1,934 research outputs found
Multipolar Planetary Nebulae: Not as Geometrically Diversified as Thought
Planetary nebulae (PNe) have diverse morphological shapes, including
point-symmetric and multipolar structures. Many PNe also have complicated
internal structures such as torus, lobes, knots, and ansae. A complete
accounting of all the morphological structures through physical models is
difficult. A first step toward such an understanding is to derive the true
three-dimensional structure of the nebulae. In this paper, we show that a
multipolar nebula with three pairs of lobes can explain many of such features,
if orientation and sensitivity effects are taken into account. Using only six
parameters - the inclination and position angles of each pair - we are able to
simulate the observed images of 20 PNe with complex structures. We suggest that
the multipolar structure is an intrinsic structure of PNe and the statistics of
multipolar PNe has been severely underestimated in the past.Comment: 36 pages, 5 figures, 2 table
Young Planetary Nebulae: Hubble Space Telescope Imaging and a New Morphological Classification System
Using Hubble Space Telescope images of 119 young planetary nebulae, most of
which have not previously been published, we have devised a comprehensive
morphological classification system for these objects. This system generalizes
a recently devised system for pre-planetary nebulae, which are the immediate
progenitors of planetary nebulae (PNs). Unlike previous classification studies,
we have focussed primarily on young PNs rather than all PNs, because the former
best show the influences or symmetries imposed on them by the dominant physical
processes operating at the first and primary stage of the shaping process.
Older PNs develop instabilities, interact with the ambient interstellar medium,
and are subject to the passage of photoionization fronts, all of which obscure
the underlying symmetries and geometries imposed early on. Our classification
system is designed to suffer minimal prejudice regarding the underlying
physical causes of the different shapes and structures seen in our PN sample,
however, in many cases, physical causes are readily suggested by the geometry,
along with the kinematics that have been measured in some systems. Secondary
characteristics in our system such as ansae indicate the impact of a jet upon a
slower-moving, prior wind; a waist is the signature of a strong equatorial
concentration of matter, whether it be outflowing or in a bound Keplerian disk,
and point symmetry indicates a secular trend, presumably precession, in the
orientation of the central driver of a rapid, collimated outflow.Comment: (to appear in The Astronomical Journal, March 2011.) The quality of
the figures as it appears in the arXiv pdf output is not up-to-par; the full
ms with high-quality figures is available by anonymous FTP at
ftp://ftp.astro.ucla.edu/pub/morris/sahai_AJ_360163.pd
The Drosophila Citrate Lyase Is Required for Cell Division during Spermatogenesis
The Drosophila melanogaster DmATPCL gene encodes for the human ATP Citrate Lyase (ACL) ortholog, a metabolic enzyme that from citrate generates glucose-derived Acetyl-CoA, which fuels central biochemical reactions such as the synthesis of fatty acids, cholesterol and acetylcholine, and the acetylation of proteins and histones. We had previously reported that, although loss of Drosophila ATPCL reduced levels of Acetyl-CoA, unlike its human counterpart, it does not affect global histone acetylation and gene expression, suggesting that its role in histone acetylation is either partially redundant in Drosophila or compensated by alternative pathways. Here, we describe that depletion of DmATPCL affects spindle organization, cytokinesis, and fusome assembly during male meiosis, revealing an unanticipated role for DmATPCL during spermatogenesis. We also show that DmATPCL mutant meiotic phenotype is in part caused by a reduction of fatty acids, but not of triglycerides or cholesterol, indicating that DmATPCL-derived Acetyl-CoA is predominantly devoted to the biosynthesis of fatty acids during spermatogenesis. Collectively, our results unveil for the first time an involvement for DmATPCL in the regulation of meiotic cell division, which is likely conserved in human cells
An Envelope Disrupted by a Quadrupolar Outflow in the Pre-Planetary Nebula IRAS19475+3119
IRAS 19475+3119 is a quadrupolar pre-planetary nebula (PPN), with two bipolar
lobes, one in the east-west (E-W) direction and one in the southeast-northwest
(SE-NW) direction. We have observed it in CO J=2-1 with the Submillimeter Array
at ~ 1" resolution. The E-W bipolar lobe is known to trace a bipolar outflow
and it is detected at high velocity. The SE-NW bipolar lobe appears at low
velocity, and could trace a bipolar outflow moving in the plane of the sky. Two
compact clumps are seen at low velocity around the common waist of the two
bipolar lobes, spatially coincident with the two emission peaks in the NIR,
tracing dense envelope material. They are found to trace the two
limb-brightened edges of a slowly expanding torus-like circumstellar envelope
produced in the late AGB phase. This torus-like envelope originally could be
either a torus or a spherical shell, and it appears as it is now because of the
two pairs of cavities along the two bipolar lobes. Thus, the envelope appears
to be disrupted by the two bipolar outflows in the PPN phase.Comment: 23 pages, 8 figure
Curves with rational chord-length parametrization
It has been recently proved that rational quadratic circles in standard Bezier form are parameterized by chord-length. If we consider that standard circles coincide with the isoparametric curves in a system of bipolar coordinates, this property comes as a straightforward consequence. General curves with chord-length parametrization are simply the analogue in bipolar coordinates of nonparametric curves. This interpretation furnishes a compact explicit expression for all planar curves with rational chord-length parametrization. In addition to straight lines and circles in standard form, they include remarkable curves, such as the equilateral hyperbola, Lemniscate of Bernoulli and Limacon of Pascal. The extension to 3D rational curves is also tackled
Three-Dimensional Adaptive Mesh Refinement Simulations of Point-Symmetric Nebulae
Previous analytical and numerical work shows that the generalized interacting
stellar winds model can explain the observed bipolar shapes of planetary
nebulae very well. However, many circumstellar nebulae have a multipolar or
point-symmetric shape. With two-dimensional calculations, Icke showed that
these seemingly enigmatic forms can be easily reproduced by a two-wind model in
which the confining disk is warped, as is expected to occur in irradiated
disks. In this contribution we present the extension to fully three-dimensional
adaptive mesh refinement simulations of such an interaction.Comment: 4 pages, 2 figures, to appear in "Asymmetrical Planetary Nebulae III"
editors M. Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series).
Movies are available at
http://www.strw.leidenuniv.nl/AstroHydro3D/movies/index.htm
New insights into the classification and nomenclature of cortical GABAergic interneurons.
A systematic classification and accepted nomenclature of neuron types is much needed but is currently lacking. This article describes a possible taxonomical solution for classifying GABAergic interneurons of the cerebral cortex based on a novel, web-based interactive system that allows experts to classify neurons with pre-determined criteria. Using Bayesian analysis and clustering algorithms on the resulting data, we investigated the suitability of several anatomical terms and neuron names for cortical GABAergic interneurons. Moreover, we show that supervised classification models could automatically categorize interneurons in agreement with experts' assignments. These results demonstrate a practical and objective approach to the naming, characterization and classification of neurons based on community consensus
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