1,724 research outputs found
Functional imaging of plants: A nuclear magnetic resonance study of a cucumber plant
Functional magnetic resonance imaging was used to study transients of biophysical parameters in a cucumber plant in response to environmental changes. Detailed flow imaging experiments showed the location of xylem and phloem in the stem and the response of the following flow characteristics to the imposed environmental changes: the total amount of water, the amount of stationary and flowing water, the linear velocity of the flowing water, and the volume flow. The total measured volume flow through the plant stem was in good agreement with the independently measured water uptake by the roots. A separate analysis of the flow characteristics for two vascular bundles revealed that changes in volume flow of the xylem sap were accounted for by a change in linear-flow velocities in the xylem vessels. Multiple-spin echo experiments revealed two water fractions for different tissues in the plant stem; the spin-spin relaxation time of the larger fraction of parenchyma tissue in the center of the stem and the vascular tissue was down by 17% in the period after cooling the roots of the plant. This could point to an increased water permeability of the tonoplast membrane of the observed cells in this period of quick recovery from severe water los
Large X-ray Flares from LMC X-4: Discovery of Milli-hertz Quasi-periodic Oscillations and QPO-modulated Pulsations
We report the discovery of milli-hertz (mHz) quasi-periodic oscillations
(QPOs) and QPO-modulated pulsations during large X-ray flares from the
high-mass X-ray binary pulsar LMC X-4 using data from the Rossi X-Ray Timing
Explorer (RXTE). The lightcurves of flares show that, in addition to ~74 mHz
coherent pulsations, there exist two more time-varying temporal structures at
frequencies of ~0.65-1.35 and ~2-20 mHz. These relatively long-term structures
appear in the power density spectra as mHz QPOs and as well-developed sidebands
around the coherent pulse frequency as well, indicating that the amplitudes of
the coherent pulsation is modulated by those of the mHz QPOs. One interesting
feature is that, while the first flare shows symmetric sidebands around the
coherent pulse frequency, the second flare shows significant excess emission in
the lower-frequency sidebands due to the ~2-20 mHz QPOs. We discuss the origin
of the QPOs using a combination of the beat-frequency model and a modified
version of the Keplerian-frequency model. According to our discussion, it seems
to be possible to attribute the origin of the ~0.65-1.35 and ~2-20 mHz QPOs to
the beating between the rotational frequency of the neutron star and the
Keplerian frequency of large accreting clumps near the corotation radius and to
the orbital motion of clumps at Keplerian radii of 2-10 times 10^9 cm,
respectively.Comment: 12 pages, including 4 figures; accepted by ApJ Letter
Spin-independent origin of the strongly enhanced effective mass in a dilute 2D electron system
We have accurately measured the effective mass in a dilute two-dimensional
electron system in silicon by analyzing temperature dependence of the
Shubnikov-de Haas oscillations in the low-temperature limit. A sharp increase
of the effective mass with decreasing electron density has been observed. Using
tilted magnetic fields, we have found that the enhanced effective mass is
independent of the degree of spin polarization, which points to a
spin-independent origin of the mass enhancement and is in contradiction with
existing theories
Universal Behavior of the Resistance Noise across the Metal-Insulator Transition in Silicon Inversion Layers
Studies of low-frequency resistance noise show that the glassy freezing of
the two-dimensional (2D) electron system in the vicinity of the metal-insulator
transition occurs in all Si inversion layers. The size of the metallic glass
phase, which separates the 2D metal and the (glassy) insulator, depends
strongly on disorder, becoming extremely small in high-mobility samples. The
behavior of the second spectrum, an important fourth-order noise statistic,
indicates the presence of long-range correlations between fluctuators in the
glassy phase, consistent with the hierarchical picture of glassy dynamics.Comment: revtex4; 4+ pages, 5 figure
Warped accretion discs and the long periods in X-ray binaries
Precessing accretion discs have long been suggested as explanations for the
long periods observed in a variety of X-ray binaries, most notably Her X-1/HZ
Her. We show that an instability of the disc's response to the radiation
reaction force from the illumination by the central source can cause the disc
to tilt out of the orbital plane and precess in something like the required
manner. The rate of precession and disc tilt obtained for realistic values of
system parameters compare favourably with the known body of data on X-ray
binaries with long periods. We explore other possible types of behaviour than
steadily precessing discs that might be observable in systems with somewhat
different parameters. At high luminosities, the inner disc tilts through more
than 90 degrees, i.e. it rotates counter to the usual direction, which may
explain the torque reversals in systems such as 4U 1626-67.Comment: submitted to MNRAS, 17-Dec-97, revised submit 2-Nov-98. 15 pages
LaTeX, 11 postscript figures in-tex
Constraints on the Formation and Evolution of Circumstellar Disks in Rotating Magnetized Cloud Cores
We use magnetic collapse models to place some constraints on the formation
and angular momentum evolution of circumstellar disks which are embedded in
magnetized cloud cores. Previous models have shown that the early evolution of
a magnetized cloud core is governed by ambipolar diffusion and magnetic
braking, and that the core takes the form of a nonequilibrium flattened
envelope which ultimately collapses dynamically to form a protostar. In this
paper, we focus on the inner centrifugally-supported disk, which is formed only
after a central protostar exists, and grows by dynamical accretion from the
flattened envelope. We estimate a centrifugal radius for the collapse of mass
shells within a rotating, magnetized cloud core. The centrifugal radius of the
inner disk is related to its mass through the two important parameters
characterizing the background medium: the background rotation rate \Omb and
the background magnetic field strength \Bref. We also revisit the issue of
how rapidly mass is deposited onto the disk (the mass accretion rate) and use
several recent models to comment upon the likely outcome in magnetized cores.
Our model predicts that a significant centrifugal disk (much larger than a
stellar radius) will be present in the very early (Class 0) stage of
protostellar evolution. Additionally, we derive an upper limit for the disk
radius as it evolves due to internal torques, under the assumption that the
star-disk system conserves its mass and angular momentum even while most of the
mass is transferred to a central star.Comment: 23 pages, 1 figure, aastex, to appear in the Astrophysical Journal
(10 Dec 1998
Holographic and Wilsonian Renormalization Groups
We develop parallels between the holographic renormalization group in the
bulk and the Wilsonian renormalization group in the dual field theory. Our
philosophy differs from most previous work on the holographic RG; the most
notable feature is the key role of multi-trace operators. We work out the forms
of various single- and double-trace flows. The key question, `what cutoff on
the field theory corresponds to a radial cutoff in the bulk?' is left
unanswered, but by sharpening the analogy between the two sides we identify
possible directions.Comment: 31 pages, 3 figures. v2: Minor clarifications. Added reference
New markers of inflammation-induced renal injury subside when endotoxin tolerance develops in humans as measured by urine proteomics
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