2,132 research outputs found
Concepts in ichnotaxonomy illustrated by small round holes in shells
La clasificación de las pistas fósiles requiere un doble sistema de nomenclatura. El nombre de la pista fósil (ichnotaxon) está basado en la morfologia de la estructura, mientras que el taxon biológico representa la posicion filogenética que se interpreta del organismo causante. Los dos sistemas de nomenclatura no se pueden intercambiar, y ambos son necesarios para la completa clasificacion dela pista. A muchas de estas pistas no les ha sido aún atribuido unichnotaxon descriptivo, pero ya que las pistas fósiles re quieren nombres si han de ser tratadas sistemáticamente, se ofrece comunmente en tales casos un biotaxon interpretativo en lugar del ichnotaxon que no se ha descritotodavia Este procedimiento tiende a desviar la atención de la verdadera naturaleza de la pista fósil e implica una falsa exactitud en la determinación filogenética, lo cual conduce a conclusiones paleobiologicas poco seguras. Estos puntos quedan bien ilustrados por el ejemplo de unas perforaciones pequeíias y redondas sobre conchas. Son pistas fósiles abundantes y, no teniendo ichnotaxon, tienden a ser relacionadas con la accion perforante de los gasteropedos-naticidos y muricidos-sobre conchas. Sin embargo, varios grupos más de gasteropodos producen perforaciones redondas, al igual que los cefalopodos octopodos, turbelarios, nemátodos y braquiópodos articulados, pero su acción es poco conocida. Antes de embarcarse en tales especulaciones, como son los organismos causantes, es necesario un ichnotaxón para atraer la atención hacia estas pistas fosiles y aumentarel rigorde su tratamiento. Sólo cuando sumorfologia y distribución sean mejor conocidas estaremos en una posición mejor para discutir sus atribuciones filogenéticas
MARINE AND NORMARINE TRACE FOSSILS AND PLANT ROOTS IN A REGRESSIONAL SETTING (PLEISTOCENE, ITALY)
The sedimentary structures occurring in one of the Pleistocene marine terraces at the Ionian coast, indicate a shoaling upward trend from the shoreface of a sandy beach to nonmarine conditions. Four main facies have been recognized. Facies A consists of hummocky cross-bedded sands representing a wave-dominated nearshore environment. Physical and biological structures of facies B are suggestive of an abrupt reduction in energy level, possibly related to the establishment of shallow protected environments. Features of facies C suggest a stable environment of upper shoreface but strongly influenced by continental supplies. Upward, this facies passes into sediments deposited in continental conditions. The last facies (D) is related to moist or wet terrestrial environments. Bioturbation levels of facies A to C are in general low. Facies D, however, is well bioturbated, having an ichnofabric dominated by Taenidium isp. Sparse, vertical roots of vascular plants extended in many cases to at least 3 m below the base of facies D. Particularly unusual is the close association of large Taenidium isp. and vertical roots, where the trace fossil is clustered tightly around the plant, in some cases to more than 1 m below the water-sediment interface
Large dimension Configuration Interaction calculations of positron binding to the group II atoms
The Configuration Interaction (CI) method is applied to the calculation of
the structures of a number of positron binding systems, including e+Be, e+Mg,
e+Ca and e+Sr. These calculations were carried out in orbital spaces containing
about 200 electron and 200 positron orbitals up to l = 12. Despite the very
large dimensions, the binding energy and annihilation rate converge slowly with
l, and the final values do contain an appreciable correction obtained by
extrapolating the calculation to the l to infinity limit. The binding energies
were 0.00317 hartree for e+Be, 0.0170 hartree for e+Mg, 0.0189 hartree for
e+Ca, and 0.0131 hartree for e+Sr.Comment: 13 pages, no figs, revtex format, Submitted to PhysRev
Practical quantum metrology in noisy environments
This is the final version. Available from American Physical Society via the DOI in this recordThe problem of estimating an unknown phase
φ
using two-level probes in the presence of unital phase-covariant noise and using finite resources is investigated. We introduce a simple model in which the phase-imprinting operation on the probes is realized by a unitary transformation with a randomly sampled generator. We determine the optimal phase sensitivity in a sequential estimation protocol and derive a general (tight-fitting) lower bound. The sensitivity grows quadratically with the number of applications
N
of the phase-imprinting operation, then attains a maximum at some
N
opt
, and eventually decays to zero. We provide an estimate of
N
opt
in terms of accessible geometric properties of the noise and illustrate its usefulness as a guideline for optimizing the estimation protocol. The use of passive ancillas and of entangled probes in parallel to improve the phase sensitivity is also considered. We find that multiprobe entanglement may offer no practical advantage over single-probe coherence if the interrogation at the output is restricted to measuring local observables.European Research CouncilRoyal Societ
A Hybrid N-body--Coagulation Code for Planet Formation
We describe a hybrid algorithm to calculate the formation of planets from an
initial ensemble of planetesimals. The algorithm uses a coagulation code to
treat the growth of planetesimals into oligarchs and explicit N-body
calculations to follow the evolution of oligarchs into planets. To validate the
N-body portion of the algorithm, we use a battery of tests in planetary
dynamics. Several complete calculations of terrestrial planet formation with
the hybrid code yield good agreement with previously published calculations.
These results demonstrate that the hybrid code provides an accurate treatment
of the evolution of planetesimals into planets.Comment: Astronomical Journal, accepted; 33 pages + 11 figure
The arctic circle boundary and the Airy process
We prove that the, appropriately rescaled, boundary of the north polar region
in the Aztec diamond converges to the Airy process. The proof uses certain
determinantal point processes given by the extended Krawtchouk kernel. We also
prove a version of Propp's conjecture concerning the structure of the tiling at
the center of the Aztec diamond.Comment: Published at http://dx.doi.org/10.1214/009117904000000937 in the
Annals of Probability (http://www.imstat.org/aop/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Line Emission from an Accretion Disk around a Black hole: Effects of Disk Structure
The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the line emission. These lines serve as powerful probes for examining
the structure of inner regions of accretion disks. Previous studies of line
emission have considered geometrically thin disks only, where the gas moves
along geodesics in the equatorial plane of a black hole. Here we extend this
work to consider effects on line profiles from finite disk thickness, radial
accretion flow and turbulence. We adopt the Novikov and Thorne (1973) solution,
and find that within this framework, turbulent broadening is the dominant new
effect. The most prominent change in the skewed, double-horned line profiles is
a substantial reduction in the maximum flux at both red and blue peaks. The
effect is most pronounced when the inclination angle is large, and when the
accretion rate is high. Thus, the effects discussed here may be important for
future detailed modeling of high quality observational data.Comment: 21 pages including 8 figures; LaTeX; ApJ format; accepted by ApJ;
short results of this paper appeared before as a conference proceedings
(astro-ph/9711214
Line Emission from an Accretion Disk around a Rotating Black Hole: Toward a Measurement of Frame Dragging
Line emission from an accretion disk and a corotating hot spot about a
rotating black hole are considered for possible signatures of the
frame-dragging effect. We explicitly compare integrated line profiles from a
geometrically thin disk about a Schwarzschild and an extreme Kerr black hole,
and show that the line profile differences are small if the inner radius of the
disk is near or above the Schwarzschild stable-orbit limit of radius 6GM/c^2.
However, if the inner disk radius extends below this limit, as is possible in
the extreme Kerr spacetime, then differences can become significant, especially
if the disk emissivity is stronger near the inner regions. We demonstrate that
the first three moments of a line profile define a three-dimensional space in
which the presence of material at small radii becomes quantitatively evident in
broad classes of disk models. In the context of the simple, thin disk paradigm,
this moment-mapping scheme suggests formally that the iron line detected by the
Advanced Satellite for Cosmology and Astrophysics mission from MCG-6-30-15
(Tanaka et al. 1995) is 3 times more likely to originate from a disk about a
rotating black hole than from a Schwarzschild system. A statistically
significant detection of black hole rotation in this way may be achieved after
only modest improvements in the quality of data. We also consider light curves
and frequency shifts in line emission as a function of time for corotating hot
spots in extreme Kerr and Schwarzschild geometries. Both the frequency-shift
profile and the light curve from a hot spot are valuable measures of orbital
parameters and might possibly be used to detect frame dragging even at radii
approaching 6GM/c^2 if the inclination angle of the orbital plane is large.Comment: 15 pages (LaTex), 7 postscript figures; color plot (Figure 1)
available at http://cfata2.harvard.edu/bromley/nu_nofun.html (This version
contains a new subsection as well as minor corrections.
Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center
We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars
traveling with such extreme velocities that dynamical ejection from a massive
black hole (MBH) is their only suggested origin. We also detect a population of
possibly bound HVSs. The significant asymmetry we observe in the velocity
distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf <
-275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main
sequence stars. Any population of hypervelocity post-main sequence stars should
contain stars falling back onto the Galaxy, contrary to the observations. The
spatial distribution of HVSs also supports the main sequence interpretation:
longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are
missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 -
4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection
mechanisms and potential models. Depending on the mass function of HVSs, we
predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs
favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is
also allowed.Comment: 10 pages, 8 figures, accepted to ApJ, minor revision
Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities
The disruption of binary stars by the tidal field of the black hole in the
Galactic Center can produce the hypervelocity stars observed in the halo. We
use numerical models to simulate the full spectrum of observable velocities of
stars ejected into the halo by this binary disruption process. Our model
includes a range of parameters for binaries with 3-4 M_Solar primaries,
consideration of radial orbits of the ejected stars through an approximate mass
distribution for the Galaxy, and the impact of stellar lifetimes. We calculate
the spectrum of ejection velocities and reproduce previous results for the mean
ejection velocity at the Galactic center. The model predicts that the full
population of ejected stars includes both the hypervelocity stars with
velocities large enough to escape from the Galaxy and a comparable number of
ejected, but bound, stars of the same stellar type. The predicted median speeds
of the population of ejected stars as a function of distance in the halo are
consistent with current observations. Combining the model with the data also
shows that interesting constraints on the properties of binaries in the
Galactic Center and on the mass distribution in the Galaxy can be obtained even
with modest samples of ejected stars.Comment: 26 pages, including 6 figures, accepted for publication in the
Astrophysical Journa
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