673 research outputs found
The Three-dimensional Evolution of Rising, Twisted Magnetic Flux Tubes in a Gravitationally Stratified Model Convection Zone
We present three-dimensional numerical simulations of the rise and
fragmentation of twisted, initially horizontal magnetic flux tubes which evolve
into emerging Omega-loops. The flux tubes rise buoyantly through an
adiabatically stratified plasma that represents the solar convection zone. The
MHD equations are solved in the anelastic approximation, and the results are
compared with studies of flux tube fragmentation in two dimensions. We find
that if the initial amount of field line twist is below a critical value, the
degree of fragmentation at the apex of a rising Omega-loop depends on its
three-dimensional geometry: the greater the apex curvature of a given
Omega-loop, the lesser the degree of fragmentation of the loop as it approaches
the photosphere. Thus, the amount of initial twist necessary for the loop to
retain its cohesion can be reduced substantially from the two-dimensional
limit. The simulations also suggest that as a fragmented flux tube emerges
through a relatively quiet portion of the solar disk, extended crescent-shaped
magnetic features of opposite polarity should form and steadily recede from one
another. These features eventually coalesce after the fragmented portion of the
Omega-loop emerges through the photosphere.Comment: 17 pages, 17 figures, uses AAS LaTeX macros v5.0. ApJ, in pres
Electronic superlattice revealed by resonant scattering from random impurities in Sr3Ru2O7
Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive
tool for the study of subtle charge, orbital, and spin superlattice orders
driven by the valence electrons, which therefore escape detection in
conventional x-ray diffraction (XRD). Although the power of REXS has been
demonstrated by numerous studies of complex oxides performed in the soft x-ray
regime, the cross section and photon wavelength of the material-specific
elemental absorption edges ultimately set the limit to the smallest
superlattice amplitude and periodicity one can probe. Here we show -- with
simulations and REXS on Mn-substituted SrRuO -- that these
limitations can be overcome by performing resonant scattering experiments at
the absorption edge of a suitably-chosen, dilute impurity. This establishes
that -- in analogy with impurity-based methods used in electron-spin-resonance,
nuclear-magnetic resonance, and M\"ossbauer spectroscopy -- randomly
distributed impurities can serve as a non-invasive, but now momentum-dependent
probe, greatly extending the applicability of resonant x-ray scattering
techniques
A Critique of Current Magnetic-Accretion Models for Classical T-Tauri Stars
Current magnetic-accretion models for classical T-Tauri stars rely on a
strong, dipolar magnetic field of stellar origin to funnel the disk material
onto the star, and assume a steady-state. In this paper, I critically examine
the physical basis of these models in light of the observational evidence and
our knowledge of magnetic fields in low-mass stars, and find it lacking.
I also argue that magnetic accretion onto these stars is inherently a
time-dependent problem, and that a steady-state is not warranted.
Finally, directions for future work towards fully-consistent models are
pointed out.Comment: 2 figure
Applied environmental stresses to enhance the levels of polyphenolics in leaves of hawthorn plants
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72331/1/j.1399-3054.2004.00332.x.pd
Application of realistic effective interactions to the structure of the Zr isotopes
We calculate the low-lying spectra of the zirconium isotopes Z=40 with
neutron numbers from N=52 to N=60 using the 1p1/20g9/2 proton and
2s1d0g7/20h11/2 neutron sub-shells to define the model space. Effective
proton-proton, neutron--neutron and proton-neutron interactions have been
derived using 88Sr as closed core and employing perturbative many-body
techniques. The starting point is the nucleon-nucleon potential derived from
modern meson exchange models. The comprehensive shell-model calculation
performed in this work provides a qualitative reproduction of essential
properties such as the sub-shell closures in 96Zr and 98Zr.Comment: To appear in Phys Rev C, june 2000, 8 figs, Revtex latex styl
Optical photometry and X-ray monitoring of the "Cool Algol" BD+05 706: Determination of the physical properties
We present new photometric observations in the BVRI bands of the double-lined
eclipsing binary BD+05 706 conducted over three observing seasons, as well as
new X-ray observations obtained with the ROSAT satellite covering a full
orbital cycle (P = 18.9 days). A detailed light-curve analysis of the optical
data shows the system to be semidetached, confirming indications from an
earlier analysis by Torres et al. (1998), with the less massive and cooler star
filling its Roche lobe. The system is a member of the rare class of cool Algol
systems, which are different from the "classical" Algol systems in that the
mass-gaining component is also a late-type star rather than a B- or A-type
star. By combining the new photometry with a reanalysis of the spectroscopic
observations reported by Torres et al. (1998) we derive accurate absolute
masses for the components of M1 = 2.633 +/- 0.028 Msun and M2 = 0.5412 +/-
0.0093 Msun, radii of R1 = 7.55 +/- 0.20 Rsun and R2 = 11.02 +/- 0.21 Rsun, as
well as effective temperatures of 5000 +/- 100 K and 4640 +/- 150 K for the
primary and secondary, respectively. There are obvious signs of activity
(spottedness) in the optical light curve of the binary. Our X-ray light curve
clearly shows the primary eclipse but not the secondary eclipse, suggesting
that the primary star is the dominant source of the activity in the system. The
depth and duration of the eclipse allow us to infer some of the properties of
the X-ray emitting region around that star.Comment: 38 pages including 8 figures and 11 tables. To appear in The
Astronomical Journal, June 200
Flower numbers, pod production, pollen viability, and pistil function are reduced and flower and pod abortion increased in chickpea (Cicer arietinum L.) under terminal drought
Terminal drought during the reproductive stage is a major constraint to yield of chickpea in many regions of the world. Termination of watering (WS) during podding in a small-seeded desi chickpea (Cicer arietinum L.) cultivar, Rupali, and a large-seeded kabuli chickpea cultivar, Almaz, induced a decrease in predawn leaf water potential (LWP), in the rate of photosynthesis, and in stomatal conductance. Compared to well-watered (WW) controls, the WS treatment reduced flower production by about two-thirds. In the WW treatment, about 15% of the flowers aborted and 42% (Rupali) and 67% (Almaz) of the pods aborted, whereas in the WS treatment 37% and 56% of the flowers aborted and 54% and 73% of the pods aborted, resulting in seed yields of 33% and 15% of the yields in WW plants in Rupali and Almaz, respectively. In vitro pollen viability and germination in Rupali decreased by 50% and 89% in the WS treatment, and pollen germination decreased by 80% in vivo when pollen from a WS plant was placed on a stigma of a WW plant. While about 37% of the germinated pollen tubes from WW plants and 22% from the WS plants reached the ovary in the WW plants, less than 3% of pollen grains reached the ovary when pollen from either WS or WW plants was placed on a stigma of a WS plant. It is concluded that, in addition to pod abortion, flower abortion is an important factor limiting yield in chickpea exposed to terminal drought and that water deficit impaired the function of the pistil/style more than the pollen
Solar cycle prediction using precursors and flux transport models
We study the origin of the predictive skill of some methods to forecast the
strength of solar activity cycles. A simple flux transport model for the
azimuthally averaged radial magnetic field at the solar surface is used, which
contains a source term describing the emergence of new flux based on
observational sunspot data. We consider the magnetic flux diffusing over the
equator as a predictor, since this quantity is directly related to the global
dipole field from which a Babcock-Leighton dynamo generates the toroidal field
for the next activity cycle. If the source is represented schematically by a
narrow activity belt drifting with constant speed over a fixed range of
latitudes between activity minima, our predictor shows considerable predictive
skill with correlation coefficients up to 0.95 for past cycles. However, the
predictive skill is completely lost when the actually observed emergence
latitudes are used. This result originates from the fact that the precursor
amplitude is determined by the sunspot activity a few years before solar
minimum. Since stronger cycles tend to rise faster to their maximum activity
(known as the Waldmeier effect), the temporal overlapping of cycles leads to a
shift of the minimum epochs that depends on the strength of the following
cycle. This information is picked up by precursor methods and also by our flux
transport model with a schematic source. Therefore, their predictive skill does
not require a memory, i.e., a physical connection between the surface
manifestations of subsequent activity cycles.Comment: Astrophys. Journal, in pres
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