43 research outputs found
Collapse of a Circular Loop of Cosmic String
We study the collapse of a circular loop of cosmic string. The gravitational
field of the string is treated using the weak field approximation. The
gravitational radiation from the loop is evaluated numerically. The memtric of
the loop near the point of collapse is found analytically.Comment: 15 page
Choptuik scaling in six dimensions
We perform numerical simulations of the critical gravitational collapse of a
spherically symmetric scalar field in 6 dimensions. The critical solution has
discrete self-similarity. We find the critical exponent \gamma and the
self-similarity period \Delta.Comment: 8 pages, 3 figures RevTe
Simulated VLBI Images From Relativistic Hydrodynamic Jet Models
A series of simulated maps showing the appearance in total intensity of flows
computed using a recently developed relativistic hydrodynamic code (Duncan \&
Hughes 1994: ApJ, 436, L119) are presented. The radiation transfer calculations
were performed by assuming the flow is permeated by a magnetic field and fast
particle distribution in energy equipartition, with energy density proportional
to the hydrodynamic energy density (i.e., pressure). We find that relativistic
flows subject to strong perturbations exhibit a density structure consisting of
a series of nested bow shocks, and that this structure is evident in the
intensity maps for large viewing angles. However, for viewing angles
, differential Doppler boosting leads to a series of axial knots
of emission, similar to the pattern exhibited by many VLBI sources. The
appearance of VLBI knots is determined primarily by the Doppler boosting of
parts of a more extended flow. To study the evolution of a perturbed jet, a
time series of maps was produced and an integrated flux light curve created.
The light curve shows features characteristic of a radio loud AGN: small
amplitude variations and a large outburst. We find that in the absence of
perturbations, jets with a modest Lorentz factor () exhibit complex
intensity maps, while faster jets (Lorentz factor ) are largely
featureless. We also study the appearance of kiloparsec jet-counterjet pairs by
producing simulated maps at relatively large viewing angles; we conclude that
observed hot spot emission is more likely to be associated with the Mach disk
than with the outer, bow shock.Comment: 27 pages, uses aasms4.sty; 18 PostScript figures (1.57Mb gziped,
8.67Mb gunziped) available from
http://www.astro.lsa.umich.edu/users/hughes/icon_dir/rad.html or by anonymous
ftp from ra.astro.lsa.umich.edu in pub/get/hughes. Submitted to Ap.
A Comparison of the Morphology and Stability of Relativistic and Nonrelativistic Jets
We compare results from a relativistic and a nonrelativistic set of 2D
axisymmetric jet simulations. For a set of five relativistic simulations that
either increase the Lorentz factor or decrease the adiabatic index we compute
nonrelativistic simulations with equal useful power or thrust. We examine these
simulations for morphological and dynamical differences, focusing on the
velocity field, the width of the cocoon, the age of the jets, and the internal
structure of the jet itself. The primary result of these comparisons is that
the velocity field of nonrelativistic jet simulations cannot be scaled up to
give the spatial distribution of Lorentz factors seen in relativistic
simulations. Since the local Lorentz factor plays a major role in determining
the total intensity for parsec scale extragalactic jets, this suggests that a
nonrelativistic simulation cannot yield the proper intensity distribution for a
relativistic jet. Another general result is that each relativistic jet and its
nonrelativistic equivalents have similar ages (in dynamical time units, =
R/a_a, where R is the initial radius of a cylindrical jet and a_a is the sound
speed in the ambient medium). In addition to these comparisons, we have
completed four new relativistic simulations to investigate the effect of
varying thermal pressure on relativistic jets. The simulations generally
confirm that faster (larger Lorentz factor) and colder jets are more stable,
with smaller amplitude and longer wavelength internal variations. The apparent
stability of these jets does not follow from linear normal mode analysis, which
suggests that there are available growing Kelvin-Helmholtz modes. (Abridged.)Comment: 32 pages, AASTEX, to appear in May 10, 1999 issue of ApJ, better
versions of Figures 1 and 6 are available at
http://crux.astr.ua.edu/~rosen/rel/rhdh.htm
Scaling of curvature in sub-critical gravitational collapse
We perform numerical simulations of the gravitational collapse of a
spherically symmetric scalar field. For those data that just barely do not form
black holes we find the maximum curvature at the position of the central
observer. We find a scaling relation between this maximum curvature and
distance from the critical solution. The scaling relation is analogous to that
found by Choptuik for black hole mass for those data that do collapse to form
black holes. We also find a periodic wiggle in the scaling exponent.Comment: Revtex, 2 figures, Discussion modified, to appear in Phys. Rev.
Numerical evolution of Brill waves
We report a numerical evolution of axisymmetric Brill waves. The numerical
algorithm has new features, including (i) a method for keeping the metric
regular on the axis and (ii) the use of coordinates that bring spatial infinity
to the edge of the computational grid. The dependence of the evolved metric on
both the amplitude and shape of the initial data is found.Comment: added more discussion of results and several reference
3D Hydrodynamic Simulations of Relativistic Extragalactic Jets
We describe a new numerical 3D relativistic hydrodynamical code, the results
of validation tests, and a comparison with earlier, 2D studies. The 3D code has
been used to study the deflection and precession of relativistic flows. We find
that even quite fast jets (gamma~10) can be significantly influenced by
impinging on an oblique density gradient, exhibiting a rotation of the Mach
disk in the jet's head. The flow is bent via a potentially strong, oblique
internal shock that arises due to asymmetric perturbation of the flow by its
cocoon. In extreme cases this cocoon can form a marginally relativistic flow
orthogonal to the jet, leading to large scale dynamics quite unlike that
normally associated with astrophysical jets. Exploration of a gamma=5 flow
subject to a large amplitude precession (semi-angle 11.25dg) shows that it
retains its integrity, with modest reduction in Lorentz factor and momentum
flux, for almost 50 jet-radii, but thereafter, the collimated flow is
disrupted. The flow is approximately ballistic, with velocity vectors not
aligned with the local jet `wall'. We consider simple estimators of the flow
emissivity in each case and conclude that a) while the oblique internal shocks
which mediate a small change in the direction of the deflected flows have
little impact on the global dynamics, significantly enhanced flow emission (by
a factor of 2-3) may be associated with such regions; and b) the convolution of
rest frame emissivity and Doppler boost in the case of the precessed jet
invariably leads to a core-jet-like structure, but that intensity fluctuations
in the jet cannot be uniquely associated with either change in internal
conditions or Doppler boost alone, but in general are a combination of both
factors.Comment: 41 pages, including 15 figures. Submitted to ApJ. Version with
complete abstract. and full resolution, color figures available from
http://www.astro.lsa.umich.edu/users/hughes/icon_dir/cfd.htm
Parameterization of a coarse-grained model of cholesterol with point-dipole electrostatics
© 2018, Springer Nature Switzerland AG. We present a new coarse-grained (CG) model of cholesterol (CHOL) for the electrostatic-based ELBA force field. A distinguishing feature of our CHOL model is that the electrostatics is modeled by an explicit point dipole which interacts through an ideal vacuum permittivity. The CHOL model parameters were optimized in a systematic fashion, reproducing the electrostatic and nonpolar partitioning free energies of CHOL in lipid/water mixtures predicted by full-detailed atomistic molecular dynamics simulations. The CHOL model has been validated by comparison to structural, dynamic and thermodynamic properties with experimental and atomistic simulation reference data. The simulation of binary DPPC/cholesterol mixtures covering the relevant biological content of CHOL in mammalian membranes is shown to correctly predict the main lipid behavior as observed experimentally
The Gene Pool Concept Applied to Crop Wild Relatives: An Evolutionary Perspective
Crop wild relatives (CWR) can provide important resources for the genetic improvement of cultivated species. Because crops are often related to many wild species and because exploration of CWR for useful traits can take many years and substantial resources, the categorization of CWR based on a comprehensive assessment of their potential for use is an important knowledge foundation for breeding programs. The initial approach for categorizing CWR was based on crossing studies to empirically establish which species were interfertile with the crop. The foundational concept of distinct gene pools published almost 50 years ago was developed from these observations. However, the task of experimentally assessing all potential CWR proved too vast; therefore, proxies based on phylogenetic and other advanced scientific information have been explored. A current major approach to categorize CWR aims to comprehensively synthesize experimental data, taxonomic information, and phylogenetic studies. This approach very often ends up relying not only on the synthesis of data but also intuition and expert opinion and is therefore difficult to apply widely in a reproducible manner. Here, we explore the potential for a stronger standardization of the categorization method, with focus on evolutionary relationships among species combined with information on patterns of interfertility between species. Evolutionary relationships can be revealed with increasing resolution via next-generation sequencing, through the application of the multispecies coalescent model and using focused analyses on species discovery and delimitation that bridge population genetics and phylogenetics fields. Evolutionary studies of reproductive isolation can inform the understanding of patterns of interfertility in plants. For CWR, prezygotic postpollination reproductive barriers and intrinsic postzygotic barriers are the most important factors and determine the probability of producing viable and fertile offspring. To further the assessment of CWR for use in plant breeding, we present observed and predicted gene pool indices. The observed index quantifies patterns of interfertility based on fertilization success, seed production, offspring viability, and hybrid fertility. The predicted gene pool index requires further development of the understanding of quantitative and qualitative relationships between reproductive barriers, measures of genetic relatedness, and other relevant characteristics for crops and their wild relatives