824 research outputs found
Density Waves Inside Inner Lindblad Resonance: Nuclear Spirals in Disk Galaxies
We analyze formation of grand-design two-arm spiral structure in the nuclear
regions of disk galaxies. Such morphology has been recently detected in a
number of objects using high-resolution near-infrared observations. Motivated
by the observed (1) continuity between the nuclear and kpc-scale spiral
structures, and by (2) low arm-interarm contrast, we apply the density wave
theory to explain the basic properties of the spiral nuclear morphology. In
particular, we address the mechanism for the formation, maintenance and the
detailed shape of nuclear spirals. We find, that the latter depends mostly on
the shape of the underlying gravitational potential and the sound speed in the
gas. Detection of nuclear spiral arms provides diagnostics of mass distribution
within the central kpc of disk galaxies. Our results are supported by 2D
numerical simulations of gas response to the background gravitational potential
of a barred stellar disk. We investigate the parameter space allowed for the
formation of nuclear spirals using a new method for constructing a
gravitational potential in a barred galaxy, where positions of resonances are
prescribed.Comment: 18 pages, 9 figures, higher resolution available at
http://www.pa.uky.edu/~ppe/papers/nucsp.ps.g
Water Footprint Differences of Producing Cultivars of Selected Crops in New Zealand
Water footprint (WF) is a measure of the amount of water used to produce goods and services. It is a very important concept on indicating how much water can be consumed to complete a process of growing or processing a product at a particular location. However, paucity of water footprint information in countries facing increased competition for water resources between industries limits market access and profit optimization. Water footprint differences of producing selected cultivars of potato, oca and pumpkin squash were determined under irrigation and rain-fed regimes. All crop husbandry practices were followed in potato, oca (3.3 plants mâ2) and pumpkin squash (2.2 plants mâ2). Water footprint was determined as the ratio of volume of evapotranspiration for irrigated and rain-fed crops plus grey water to total yield. The consumptive water use for the rain-fed crop was 75, 65 and 69% of the irrigated oca, potato and pumpkin squash, respectively, with high water consumption in heritage cultivars. The water footprint was low in pumpkin squash and highest in oca, while potato cultivars were intermediate. Irrigation reduced water footprint especially in crops more responsive to irrigation. Farmers should focus on improving the harvest index and irrigation to reduce water footprint
Comparing life histories across taxonomic groups in multiple dimensions: how mammal-like are insects?
Explaining variation in life histories remains a major challenge because they are multi-dimensional and there are many competing explanatory theories and paradigms. An influential concept in life history theory is the âfast-slow continuumâ, exemplified by mammals. Determining the utility of such concepts across taxonomic groups requires comparison of the groupsâ life histories in multidimensional space. Insects display enormous species richness and phenotypic diversity, but testing hypotheses like the âfast-slow continuumâ has been inhibited by incomplete trait data. We use phylogenetic imputation to generate complete datasets of seven life history traits in orthopterans (grasshoppers and crickets) and examine the robustness of these imputations for our findings. Three phylogenetic principal components explain 83-96% of variation in these data. We find consistent evidence of an axis mostly following expectations of a âfast-slow continuumâ, except that âslowâ species produce larger, not smaller, clutches of eggs. We show that the principal axes of variation in orthopterans and reptiles are mutually explanatory, as are those of mammals and birds. Essentially, trait covariation in Orthoptera, with âslowâ species producing larger clutches, is more reptile-like than mammal-or-bird-like. We conclude that the âfast-slow continuumâ is less pronounced in Orthoptera than in birds and mammals, reducing the universal relevance of this pattern, and the theories that predict it
Random and ordered phases of off-lattice rhombus tiles
We study the covering of the plane by non-overlapping rhombus tiles, a
problem well-studied only in the limiting case of dimer coverings of regular
lattices. We go beyond this limit by allowing tiles to take any position and
orientation on the plane, to be of irregular shape, and to possess different
types of attractive interactions. Using extensive numerical simulations we show
that at large tile densities there is a phase transition from a fluid of
rhombus tiles to a solid packing with broken rotational symmetry. We observe
self-assembly of broken-symmetry phases, even at low densities, in the presence
of attractive tile-tile interactions. Depending on tile shape and interactions
the solid phase can be random, possessing critical orientational fluctuations,
or crystalline. Our results suggest strategies for controlling tiling order in
experiments involving `molecular rhombi'.Comment: Supp. Info. and version with high-res figures at
http://nanotheory.lbl.gov/people/rhombus_paper/rhombus.htm
Dynamics of Line-Driven Winds from Disks in Cataclysmic Variables. II. Mass Loss Rates and Velocity Laws
We analyze the dynamics of 2D stationary line-driven winds from accretion
disks in cataclysmic variables (CVs), by generalizing the Castor, Abbott and
Klein theory. In paper 1, we have solved the wind Euler equation, derived its
two eigenvalues, and addressed the solution topology and wind geometry. Here,
we focus on mass loss and velocity laws. We find that disk winds, even in
luminous novalike variables, have low optical depth, even in the strongest
driving lines. This suggests that thick-to-thin transitions in these lines
occur. For disks with a realistic radial temperature, the mass loss is
dominated by gas emanating from the inner decade in r. The total mass loss rate
associated with a luminosity 10 Lsun is 10^{-12} Msun/yr, or 10^{-4} of the
mass accretion rate. This is one order of magnitude below the lower limit
obtained from P Cygni lines, when the ionizing flux shortwards of the Lyman
edge is supressed. The difficulties with such small mass loss rates in CVs are
principal, and confirm our previous work. We conjecture that this issue may be
resolved by detailed nonLTE calculations of the line force within the context
of CV disk winds, and/or better accounting for the disk energy distribution and
wind ionization structure. We find that the wind velocity profile is well
approximated by the empirical law used in kinematical modeling. The
acceleration length scale is given by the footpoint radius of the wind
streamline in the disk. This suggests an upper limit of 10 Rwd to the
acceleration scale, which is smaller by factors of a few as compared to values
derived from line fitting.Comment: 14 pages, 3 Postscript figures, also from
http://www.pa.uky.edu/~shlosman/publ.html. Astrophysical Journal, submitte
A surface electrode point Paul trap
We present a model as well as experimental results for a surface electrode
radio-frequency Paul trap that has a circular electrode geometry well-suited
for trapping of single ions and two-dimensional planar ion crystals. The trap
design is compatible with microfabrication and offers a simple method by which
the height of the trapped ions above the surface may be changed \emph{in situ}.
We demonstrate trapping of single and few Sr+ ions over an ion height range of
200-1000 microns for several hours under Doppler laser cooling, and use these
to characterize the trap, finding good agreement with our model.Comment: 10 pages, 11 figures, 1 tabl
Optical and ultraviolet spectroscopic analysis of SN 2011fe at late times
We present optical spectra of the nearby Type Ia supernova SN 2011fe at 100,
205, 311, 349, and 578 days post-maximum light, as well as an ultraviolet
spectrum obtained with Hubble Space Telescope at 360 days post-maximum light.
We compare these observations with synthetic spectra produced with the
radiative transfer code PHOENIX. The day +100 spectrum can be well fit with
models which neglect collisional and radiative data for forbidden lines.
Curiously, including this data and recomputing the fit yields a quite similar
spectrum, but with different combinations of lines forming some of the stronger
features. At day +205 and later epochs, forbidden lines dominate much of the
optical spectrum formation; however, our results indicate that recombination,
not collisional excitation, is the most influential physical process driving
spectrum formation at these late times. Consequently, our synthetic optical and
UV spectra at all epochs presented here are formed almost exclusively through
recombination-driven fluorescence. Furthermore, our models suggest that the
ultraviolet spectrum even as late as day +360 is optically thick and consists
of permitted lines from several iron-peak species. These results indicate that
the transition to the "nebular" phase in Type Ia supernovae is complex and
highly wavelength-dependent.Comment: 22 pages, 21 figuress, 1 table, submitted to MNRA
Common physical framework explains phase behavior and dynamics of atomic, molecular, and polymeric network formers
We show that the self-assembly of a diverse collection of building blocks can be understood within a common physical framework. These building blocks, which form periodic honeycomb networks and nonperiodic variants thereof, range in size from atoms to micron-scale polymers and interact through mechanisms as different as hydrogen bonds and covalent forces. A combination of statistical mechanics and quantum mechanics shows that one can capture the physics that governs the assembly of these networks by resolving only the geometry and strength of building-block interactions. The resulting framework reproduces a broad range of phenomena seen experimentally, including periodic and nonperiodic networks in thermal equilibrium, and nonperiodic supercooled and glassy networks away from equilibrium. Our results show how simple âdesign criteriaâ control the assembly of a wide variety of networks and suggest that kinetic trapping can be a useful way of making functional assemblies
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