3,172 research outputs found
The Accretion Flows and Evolution of Magnetic Cataclysmic Variables
We have used a model of magnetic accretion to investigate the accretion flows
of magnetic cataclysmic variables. Numerical simulations demonstrate that four
types of flow are possible: discs, streams, rings and propellers. The
fundamental observable determining the accretion flow, for a given mass ratio,
is the spin-to-orbital period ratio of the system. If IPs are accreting at
their equilibrium spin rates, then for a mass ratio of 0.5, those with
Pspin/Porb < 0.1 will be disc-like, those with 0.1 < Pspin/Porb < 0.6 will be
stream-like, and those with Pspin/Porb ~ 0.6 will be ring-like. The spin to
orbital period ratio at which the systems transition between these flow types
increases as the mass ratio of the stellar components decreases.
For the first time we present evolutionary tracks of mCVs which allow
investigation of how their accretion flow changes with time. As systems evolve
to shorter orbital periods and smaller mass ratios, in order to maintain spin
equilibrium, their spin-to-orbital period ratio will generally increase. As a
result, the relative occurrence of ring-like flows will increase, and the
occurrence of disc-like flows will decrease, at short orbital periods. The
growing number of systems observed at high spin-to-orbital period ratios with
orbital periods below 2h, and the observational evidence for ring-like
accretion in EX Hya, are fully consistent with this picture.Comment: Accepted for publication in ApJ. 6 figures - included here at low
resolutio
A COMPARATIVE STUDY OF THE ULTRASTRUCTURE OF MICROVILLI IN THE EPITHELIUM OF SMALL AND LARGE INTESTINE OF MICE
A comparative analysis of the fine structure of the microvilli on jejunal and colonic epithelial cells of the mouse intestine has been made. The microvilli in these two locations demonstrate a remarkably similar fine structure with respect to the thickness of the plasma membrane, the extent of the filament-free zone, and the characteristics of the microfilaments situated within the microvillous core. Some of the core microfilaments appear to continue across the plasma membrane limiting the tip of the microvillus. The main difference between the microvilli of small intestine and colon is in the extent and organization of the surface coat. In the small intestine, in addition to the commonly observed thin surface "fuzz," occasional areas of the jejunal villus show a more conspicuous surface coat covering the tips of the microvilli. Evidence has been put forward which indicates that the surface coat is an integral part of the epithelial cells. In contrast to the jejunal epithelium, the colonic epithelium is endowed with a thicker surface coat. Variations in the organization of the surface coat at different levels of the colonic crypts have also been noted. The functional significance of these variations in the surface coat is discussed
Experimental evaluation of compliant surfaces at low speeds
An experimental study was devised to determine the profile drag on a subsonic wind tunnel model partially covered with various compliant surface materials. The model consisted of a large section of constant thickness bounded fore and aft by symmetric airfoil fairings. A flat rigid plate, the control surface, could be exchanged for four contiguous complaint panels. The flexible media generally consisted of thin polyvinylchloride membranes stretched to various tensions over trapped fluid cavities of air, water, or polyethelene oxide solution, or over dry or flooded open-celled polyurethane foams. On the basis of very accurate direct-force cell measurements, all configurations were found to yield profile drags equal to or slightly higher than that obtained for the conventional rigid surface case. The results indicate that the potential for practical drag-reducing applications will be limited to either high-density or high-speed flows where more sensible pressure perturbations occur
Youthful Processing Speed in Older Adults: Genetic, Biological, and Behavioral Predictors of Cognitive Processing Speed Trajectories in Aging.
Objective: To examine the impact of genetic, inflammatory, cardiovascular, lifestyle, and neuroanatomical factors on cognitive processing speed (CPS) change over time in functionally intact older adults. Methods: This observational study conducted over two time points, included 120 community dwelling cognitively normal older adults between the ages of 60 and 80 from the University of California San Francisco Memory and Aging Center. Participants were followed with composite measures of CPS, calculated based on norms for 20-30 year-olds. Variables of interest were AD risk genes (APOE, CR1), markers of inflammation (interleukin 6) and cardiovascular health (BMI, LDL, HDL, mean arterial pressure, fasting insulin), self-reported physical activity, and corpus callosum (CC) volumes. The sample was divided into three groups: 17 "resilient-agers" with fast and stable processing speed; 56 "average-agers" with average and stable processing speed; and 47 "sub-agers" with average baseline speed who were slower at follow-up. Results: Resilient-agers had larger baseline CC volumes than sub-agers (p < 0.05). Resilient-agers displayed lower levels of interleukin-6 (IL-6) and insulin (ps < 0.05) than sub-agers, and reported more physical activity than both average- and sub-agers (ps < 0.01). In a multinomial logistic regression, physical activity and IL-6 predicted average- and sub-ager groups. Resilient-agers displayed a higher frequency of APOE e4 and CR1 AA/AG alleles. Conclusion: Robust and stable CPS is associated with larger baseline CC volumes, lower levels of inflammation and insulin, and greater self-reported physical activity. These findings highlight the relevance of neuroanatomical, biological, and lifestyle factors in the identification and prediction of heterogeneous cognitive aging change over time
Infrared and radio observations of W51: Another Orion-KL at a distance of 7kpc
The bright infrared sources W51-IRS2 has at least three components with different physical and evolutionary properties. The spatial distribution and the near infrared spectra of the components in IRS2 are remarkably similar to, but more luminous than those found in Orion, where an H2 region of comparable linear size is also located close to a cluster of compact infrared sources. The characteristics of the nearby W51-NORTH H2O maser source, and the detection of 2 micro m H2 quadrupole emission in IRS2 indicate that the mass loss phenomena found in Orion-KL also exist in W51
Binary evolution with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of very faint X-ray binaries,
orbital period distribution of black hole X-ray binaries and neutron star spin
up. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timing. (v2 few typos corrected
Origin of electron cyclotron maser-induced radio emissions at ultra-cool dwarfs: magnetosphere-ionosphere coupling currents
A number of ultra-cool dwarfs emit circularly polarised radio waves generated
by the electron cyclotron maser instability. In the solar system such radio is
emitted from regions of strong auroral magnetic field-aligned currents. We thus
apply ideas developed for Jupiter's magnetosphere, being a well-studied
rotationally-dominated analogue in our solar system, to the case of
fast-rotating UCDs. We explain the properties of the radio emission from UCDs
by showing that it would arise from the electric currents resulting from an
angular velocity shear in the fast-rotating magnetic field and plasma, i.e. by
an extremely powerful analogue of the process which causes Jupiter's auroras.
Such a velocity gradient indicates that these bodies interact significantly
with their space environment, resulting in intense auroral emissions. These
results strongly suggest that auroras occur on bodies outside our solar system.Comment: Accepted for publication in the Astrophysical Journa
Hubble Space Telescope observations of the NUV transit of WASP-12b
We present new observations of four closely-spaced NUV transits of the hot
Jupiter-like exoplanet WASP-12b using HST/COS, significantly increasing the
phase resolution of the observed NUV light curve relative to previous
observations, while minimising the temporal variation of the system. We observe
significant excess NUV absorption during the transit, with mean normalised
in-transit fluxes of , i.e. 2-5
deeper than the optical transit level of for a uniform stellar
disk (the exact confidence level depending on the normalisation method used).
We further observe an asymmetric transit shape, such that the post-conjunction
fluxes are overall 2-3 higher than pre-conjunction values, and
characterised by rapid variations in count rate between the pre-conjunction and
out of transit levels. We do not find evidence for an early ingress to the NUV
transit as suggested by earlier HST observations. However, we show that the NUV
count rate observed prior to the optical transit is highly variable, but
overall 2.2-3.0 below the post-transit values and comparable
in depth to the optical transit, possibly forming a variable region of NUV
absorption from at least phase 0.83, limited by the data coverage.Comment: Accepted into the Astrophysical Journa
Observation of an unusual field dependent slow magnetic relaxation and two distinct transitions in a family of new complexes
An unusual field dependent slow magnetic relaxation and two distinct
transitions were observed in a family of new rare earth-transition metal
complexes, [Ln (bipy) (HO) M(CN)] 1.5 (bipy) 4HO (bipy = 2,2'-bipyridine; Ln = Gd,Y; M = Fe,
Co). The novel magnetic relaxation, which is quite different from those
in normal spin glasses and superparamagnets but very resembles qualitatively
those in single-molecule magnet Mn-Ac even if they possess different
structures, might be attributed to the presence of frustration that is
incrementally unveiled by the external magnetic field. The two distinct
transitions in [GdFe] were presumed from DC and AC susceptibility as well as
heat capacity measurements.Comment: Revtex, 6 figure
High-temperature environments of human evolution in East Africa based on bond ordering in paleosol carbonates
Many important hominid-bearing fossil localities in East Africa are in regions that are extremely hot and dry. Although humans are well adapted to such conditions, it has been inferred that East African environments were cooler or more wooded during the Pliocene and Pleistocene when this region was a central stage of human evolution. Here we show that the Turkana Basin, Kenya—today one of the hottest places on Earth—has been continually hot during the past 4 million years. The distribution of ^(13)C-^(18)O bonds in paleosol carbonates indicates that soil temperatures during periods of carbonate formation were typically above 30 °C and often in excess of 35 °C. Similar soil temperatures are observed today in the Turkana Basin and reflect high air temperatures combined with solar heating of the soil surface. These results are specific to periods of soil carbonate formation, and we suggest that such periods composed a large fraction of integrated time in the Turkana Basin. If correct, this interpretation has implications for human thermophysiology and implies a long-standing human association with marginal environments
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