1,135,295 research outputs found
Current singularities at finitely compressible three-dimensional magnetic null points
The formation of current singularities at line-tied two- and three-dimensional (2D and 3D, respectively) magnetic null points in a nonresistive magnetohydrodynamic environment is explored. It is shown that, despite the different separatrix structures of 2D and 3D null points, current singularities may be initiated in a formally equivalent manner. This is true no matter whether the collapse is triggered by flux imbalance within closed, line-tied null points or driven by externally imposed velocity fields in open, incompressible geometries. A Lagrangian numerical code is used to investigate the finite amplitude perturbations that lead to singular current sheets in collapsing 2D and 3D null points. The form of the singular current distribution is analyzed as a function of the spatial anisotropy of the null point, and the effects of finite gas pressure are quantified. It is pointed out that the pressure force, while never stopping the formation of the singularity, significantly alters the morphology of the current distribution as well as dramatically weakening its strength. The impact of these findings on 2D and 3D magnetic reconnection models is discussed
A novel 3D human glioblastoma cell culture system for modeling drug and radiation responses
Background. Glioblastoma (GBM) is the most common primary brain tumor, with dismal prognosis. The failure of drug–radiation
combinations with promising preclinical data to translate into effective clinical treatments may relate to the use of simplified
2-dimensional in vitro GBM cultures.
Methods. We developed a customized 3D GBM culture system based on a polystyrene scaffold (Alvetex) that recapitulates key
histological features of GBM and compared it with conventional 2D cultures with respect to their response to radiation and to
molecular targeted agents for which clinical data are available.
Results. In 3 patient-derived GBM lines, no difference in radiation sensitivity was observed between 2D and 3D cultures, as
measured by clonogenic survival. Three different molecular targeted agents, for which robust clinical data are available were
evaluated in 2D and 3D conditions: (i) temozolomide, which improves overall survival and is standard of care for GBM, exhibited
statistically significant effects on clonogenic survival in both patient-derived cell lines when evaluated in the 3D model compared
with only one cell line in 2D cells; (ii) bevacizumab, which has been shown to increase progression-free survival when added to
standard chemoradiation in phase III clinical trials, exhibited marked radiosensitizing activity in our 3D model but had no effect
on 2D cells; and (iii) erlotinib, which had no efficacy in clinical trials, displayed no activity in our 3D GBM model, but radiosensitized
2D cells.
Conclusions. Our 3D model reliably predicted clinical efficacy, strongly supporting its clinical relevance and potential value in
preclinical evaluation of drug–radiation combinations for GBM
CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital
rocket experiment that on 3rd September 2015 measured the linear polarization
produced by scattering processes in the hydrogen Ly- line of the solar
disk radiation, whose line-center photons stem from the chromosphere-corona
transition region (TR). These unprecedented spectropolarimetric observations
revealed an interesting surprise, namely that there is practically no
center-to-limb variation (CLV) in the line-center signals. Using an
analytical model, we first show that the geometrical complexity of the
corrugated surface that delineates the TR has a crucial impact on the CLV of
the and line-center signals. Secondly, we introduce a statistical
description of the solar atmosphere based on a three-dimensional (3D) model
derived from a state-of-the-art radiation magneto-hydrodynamic simulation. Each
realization of the statistical ensemble is a 3D model characterized by a given
degree of magnetization and corrugation of the TR, and for each such
realization we solve the full 3D radiative transfer problem taking into account
the impact of the CLASP instrument degradation on the calculated polarization
signals. Finally, we apply the statistical inference method presented in a
previous paper to show that the TR of the 3D model that produces the best
agreement with the CLASP observations has a relatively weak magnetic field and
a relatively high degree of corrugation. We emphasize that a suitable way to
validate or refute numerical models of the upper solar chromosphere is by
confronting calculations and observations of the scattering polarization in
ultraviolet lines sensitive to the Hanle effect.Comment: Accepted for publication in The Astrophysical Journal Letter
IRIS: A Generic Three-Dimensional Radiative Transfer Code
We present IRIS, a new generic three-dimensional (3D) spectral radiative
transfer code that generates synthetic spectra, or images. It can be used as a
diagnostic tool for comparison with astrophysical observations or laboratory
astrophysics experiments. We have developed a 3D short-characteristic solver
that works with a 3D nonuniform Cartesian grid. We have implemented a piecewise
cubic, locally monotonic, interpolation technique that dramatically reduces the
numerical diffusion effect. The code takes into account the velocity gradient
effect resulting in gradual Doppler shifts of photon frequencies and subsequent
alterations of spectral line profiles. It can also handle periodic boundary
conditions. This first version of the code assumes Local Thermodynamic
Equilibrium (LTE) and no scattering. The opacities and source functions are
specified by the user. In the near future, the capabilities of IRIS will be
extended to allow for non-LTE and scattering modeling. IRIS has been validated
through a number of tests. We provide the results for the most relevant ones,
in particular a searchlight beam test, a comparison with a 1D plane-parallel
model, and a test of the velocity gradient effect. IRIS is a generic code to
address a wide variety of astrophysical issues applied to different objects or
structures, such as accretion shocks, jets in young stellar objects, stellar
atmospheres, exoplanet atmospheres, accretion disks, rotating stellar winds,
cosmological structures. It can also be applied to model laboratory
astrophysics experiments, such as radiative shocks produced with high power
lasers.Comment: accepted for publication in A&A; 17 pages, 9 figures, 2 table
Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field Structure of FeSt 1-457
Three dimensional (3D) magnetic field information on molecular clouds and
cores is important for revealing their kinematical stability (magnetic support)
against gravity which is fundamental for studying the initial conditions of
star formation. In the present study, the 3D magnetic field structure of the
dense starless core FeSt 1-457 is determined based on the near-infrared
polarimetric observations of the dichroic polarization of background stars and
simple 3D modeling. With an obtained angle of line-of-sight magnetic
inclination axis of and previously
determined plane-of-sky magnetic field strength of
, the total magnetic field strength for FeSt 1-457 is derived to be
. The critical mass of FeSt 1-457, evaluated using
both magnetic and thermal/turbulent support is
, which is identical to the observed core mass, . We thus conclude that the stability of
FeSt 1-457 is in a condition close to the critical state. Without infalling gas
motion and no associated young stars, the core is regarded to be in the
earliest stage of star formation, i.e., the stage just before the onset of
dynamical collapse following the attainment of a supercritical condition. These
properties would make FeSt 1-457 one of the best starless cores for future
studies of the initial conditions of star formation.Comment: Accepted to the Astrophysical Journal (ApJ
On Tetrahedralisations Containing Knotted and Linked Line Segments
This paper considers a set of twisted line segments in 3d such that they form a knot (a closed curve) or a link of two closed curves. Such line segments appear on the boundary of a family of 3d indecomposable polyhedra (like the Schönhardt polyhedron) whose interior cannot be tetrahedralised without additional vertices added. On the other hand, a 3d (non-convex) polyhedron whose boundary contains such line segments may still be decomposable as long as the twist is not too large. It is therefore interesting to consider the question: when there exists a tetrahedralisation contains a given set of knotted or linked line segments?
In this paper, we studied a simplified question with the assumption that all vertices of the line segments are in convex position. It is straightforward to show that no tetrahedralisation of 6 vertices (the three-line-segments case) can contain a trefoil knot. Things become interesting when the number of line segments increases. Since it is necessary to create new interior edges to form a tetrahedralisation. We provided a detailed analysis for the case of a set of 4 line segments. This leads to a crucial condition on the orientation of pairs of new interior edges which determines whether this set is decomposable or not. We then prove a new theorem about the decomposability for a set of n (n ≥ 3) knotted or linked line segments. This theorem implies that the family of polyhedra generalised from the Schonhardt polyhedron by Rambau [1] are all indecomposable
The three-dimensional structure of the Eta Carinae Homunculus
We investigate, using the modeling code SHAPE, the three-dimensional
structure of the bipolar Homunculus nebula surrounding Eta Carinae, as mapped
by new ESO VLT/X-Shooter observations of the H2 micron
emission line. Our results reveal for the first time important deviations from
the axisymmetric bipolar morphology: 1) circumpolar trenches in each lobe
positioned point-symmetrically from the center and 2) off-planar protrusions in
the equatorial region from each lobe at longitudinal (~55 degrees) and
latitudinal (10-20 degrees) distances from the projected apastron direction of
the binary orbit. The angular distance between the protrusions (~110 degrees)
is similar to the angular extent of each polar trench (~130 degrees) and nearly
equal to the opening angle of the wind-wind collision cavity (~110 degrees). As
in previous studies, we confirm a hole near the centre of each polar lobe and
no detectable near-IR H2 emission from the thin optical skirt seen prominently
in visible imagery. We conclude that the interaction between the outflows
and/or radiation from the central binary stars and their orientation in space
has had, and possibly still has, a strong influence on the Homunculus. This
implies that prevailing theoretical models of the Homunculus are incomplete as
most assume a single star origin that produces an axisymmetric nebula. We
discuss how the newly found features might be related to the Homunculus
ejection, the central binary and the interacting stellar winds. We also include
a 3D printable version of our Homunculus model.Comment: 14 pages, 7 color figures, 1 interactive 3D figure (Figure 5,
requires Adobe Reader), published in MNRAS. A 3D printable version of our
Homunculus model can be downloaded from
http://svs.gsfc.nasa.gov/vis/a010000/a011500/a011568/Eta_Car_Homunuculus_3D_model.zip
or from the 'Supporting Information' link in the electronic version of the
MNRAS articl
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