4,121 research outputs found
The 8-13 micron observations of Titan
Narrow band observations of Titan at selected wavelengths in the 8-13 micron range show evidence for a strong temperature inversion and the existence of at least one more spectroscopically active component in the atmosphere in addition to H2 and CH4
Infrared Observations of Late Type Stars
Substantive mass loss resulting in appreciable circumstellar dust envelopes is common in late-type stars. The evolutionary history and physical state of a cool star determine the chemistry within the outer stellar atmosphere mirrored by the molecular and particulate material present in the envelope. The observational consequences of this debris determined by moderate spectral resolution infrared spectrophotometry are reviewed. Significant information is provided by observations of the emergent energy flux of both the cool stellar photosphere and of the circumstellar dust envelope. The observation suggests that mass-loss occurs to some degree throughout late stellar evolutionary phases and that occasional periods of high mass loss are not uncommon
Multi-qubit compensation sequences
The Hamiltonian control of n qubits requires precision control of both the
strength and timing of interactions. Compensation pulses relax the precision
requirements by reducing unknown but systematic errors. Using composite pulse
techniques designed for single qubits, we show that systematic errors for n
qubit systems can be corrected to arbitrary accuracy given either two
non-commuting control Hamiltonians with identical systematic errors or one
error-free control Hamiltonian. We also examine composite pulses in the context
of quantum computers controlled by two-qubit interactions. For quantum
computers based on the XY interaction, single-qubit composite pulse sequences
naturally correct systematic errors. For quantum computers based on the
Heisenberg or exchange interaction, the composite pulse sequences reduce the
logical single-qubit gate errors but increase the errors for logical two-qubit
gates.Comment: 9 pages, 5 figures; corrected reference formattin
An inquiry-based learning approach to teaching information retrieval
The study of information retrieval (IR) has increased in interest and importance with the explosive growth of online information in recent years. Learning about IR within formal courses of study enables users of search engines to use
them more knowledgeably and effectively, while providing the starting point for the explorations of new researchers into novel search technologies. Although IR can be taught in a traditional manner of formal classroom instruction with students being led through the details of the subject and expected to reproduce this in assessment, the nature of IR as a topic makes it an ideal subject for inquiry-based learning approaches to teaching. In an inquiry-based learning approach students are introduced to the principles of a subject and then encouraged to develop their understanding by solving structured or open problems. Working through solutions in subsequent class discussions enables students to appreciate the availability of alternative solutions as proposed by their classmates. Following this approach students not only learn the details of IR techniques, but significantly, naturally learn to apply them in solution of problems. In doing this they not only gain an appreciation of alternative solutions to a problem, but also how to assess their relative strengths and weaknesses. Developing confidence and skills in problem solving enables student assessment to be structured around solution of problems. Thus students can be assessed on the basis of their understanding and ability to apply techniques, rather simply their skill at reciting facts. This has the additional benefit of encouraging general problem solving skills which can be of benefit in other subjects. This approach to teaching IR was successfully implemented in an undergraduate module where students were
assessed in a written examination exploring their knowledge and understanding of the principles of IR and their ability to apply them to solving problems, and a written assignment based on developing an individual research proposal
Non-Equilibrium Dynamics and Superfluid Ring Excitations in Binary Bose-Einstein Condensates
We revisit a classic study [D. S. Hall {\it et al.}, Phys. Rev. Lett. {\bf
81}, 1539 (1998)] of interpenetrating Bose-Einstein condensates in the
hyperfine states and of Rb and observe striking new non-equilibrium
component separation dynamics in the form of oscillating ring-like structures.
The process of component separation is not significantly damped, a finding that
also contrasts sharply with earlier experimental work, allowing a clean first
look at a collective excitation of a binary superfluid. We further demonstrate
extraordinary quantitative agreement between theoretical and experimental
results using a multi-component mean-field model with key additional features:
the inclusion of atomic losses and the careful characterization of trap
potentials (at the level of a fraction of a percent).Comment: 4 pages, 3 figures (low res.), to appear in PR
On the Asymmetries of Extended X-ray Emission from Planetary Nebulae
Chandra X-ray Observatory (CXO) images have revealed that the X-ray emitting
regions of the molecule-rich young planetary nebulae (PNs) BD+30 3639 (BD+30)
and NGC 7027 are much more asymmetric than their optical nebulosities. To
evaluate the potential origins of these X-ray asymmetries, we analyze X-ray
images of BD+30, NGC 7027, and another planetary nebula resolved by CXO, NGC
6543, within specific energy bands. Image resolution has been optimized by
sub-pixel repositioning of individual X-ray events. The resulting
subarcsecond-resolution images reveal that the soft (E < 0.7 keV) X-ray
emission from BD+30 is more uniform than the harder emission, which is largely
confined to the eastern rim of the optical nebula. In contrast, soft X-rays
from NGC 7027 are highly localized and this PN is more axially symmetric in
harder emission. The broad-band X-ray morphologies of BD+30 and NGC 7027 are
highly anticorrelated with their distributions of visual extinction, as
determined from high-resolution, space- and ground-based optical and infrared
imaging. Hence, it is likely that the observed X-ray asymmetries of these
nebulae are due in large part to the effects of nonuniform intranebular
extinction. However, the energy-dependent X-ray structures in both nebulae and
in NGC 6543 -- which is by far the least dusty and molecule-rich of the three
PNs, and displays very uniform intranebular extinction -- suggests that other
mechanisms, such as the action of collimated outflows and heat conduction, are
also important in determining the detailed X-ray morphologies of young
planetary nebulae.Comment: 33 pages, 12 figures; to appear in the Astrophysical Journa
Mid-infrared spectra of late-type stars: Long-term evolution
Recent ground-based mid-infrared spectra of 29 late-type stars, most with
substantial dust shells, are compared to ground-based spectra of these stars
from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra
of about half the stars show no detectable changes, implying that their
distributions of circumstellar material and associated dust grain properties
have changed little over this time interval. However, many of the stars with
strong silicate features showed marked changes. In nearly all cases the
silicate peak has strengthened with respect to the underlying continuum,
although there is one case (VY~CMa) in which the silicate feature has almost
completely disappeared. This suggests that, in general, an oxygen-rich star
experiences long periods of gradual silicate feature strengthening, punctuated
by relatively rare periods when the feature weakens. We discuss various
mechanisms for producing the changes, favoring the slow evolution of the
intrinsic dust properties (i.e., the chemical composition or grain structure).
Although most IRAS spectra agree well with ground-based spectra, there are a
number of cases where they fall well outside the expected range of uncertainty.
In almost all such cases the slopes of the red and blue LRS spectra do not
match in their region of overlap.Comment: Accepted in ApJ, 20 pages, 5 figures, 1 tabl
Jazz Baby / music by M. K. Jerome; words by Blanche Merrill
Cover: drawing of Caucasian couples dancing in at a Ball; Publisher: Waterson Berlin and Snyder Co. (New York)https://egrove.olemiss.edu/sharris_c/1167/thumbnail.jp
Jazz Baby / music by M. K. Jerome; words by Blanche Merrill
Cover: drawing of well-dressed Caucasian couples dancing; Publisher: Waterson Berlin and Snyder Co. (New York)https://egrove.olemiss.edu/sharris_c/1161/thumbnail.jp
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