1,066,444 research outputs found
Sherlock: An Automated Follow-Up Telescope for Wide-Field Transit Searches
The most significant challenge currently facing photometric surveys for
transiting gas-giant planets is that of confusion with eclipsing binary systems
that mimic the photometric signature. A simple way to reject most forms of
these false positives is high-precision, rapid-cadence monitoring of the
suspected transit at higher angular resolution and in several filters. We are
currently building a system that will perform higher-angular-resolution,
multi-color follow-up observations of candidate systems identified by Sleuth
(our wide-field transit survey instrument at Palomar), and its two twin system
instruments in Tenerife and northern Arizona.Comment: 4 pages, 3 figures, to appear in AIP Conf Proc: The Search for Other
Worlds, eds. S. S. Holt & D. Demin
Characterising the magnetic fields of the Herbig Ae/Be stars HD97048, HD150193, HD176386, and MWC480
Our knowledge of the presence and the role of magnetic fields in
intermediate-mass pre-main-sequence stars remains very poor.
We present the magnetic properties of four Herbig Ae/Be stars that have not
been previously studied in detail.
Our results for the three Herbig Ae/Be stars HD97048, HD150193, and HD176386
are based on multi-epoch low-resolution spectra obtained in spectropolarimetric
mode with FORS2 mounted on the VLT. New high-resolution polarimetric spectra
were obtained for MWC480 with the SOFIN spectrograph installed at the Nordic
Optical Telescope. We discuss 41 FORS2 low-resolution observations of the
Herbig Ae/Be stars HD97048, HD150193, and HD176386 and determine their
rotational periods. Using stellar fundamental parameters and the longitudinal
magnetic field phase curves, we place constraints on the magnetic field
geometry. Three high-resolution circularly polarised SOFIN spectra obtained for
MWC480 were measured using the moment technique where wavelength shifts between
right- and left-hand side circularly polarised spectra are interpreted in terms
of a longitudinal magnetic field .
Our search for periodicities resulted in P=0.693d for HD97048, P=1.317d for
HD150193, and P=0.899d for HD176386. The magnetic field geometry can likely be
described by a centred dipole with a polar magnetic field strength B_d of
several hundred Gauss. The longitudinal magnetic-field measurements of MWC480
reveal the presence of a strong kG field, which was undetected in our previous
low-resolution polarimetric observations with FORS1. A weak magnetic field was
detected in the circumstellar components of the Ca II H&K lines and the Na I D
lines, indicating a complex interaction between the stellar magnetic field and
the circumstellar environment.Comment: 8 pages, 9 figures, 5 tables, accepted for publication in A&
A new numerical strategy with space-time adaptivity and error control for multi-scale streamer discharge simulations
This paper presents a new resolution strategy for multi-scale streamer
discharge simulations based on a second order time adaptive integration and
space adaptive multiresolution. A classical fluid model is used to describe
plasma discharges, considering drift-diffusion equations and the computation of
electric field. The proposed numerical method provides a time-space accuracy
control of the solution, and thus, an effective accurate resolution independent
of the fastest physical time scale. An important improvement of the
computational efficiency is achieved whenever the required time steps go beyond
standard stability constraints associated with mesh size or source time scales
for the resolution of the drift-diffusion equations, whereas the stability
constraint related to the dielectric relaxation time scale is respected but
with a second order precision. Numerical illustrations show that the strategy
can be efficiently applied to simulate the propagation of highly nonlinear
ionizing waves as streamer discharges, as well as highly multi-scale nanosecond
repetitively pulsed discharges, describing consistently a broad spectrum of
space and time scales as well as different physical scenarios for consecutive
discharge/post-discharge phases, out of reach of standard non-adaptive methods.Comment: Support of Ecole Centrale Paris is gratefully acknowledged for
several month stay of Z. Bonaventura at Laboratory EM2C as visiting
Professor. Authors express special thanks to Christian Tenaud (LIMSI-CNRS)
for providing the basis of the multiresolution kernel of MR CHORUS, code
developed for compressible Navier-Stokes equations (D\'eclaration d'Invention
DI 03760-01). Accepted for publication; Journal of Computational Physics
(2011) 1-2
On Noncommutative Multi-solitons
We find the moduli space of multi-solitons in noncommutative scalar field
theories at large theta, in arbitrary dimension. The existence of a non-trivial
moduli space at leading order in 1/theta is a consequence of a Bogomolnyi bound
obeyed by the kinetic energy of the theta=infinity solitons. In two spatial
dimensions, the parameter space for k solitons is a Kahler de-singularization
of the symmetric product (R^2)^k/S_k. We exploit the existence of this moduli
space to construct solitons on quotient spaces of the plane: R^2/Z_k, cylinder,
and T^2. However, we show that tori of area less than or equal to (2 pi theta)
do not admit stable solitons. In four dimensions the moduli space provides an
explicit Kahler resolution of (R^4)^k/S_k. In general spatial dimension 2d, we
show it is isomorphic to the Hilbert scheme of k points in C^d, which for d > 2
(and k > 3) is not smooth and can have multiple branches.Comment: 33 pages, 6 figures, harvmac; v2: minor corrections and improvements
to presentation, references adde
2D Multi-Angle, Multi-Group Neutrino Radiation-Hydrodynamic Simulations of Postbounce Supernova Cores
We perform axisymmetric (2D) multi-angle, multi-group neutrino
radiation-hydrodynamic calculations of the postbounce phase of core-collapse
supernovae using a genuinely 2D discrete-ordinate (S_n) method. We follow the
long-term postbounce evolution of the cores of one nonrotating and one
rapidly-rotating 20-solar-mass stellar model for ~400 milliseconds from 160 ms
to ~550 ms after bounce. We present a multi-D analysis of the multi-angle
neutrino radiation fields and compare in detail with counterpart simulations
carried out in the 2D multi-group flux-limited diffusion (MGFLD) approximation
to neutrino transport. We find that 2D multi-angle transport is superior in
capturing the global and local radiation-field variations associated with
rotation-induced and SASI-induced aspherical hydrodynamic configurations. In
the rotating model, multi-angle transport predicts much larger asymptotic
neutrino flux asymmetries with pole to equator ratios of up to ~2.5, while
MGFLD tends to sphericize the radiation fields already in the optically
semi-transparent postshock regions. Along the poles, the multi-angle
calculation predicts a dramatic enhancement of the neutrino heating by up to a
factor of 3, which alters the postbounce evolution and results in greater polar
shock radii and an earlier onset of the initially rotationally weakened SASI.
In the nonrotating model, differences between multi-angle and MGFLD
calculations remain small at early times when the postshock region does not
depart significantly from spherical symmetry. At later times, however, the
growing SASI leads to large-scale asymmetries and the multi-angle calculation
predicts up to 30% higher average integral neutrino energy deposition rates
than MGFLD.Comment: 20 pages, 21 figures. Minor revisions. Accepted for publication in
ApJ. A version with high-resolution figures may be obtained from
http://www.stellarcollapse.org/papers/Ott_et_al2008_multi_angle.pd
Multi-Channel SQUID System for MEG and Ultra-Low-Field MRI
A seven-channel system capable of performing both magnetoencephalography
(MEG) and ultra-low-field magnetic resonance imaging (ULF MRI) is described.
The system consists of seven second-order SQUID gradiometers with 37 mm
diameter and 60 mm baseline, having magnetic field resolution of 1.2-2.8
fT/rtHz. It also includes four sets of coils for 2-D Fourier imaging with
pre-polarization. The system's MEG performance was demonstrated by measurements
of auditory evoked response. The system was also used to obtain a multi-channel
2-D image of a whole human hand at the measurement field of 46 microtesla with
3 by 3 mm resolution.Comment: To appear in Proceedings of 2006 Applied Superconductivity Conferenc
A Two-Dimensional MagnetoHydrodynamics Scheme for General Unstructured Grids
We report a new finite-difference scheme for two-dimensional
magnetohydrodynamics (MHD) simulations, with and without rotation, in
unstructured grids with quadrilateral cells. The new scheme is implemented
within the code VULCAN/2D, which already includes radiation-hydrodynamics in
various approximations and can be used with arbitrarily moving meshes (ALE).
The MHD scheme, which consists of cell-centered magnetic field variables,
preserves the nodal finite difference representation of div(\bB) by
construction, and therefore any initially divergence-free field remains
divergence-free through the simulation. In this paper, we describe the new
scheme in detail and present comparisons of VULCAN/2D results with those of the
code ZEUS/2D for several one-dimensional and two-dimensional test problems. The
code now enables two-dimensional simulations of the collapse and explosion of
the rotating, magnetic cores of massive stars. Moreover, it can be used to
simulate the very wide variety of astrophysical problems for which multi-D
radiation-magnetohydrodynamics (RMHD) is relevant.Comment: 22 pages, including 11 figures; Accepted to the Astrophysical
Journal. Higher resolution figures available at
http://zenith.as.arizona.edu/~burrows/mhd-code
- …