314 research outputs found
Compactification, topology change and surgery theory
We study the process of compactification as a topology change. It is shown
how the mediating spacetime topology, or cobordism, may be simplified through
surgery. Within the causal Lorentzian approach to quantum gravity, it is shown
that any topology change in dimensions may be achieved via a causally
continuous cobordism. This extends the known result for 4 dimensions.
Therefore, there is no selection rule for compactification at the level of
causal continuity. Theorems from surgery theory and handle theory are seen to
be very relevant for understanding topology change in higher dimensions.
Compactification via parallelisable cobordisms is particularly amenable to
study with these tools.Comment: 1+19 pages. LaTeX. 9 associated eps files. Discussion of disconnected
case adde
A Spectroscopic Orbit for Regulus
We present a radial velocity study of the rapidly rotating B-star Regulus
that indicates the star is a single-lined spectroscopic binary. The orbital
period (40.11 d) and probable semimajor axis (0.35 AU) are large enough that
the system is not interacting at present. However, the mass function suggests
that the secondary has a low mass (M_2 > 0.30 M_sun), and we argue that the
companion may be a white dwarf. Such a star would be the remnant of a former
mass donor that was the source of the large spin angular momentum of Regulus
itself.Comment: 18 pages, 2 figures, ApJL in pres
Measurement of Muscular Activity Associated With Peristalsis in the Human Gut Using Fiber Bragg Grating Arrays
Author version made available under Publisher copyright policy.Diagnostic catheters based on fibre Bragg gratings (FBG’s) are proving to be highly effective for measurement of the muscular activity associated with peristalsis in the human gut. The primary muscular contractions that generate peristalsis are circumferential in nature; however, it has long been known that there is also a component of longitudinal contractility present, acting in harmony with the circumferential component to improve the overall efficiency of material movement. We report on the development of, and latest results from, catheter based sensors capable of detecting both forms of muscular activity. While detection of the circumferential contractions has been possible using solid state, hydraulic, and pneumatic sensor arrays in the oesophagus and anorectum, FBG based devices allow access into the complex and convoluted regions of the gut below the stomach. We report early results from FBG catheters used during trials of novel therapies in patients with both slow transit constipation and faecal incontinence. In addition, there have been relatively few reports on the measurement or inference of longitudinal contractions in humans. This is due to the lack of a viable recording technique suitable for real-time in-vivo measurement of this type of activity over extended lengths of the gut. We report preliminary data on the detection of longitudinal motion in lengths of excised mammalian colon using an FBG technique that should be viable for similar detection in humans. The longitudinal sensors have been combined with pressure sensing elements to form a composite catheter that allows the relative phase between the two components to be detected. The output of both types of catheter has been validated using digital video mapping in an ex-vivo animal preparation using lengths of rabbit ileum
A handlebody calculus for topology change
We consider certain interesting processes in quantum gravity which involve a
change of spatial topology. We use Morse theory and the machinery of
handlebodies to characterise topology changes as suggested by Sorkin. Our
results support the view that that the pair production of Kaluza-Klein
monopoles and the nucleation of various higher dimensional objects are allowed
transitions with non-zero amplitude.Comment: Latex, 32 pages, 7 figure
The JCMT Gould Belt Survey: Evidence for radiative heating in Serpens MWC 297 and its influence on local star formation
We present SCUBA-2 450micron and 850micron observations of the Serpens MWC
297 region, part of the JCMT Gould Belt Survey of nearby star-forming regions.
Simulations suggest that radiative feedback influences the star-formation
process and we investigate observational evidence for this by constructing
temperature maps. Maps are derived from the ratio of SCUBA-2 fluxes and a two
component model of the JCMT beam for a fixed dust opacity spectral index of
beta = 1.8. Within 40 of the B1.5Ve Herbig star MWC 297, the submillimetre
fluxes are contaminated by free-free emission with a spectral index of
1.03+-0.02, consistent with an ultra-compact HII region and polar winds/jets.
Contamination accounts for 73+-5 per cent and 82+-4 per cent of peak flux at
450micron and 850micron respectively. The residual thermal disk of the star is
almost undetectable at these wavelengths. Young Stellar Objects are confirmed
where SCUBA-2 850micron clumps identified by the fellwalker algorithm coincide
with Spitzer Gould Belt Survey detections. We identify 23 objects and use Tbol
to classify nine YSOs with masses 0.09 to 5.1 Msun. We find two Class 0, one
Class 0/I, three Class I and three Class II sources. The mean temperature is
15+-2K for the nine YSOs and 32+-4K for the 14 starless clumps. We observe a
starless clump with an abnormally high mean temperature of 46+-2K and conclude
that it is radiatively heated by the star MWC 297. Jeans stability provides
evidence that radiative heating by the star MWC 297 may be suppressing clump
collapse.Comment: 24 pages, 13 figures, 7 table
First Results from the CHARA Array. I. An Interferometric and Spectroscopic Study of the Fast Rotator alpha Leonis (Regulus)
We report on K-band interferometric observations of the bright, rapidly
rotating star Regulus (type B7 V) made with the CHARA Array on Mount Wilson,
California. Through a combination of interferometric and spectroscopic
measurements, we have determined for Regulus the equatorial and polar diameters
and temperatures, the rotational velocity and period, the inclination and
position angle of the spin axis, and the gravity darkening coefficient. These
first results from the CHARA Array provide the first interferometric
measurement of gravity darkening in a rapidly rotating star and represent the
first detection of gravity darkening in a star that is not a member of an
eclipsing binary system.Comment: Accepted for publication in The Astrophysical Journal, 44 pages w/ 14
figure
The JCMT Gould Belt Survey: A First Look at the Auriga–California Molecular Cloud with SCUBA-2
We present 850 and 450 μm observations of the dense regions within the Auriga–California molecular cloud using SCUBA-2 as part of the JCMT Gould Belt Legacy Survey to identify candidate protostellar objects, measure the masses of their circumstellar material (disk and envelope), and compare the star formation to that in the Orion A molecular cloud. We identify 59 candidate protostars based on the presence of compact submillimeter emission, complementing these observations with existing Herschel/SPIRE maps. Of our candidate protostars, 24 are associated with young stellar objects (YSOs) in the Spitzer and Herschel/PACS catalogs of 166 and 60 YSOs, respectively (177 unique), confirming their protostellar nature. The remaining 35 candidate protostars are in regions, particularly around LkHα 101, where the background cloud emission is too bright to verify or rule out the presence of the compact 70 μm emission that is expected for a protostellar source. We keep these candidate protostars in our sample but note that they may indeed be prestellar in nature. Our observations are sensitive to the high end of the mass distribution in Auriga–Cal. We find that the disparity between the richness of infrared star-forming objects in Orion A and the sparsity in Auriga–Cal extends to the submillimeter, suggesting that the relative star formation rates have not varied over the Class II lifetime and that Auriga–Cal will maintain a lower star formation efficiency
The effect of luminal content and rate of occlusion on the interpretation of colonic manometry
This is the accepted version of the following article: [Arkwright, J. W., Dickson, A., Maunder, S. A., Blenman, N. G., Lim, J., O’Grady, G., Archer, R., Costa, M., Spencer, N. J., Brookes, S., Pullan, A. and Dinning, P. G. (2013), The effect of luminal content and rate of occlusion on the interpretation of colonic manometry. Neurogastroenterology & Motility, 25: e52–e59.], which has been published in final form at [http://dx.doi.org/10.1111/nmo.12051]. In addition, authors may also transmit, print and share copies with colleagues, provided that there is no systematic distribution of the submitted version, e.g. posting on a listserve, network or automated delivery.Background Manometry is commonly used for diagnosis of esophageal and anorectal motility disorders. In the colon, manometry is a useful tool, but clinical application remains uncertain. This uncertainty is partly based on the belief that manometry cannot reliably detect non-occluding colonic contractions and, therefore, cannot identify reliable markers of dysmotility. This study tests the ability of manometry to record pressure signals in response to non-lumen-occluding changes in diameter, at different rates of wall movement and with content of different viscosities. Methods A numerical model was built to investigate pressure changes caused by localized, non-lumen-occluding reductions in diameter, similar to those caused by contraction of the gut wall. A mechanical model, consisting of a sealed pressure vessel which could produce localized reductions in luminal diameter, was used to validate the model using luminal segments formed from; (i) natural latex; and (ii) sections of rabbit proximal colon. Fluids with viscosities ranging from 1 to 6800 mPa s-1 and luminal contraction rates over the range 5-20 mmHg s-1 were studied. Key Results Manometry recorded non-occluding reductions in diameter, provided that they occurred with sufficiently viscous content. The measured signal was linearly dependent on the rate of reduction in luminal diameter and also increased with increasing viscosity of content (R2 = 0.62 and 0.96 for 880 and 1760 mPa s-1, respectively). Conclusions & Inferences Manometry reliably registers non-occluding contractions in the presence of viscous content, and is therefore a viable tool for measuring colonic motility. Interpretation of colonic manometric data, and definitions based on manometric results, must consider the viscosity of luminal content.Australian National Health & Medical Research Counci
- …