1,367 research outputs found
Quasiperiodic graphs: structural design, scaling and entropic properties
A novel class of graphs, here named quasiperiodic, are constructed via
application of the Horizontal Visibility algorithm to the time series generated
along the quasiperiodic route to chaos. We show how the hierarchy of
mode-locked regions represented by the Farey tree is inherited by their
associated graphs. We are able to establish, via Renormalization Group (RG)
theory, the architecture of the quasiperiodic graphs produced by irrational
winding numbers with pure periodic continued fraction. And finally, we
demonstrate that the RG fixed-point degree distributions are recovered via
optimization of a suitably defined graph entropy
Correlating electronic and magnetic coupling in large magnetic molecules via scanning tunneling microscopy
Contains fulltext :
175413.pdf (publisher's version ) (Open Access)7 p
Rapid bottom-water circulation changes during the last glacial cycle in the coastal low-latitude NE Atlantic
Previous paleoceanographic studies along the NW African margin focused on the dynamics of surface and intermediate waters, whereas little attention has been devoted to deep-water masses. Currently, these deep waters consist mainly of North Atlantic Deep Waters as part of the Atlantic Meridional Overturning Circulation (AMOC). However, this configuration was altered during periods of AMOC collapse. We present a highresolution reconstruction of bottom-water ventilation and current evolution off Mauritania from the last glacial maximum into the early Holocene. Applying redox proxies (Mo, U and Mn) measured on sediments from off Mauritania, we describe changes in deep-water oxygenation and we infer the evolution of deep-water conditions during millennial-scale climate/oceanographic events in the area. The second half of Heinrich Event 1 and the Younger Dryas were recognized as periods of reduced ventilation, coinciding with events of AMOC reduction. We propose that these weakening circulation events induced deficient deep-water oxygenation in the Mauritanian upwelling region, which together with increased productivity promoted reducing conditions and enhanced organic-matter preservation. This is the first time the effect of AMOC collapse in the area is described at high resolution, broadening the knowledge on basin-wide oceanographic changes associated with rapid climate variability during the last deglaciation
The Potential-Density Phase Shift Method for Determining the Corotation Radii in Spiral and Barred Galaxies
We have developed a new method for determining the corotation radii of
density waves in disk galaxies, which makes use of the radial distribution of
an azimuthal phase shift between the potential and density wave patterns. The
approach originated from improved theoretical understandings of the relation
between the morphology and kinematics of galaxies, and on the dynamical
interaction between density waves and the basic-state disk stars which results
in the secular evolution of disk galaxies. In this paper, we present the
rationales behind the method, and the first application of it to several
representative barred and grand-design spiral galaxies, using near-infrared
images to trace the mass distributions, as well as to calculate the potential
distributions used in the phase shift calculations. We compare our results with
those from other existing methods for locating the corotations, and show that
the new method both confirms the previously-established trends of bar-length
dependence on galaxy morphological types, as well as provides new insights into
the possible extent of bars in disk galaxies. Application of the method to a
larger sample and the preliminary analysis of which show that the phase shift
method is likely to be a generally-applicable, accurate, and essentially
model-independent method for determining the pattern speeds and corotation
radii of single or nested density wave patterns in galaxies. Other implications
of this work are: most of the nearby bright disk galaxies appear to possess
quasi-stationary spiral modes; that these density wave modes and the associated
basic state of the galactic disk slowly transform over time; and that
self-consistent N-particle systems contain physics not revealed by the passive
orbit analysis approaches.Comment: 48 pages, 16 figures. Accepted for publication in the Astronomical
Journa
Atomic Hydrogen and Star Formation in the Bridge/Ring Interacting Galaxy Pair NGC 7714/7715 (Arp 284)
We present high spatial resolution 21 cm HI maps of the interacting galaxy
pair NGC 7714/7715. We detect a massive (2 x 10**9 M(sun)) HI bridge connecting
the galaxies that is parallel to but offset from the stellar bridge. A chain of
HII regions traces the gaseous bridge, with H-alpha peaks near but not on the
HI maxima. An HI tidal tail is also detected to the east of the smaller galaxy
NGC 7715, similarly offset from a stellar tail. The strong partial stellar ring
on the eastern side of NGC 7714 has no HI counterpart, but on the opposite side
of NGC 7714 there is a 10**9 M(sun) HI loop 11 kpc in radius. Within the NGC
7714 disk, clumpy HI gas is observed associated with star formation regions.
Redshifted HI absorption is detected towards the starburst nucleus. We compare
the observed morphology and gas kinematics with gas dynamical models in which a
low-mass companion has an off-center prograde collision with the outer disk of
a larger galaxy. These simulations suggest that the bridge in NGC 7714/7715 is
a hybrid between bridges seen in systems like M51 and the purely gaseous
`splash' bridges found in ring galaxies like the Cartwheel. The offset between
the stars and gas in the bridge may be due to dissipative cloud-cloud
collisions occuring during the impact of the two gaseous disks.Comment: 31 pages, Latex, 11 figures, to be published in the July 10, 1997
issue of the Astrophysical Journa
Physical Properties of Tidal Features in Interacting Disk Galaxies
We explore tidal interactions of a galactic disk with Toomre parameter Q ~ 2
embedded in rigid halo/bulge with a point mass companion moving in a prescribed
parabolic orbit. Tidal interactions produce well-defined spiral arms and
extended tidal features such as bridge and tail that are all transient, but
distinct in nature. In the extended disks, strong tidal force is able to lock
the perturbed epicycle phases of the near-side particles to the perturber,
shaping them into a tidal bridge that corotates with the perturber. A tidal
tail develops at the opposite side as strongly-perturbed, near-side particles
overtake mildly-perturbed, far-side particles. The tail is essentially a narrow
material arm with a roughly logarithmic shape, dissolving with time because of
large velocity dispersions. Inside the disks where tidal force is relatively
weak, on the other hand, a two-armed logarithmic spiral pattern emerges due to
the kinematic alignment of perturbed particle orbits. While self-gravity makes
the spiral arms a bit stronger, the arms never become fully self-gravitating,
wind up progressively with time, and decay after the peak almost exponentially
in a time scale of ~ 1 Gyr. The arm pattern speed varying with both radius and
time converges to Omega-kappa/2 at late time, suggesting that the pattern speed
of tidally-driven arms may depend on radius in real galaxies. We present the
parametric dependences of various properties of tidal features on the tidal
strength, and discuss our findings in application to tidal spiral arms in
grand-design spiral galaxies. (Abridged)Comment: 49 pages, 17 figures, 1 table. Accepted for publication in
Astrophysical Journal. PDF version with higher resolution figures is
available at
http://astro.snu.ac.kr/~shoh/research/publications/astroph/Tidally_Induced_Spiral_Structure.pd
Dominant patterns of boreal summer interactions between tropics and mid-latitude: causal relationships and the role of timescales
Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemisphere. In winter, the El NiñoâSouthern Oscillation (ENSO) is the dominant source of predictability in the tropics and extra-tropics, but its role in summer is much less pronounced and the exact teleconnection pathways are not well understood. Here, we assess how tropical convection interacts with mid-latitude summer circulation at different intraseasonal time-scales and how ENSO affects these interactions. First, we apply maximum covariance analysis (MCA) between tropical convective activity and mid-latitude geopotential height fields to identify the dominant modes of interaction. The first MCA mode connects the South Asian monsoon with the mid-latitude circumglobal teleconnection pattern. The second MCA mode connects the western North Pacific summer monsoon in the tropics with a wave-5 pattern centred over the North Pacific High in the mid-latitudes. We show that the MCA patterns are fairly insensitive to the selected intraseasonal time-scale from weekly to 4-weekly data. To study the potential causal interdependencies between these modes and with other atmospheric fields, we apply causal effect networks (CEN) at different time-scales. CENs extend standard correlation analysis by removing the confounding effects of autocorrelation, indirect links and common drivers. In general, there is a two-way causal interaction between the tropics and mid-latitudes but the strength and sometimes sign of the causal link are time-scale dependent. We introduce causal maps that plot the regionally specific causal effect from each MCA mode. Those maps confirm the dominant patterns of interaction and in addition, highlight specific mid-latitude regions that are most strongly connected to tropical convection. In general, the identified causal teleconnection patterns are only mildly affected by ENSO and the tropical-mid-latitude linkages remain similar. Still, La Niña strengthens the South Asian monsoon generating a stronger response in the mid-latitudes, while during El Niño years, the Pacific pattern is reinforced. This study paves the way for process-based validation of boreal summer teleconnections in (sub)seasonal forecast models and climate models and therefore helps to improve sub-seasonal and climate projections
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