1,015 research outputs found
New or Noteworthy Species of Flowering Plants from the Sierra Madre de Sur of Guerrero and Michoacán, Mexico
The Sierra Madre del Sur of Pacific coastal Mexico has been of limited accessibility and therefore not well explored botanically. Only G. B. Hinton and E. Langlasse have made significant collections from the area. More recent collections by the authors have resulted in the recognition of nine new species, described herein as follows: Acanthaceae—Elytraria rnexieana; Malvaceae—Bastardiastrurn batesii, Gossypiurn schwendirnanii, Hibiscus zygornorphus, Kosteletzkya flavicentrum, Periptera lobelioides, Sida fastuosa, and Sida prolifiea; and Tumeraceae—Piriquetia mexieana. Other species meriting specific comment include Dioseorea insignis (Dioscoreaceae), Anotea flavida (Malvaceae), and Helieteres rekoi (Sterculiaceae). Illustrations of the new species and a key to the described Mexican species of Elytraria are included
On the Scale Dependence of Foraging in Terrestrial Herbivores
Meaningful modeling of the spatial and trophic dynamics of terrestrial herbivores demands understanding of the constraints and fitness objectives that presumably underlie behavior. This is complex in terrestrial herbivores, because of scale-dependent constraints on nutrient or energy gain. Mechanistic processes of forage cropping, forage mastication, movements between feeding stations, and forage digestion each have unique constraints that apply on different time, size, and spatial scales. Moreover, competing activities are rarely taken into account. Experimental testing of foraging objectives is therefore clouded by uncertainty regarding which time scale is most relevant from the animal’s perspective, leading to confusion and misrepresentation in the foraging literature. We illustrate these arguments from both theoretical and empirical points of view, based on our work with wild ungulates as well as the contemporary literature
Jet trails and Mach cones: The interaction of microquasars with the ISM
A sub-set of microquasars exhibit high peculiar velocity with respect to the
local standard of rest due to the kicks they receive when being born in
supernovae. The interaction between the radio plasma released by microquasar
jets from such high-velocity binaries with the ISM must lead to the production
of trails and bow shocks similar to what is observed in narrow-angle tailed
radio galaxies and pulsar wind nebulae. We present a set of numerical
simulations of this interaction that illuminate the long term dynamical
evolution and the observational properties of these microquasar bow shock
nebulae and trails. We find that this interaction always produces a structure
that consists of a bow shock, a trailing neck, and an expanding bubble. Using
our simulations to model emission, we predict that the shock surrounding the
bubble and the neck should be visible in H{\alpha} emission, the interior of
the bubble should be visible in synchrotron radio emission, and only the bow
shock is likely to be detectable in X-ray emission. We construct an analytic
model for the evolution of the neck and bubble shape and compare this model
with observations of X-ray binary SAX J1712.6-3739.Comment: 33 pages, 13 figures, 1 table; Accepted to Ap
Rings of Dark Matter in Collisions Between Clusters of Galaxies
Several lines of evidence suggest that the galaxy cluster Cl0024+17, an
apparently relaxed system, is actually a collision of two clusters, the
interaction occurring along our line of sight. Recent lensing observations
suggest the presence of a ring-like dark matter structure, which has been
interpreted as the result of such a collision. In this paper we present
-body simulations of cluster collisions along the line of sight to
investigate the detectability of such features. We use realistic dark matter
density profiles as determined from cosmological simulations. Our simulations
show a "shoulder" in the dark matter distribution after the collision, but no
ring feature even when the initial particle velocity distribution is highly
tangentially anisotropic (). Only when the initial
particle velocity distribution is circular do our simulations show such a
feature. Even modestly anisotropic velocity distributions are inconsistent with
the halo velocity distributions seen in cosmological simulations, and would
require highly fine-tuned initial conditions. Our investigation leaves us
without an explanation for the dark matter ring-like feature in Cl 0024+17
suggested by lensing observations.Comment: 7 pages (emulateapj), 9 figures. Expanded figures and text to match
accepted versio
Large-Scale Simulations of Clusters of Galaxies
We discuss some of the computational challenges encountered in simulating the
evolution of clusters of galaxies. Eulerian adaptive mesh refinement (AMR)
techniques can successfully address these challenges but are currently being
used by only a few groups. We describe our publicly available AMR code, FLASH,
which uses an object-oriented framework to manage its AMR library, physics
modules, and automated verification. We outline the development of the FLASH
framework to include collisionless particles, permitting it to be used for
cluster simulation.Comment: 3 pages, 3 figures, to appear in Proceedings of the VII International
Workshop on Advanced Computing and Analysis Techniques in Physics Research
(ACAT 2000), Fermilab, Oct. 16-20, 200
Genetic diversity within and genetic differentiation between blooms of a microalgal species
The field of genetic diversity in protists, particularly phytoplankton, is under expansion. However, little is known regarding variation in genetic diversity within populations over time. The aim of our study was to investigate intrapopulation genetic diversity and genetic differentiation in the freshwater bloom-forming microalga Gonyostomum semen (Raphidophyceae). The study covered a 2-year period including all phases of the bloom. Amplified fragment length polymorphism (AFLP) was used to determine the genetic structure and diversity of the population. Our results showed a significant differentiation between samples collected during the two blooms from consecutive years. Also, an increase of gene diversity and a loss of differentiation among sampling dates were observed over time within a single bloom. The latter observations may reflect the continuous germination of cysts from the sediment. The life cycle characteristics of G. semen, particularly reproduction and recruitment, most likely explain a high proportion of the observed variation. This study highlights the importance of the life cycle for the intraspecific genetic diversity of microbial species, which alternates between sexual and asexual reproduction.Postprin
Comptonization of the cosmic microwave background by high energy particles residing in AGN cocoons
X-ray cavities and extended radio sources (`cocoons') surrounding active
galactic nuclei (AGN) have been detected by the Chandra X-ray mission and radio
interferometers. A joint analysis of X-ray and radio maps suggests that
pressure values of non-thermal radio-emitting particles derived from the radio
maps are not sufficient to inflate the X-ray cavities. We propose using the
Sunyaev-Zel'dovich (SZ) effect, whose intensity strongly depends on the
pressure, to find the hitherto undetected, dynamically-dominant component in
the radio cocoons.
We demonstrate that the spectral function at a frequency of 217 GHz has an
absolute maximum at a temperature higher than K, therefore the
measurement of the SZ effect at this frequency is a powerful tool for
potentially revealing the dynamically-dominant component inside AGN jet-driven
radio cocoons. A new method is proposed for excluding the contribution from the
low energy, non-relativistic electrons to the SZ effect by means of
observations at two frequencies. We show how one may correct for a possible
contribution from the kinematic SZ effect. The intensity maps of the SZ effect
are calculated for the self-similar Sedov solution, and application of a
predicted ring-like structure on the SZ map at a frequency of 217 GHz is
proposed to determine the energy released during the active jet stage. The SZ
intensity map for an AGN cocoon in a distant elliptical is calculated using a
2-D numerical simulation and including relativistic corrections to the SZ
effect. We show the intensity spectrum of the SZ effect is flat at high
frequencies if gas temperature is as high as
keV.Comment: 12 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
Predicted Impact of Barriers to Migration on the Serengeti Wildebeest Population
The Serengeti wildebeest migration is a rare and spectacular example of a once-common biological phenomenon. A proposed road project threatens to bisect the Serengeti ecosystem and its integrity. The precautionary principle dictates that we consider the possible consequences of a road completely disrupting the migration. We used an existing spatially-explicit simulation model of wildebeest movement and population dynamics to explore how placing a barrier to migration across the proposed route (thus creating two disjoint but mobile subpopulations) might affect the long-term size of the wildebeest population. Our simulation results suggest that a barrier to migration—even without causing habitat loss—could cause the wildebeest population to decline by about a third. The driver of this decline is the effect of habitat fragmentation (even without habitat loss) on the ability of wildebeest to effectively track temporal shifts in high-quality forage resources across the landscape. Given the important role of the wildebeest migration for a number of key ecological processes, these findings have potentially important ramifications for ecosystem biodiversity, structure, and function in the Serengeti
External Mass Accumulation onto Core Potentials: Implications for Star Clusters, Galaxies and Galaxy Clusters
Accretion studies have been focused on the flow around bodies with point mass
gravitational potentials, but few general results are available for non-point
mass distributions. Here, we study the accretion flow onto non-divergent, core
potentials moving through a background medium. We use Plummer and Hernquist
potentials as examples to study gas accretion onto star clusters, dwarf and
large galaxy halos and galaxy clusters in a variety of astrophysical
environments. The general conditions required for a core potential to
collectively accrete large quantities of gas from the external medium are
derived using both simulations and analytic results. The consequences of large
mass accumulation in galaxy nuclei, dwarf galaxies and star clusters are
twofold. First, if the gas cools effectively star formation can be triggered,
generating new stellar members in the system. Second, if the collective
potential of the system is able to alter the ambient gas properties before the
gas is accreted onto the individual core members, the augmented mass supply
rates could significantly alter the state of the various accreting stellar
populations and result in an enhanced central black hole accretion luminosity.Comment: 24 pages, 15 figures, accepted to Ap
X-ray flares from propagation instabilities in long Gamma-Ray Burst jets
We present a numerical simulation of a gamma-ray burst jet from a
long-lasting engine in the core of a 16 solar mass Wolf-Rayet star. The engine
is kept active for 6000 s with a luminosity that decays in time as a power-law
with index -5/3. Even though there is no short time-scale variability in the
injected engine luminosity, we find that the jet's kinetic luminosity outside
the progenitor star is characterized by fluctuations with relatively short time
scale. We analyze the temporal characteristics of those fluctuations and we
find that they are consistent with the properties of observed flares in X-ray
afterglows. The peak to continuum flux ratio of the flares in the simulation is
consistent with some, but not all, the observed flares. We propose that
propagation instabilities, rather than variability in the engine luminosity,
are responsible for the X-ray flares with moderate contrast. Strong flares such
as the one detected in GRB 050502B, instead, cannot be reproduced by this model
and require strong variability in the engine activity.Comment: 6 pages, MNRAS in pres
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