heating of the outer atmosphere by large-scale magnetic fields.
We find that rotation may play a role in generating the fields as evidenced by
a tentative correlation between radio activity and rotation velocity. The X-ray
emission, however, shows evidence for super-saturation at vsini>25 km/s.Comment: Submitted to Ap
Unlocking new contrast in a scanning helium microscope.
Delicate structures (such as biological samples, organic films for polymer electronics and adsorbate layers) suffer degradation under the energetic probes of traditional microscopies. Furthermore, the charged nature of these probes presents difficulties when imaging with electric or magnetic fields, or for insulating materials where the addition of a conductive coating is not desirable. Scanning helium microscopy is able to image such structures completely non-destructively by taking advantage of a neutral helium beam as a chemically, electrically and magnetically inert probe of the sample surface. Here we present scanning helium micrographs demonstrating image contrast arising from a range of mechanisms including, for the first time, chemical contrast observed from a series of metal-semiconductor interfaces. The ability of scanning helium microscopy to distinguish between materials without the risk of damage makes it ideal for investigating a wide range of systems.This research was supported under the Australian Research Councils Discovery Projects (Project No. DP08831308) funding scheme. Postgraduate research scholarships (M.B., A.F.) from the University of Newcastle gratefully acknowledged. We thank the Newcastle and Cavendish workshops, Donald MacLaren and Kane O’Donnell for technical support, insightful discussions and assistance. This work was performed in part at both the Materials and ACT nodes of the Australian National Fabrication Facility, which is a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia’s researchers.This is the final version of the article. It was first available from NPG via http://dx.doi.org/10.1038/ncomms1018
Magnetic topologies of cool stars
Stellar magnetic fields can be investigated using several, very complementary
approaches. While conventional spectroscopy is capable of estimating the
average magnetic strength of potentially complex field configurations thanks to
its low sensitivity to the vector properties of the field, spectropolarimetry
can be used to map the medium- and large-scale structure of magnetic
topologies. In particular, the latter approach allows one to retrieve
information about the poloidal and toroidal components of the large-scale
dynamo fields in low-mass stars, and thus to investigate the physical processes
that produce them. Similarly, this technique can be used to explore how
magnetic fields couple young stars to their massive accretion disc and thus to
estimate how much mass and angular momentum are transfered to the newly-born
low-mass star. We present here the latest results in this field obtained with
spectropolarimetry, with special emphasis on the surprising discoveries
obtained on very-low mass fully-convective stars and classical T Tauri stars
thanks to the ESPaDOnS spectropolarimeter recently installed on the 3.6m
Canada-France-Hawaii Telescope.Comment: 10p invited review paper, 3 figures, to be published in the
proceedings of the 14th Cambridge Workshop on Cool Stars, Stellar Systems,
and the Sun, November 6-10, 2006, ed. G. van Belle (ASP Conf Ser
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