22,780 research outputs found
Do Magnetic Fields Prevent Hydrogen from Accreting onto Cool Metal-line White Dwarf Stars?
It is generally assumed that metals detected in the spectra of a few cool
white dwarfs cannot be of primordial origin and must be accreted from the
interstellar medium. However, the observed abundances of hydrogen, which should
also be accreted from the interstellar medium, are lower than expected from
metal accretion. Magnetic fields are thought to be the reason for this
discrepancy. We have therefore obtained circular polarization spectra of the
helium-rich white dwarfs GD40 and L745-46A, which both show strong metal lines
as well as hydrogen. Whereas L745-46A might have a magnetic field of about
-6900 G, which is about two times the field strength of 3000G necessary to
repell hydrogen at the Alfen radius, only an upper limit for the field strength
of GD40 of 4000G (with 99% confidence) can be set which is far off the minimum
field strength of 144000G to repell hydrogen.Comment: 4 LaTeX pages, 4 eps figures, to appear in the proceedings of the
14th European Workshop on White Dwarfs, eds. D. Koester and S. Moehler, ASP
Conf. Serie
An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853
The coupling of the electromagnetic field directly with gravitational gauge
fields leads to new physical effects that can be tested using astronomical
data. Here we consider a particular case for closer scrutiny, a specific
nonminimal coupling of torsion to electromagnetism, which enters into a
metric-affine geometry of space-time. We show that under the assumption of this
nonminimal coupling, spacetime is birefringent in the presence of such a
gravitational field. This leads to the depolarization of light emitted from
extended astrophysical sources. We use polarimetric data of the magnetic white
dwarf to set strong constraints on the essential coupling
constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in
Phys.Rev.
Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS
We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with
magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000
K and 50000 K. Through a least-squares minimization scheme with an evolutionary
algorithm, we have analyzed the spectra of 114 magnetic DAs from the SDSS (95
previously published plus 14 newly discovered within SDSS, and five discovered
by SEGUE). Since we were limited to a single spectrum for each object we used
only centered magnetic dipoles or dipoles which were shifted along the magnetic
dipole axis. We also statistically investigated the distribution of
magnetic-field strengths and geometries of our sample.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
Accurate Reaction-Diffusion Operator Splitting on Tetrahedral Meshes for Parallel Stochastic Molecular Simulations
Spatial stochastic molecular simulations in biology are limited by the
intense computation required to track molecules in space either in a discrete
time or discrete space framework, meaning that the serial limit has already
been reached in sub-cellular models. This calls for parallel simulations that
can take advantage of the power of modern supercomputers; however exact methods
are known to be inherently serial. We introduce an operator splitting
implementation for irregular grids with a novel method to improve accuracy, and
demonstrate potential for scalable parallel simulations in an initial MPI
version. We foresee that this groundwork will enable larger scale, whole-cell
stochastic simulations in the near future.Comment: 33 pages, 10 figure
Thermal correlators of anyons in two dimensions
The anyon fields have trivial -commutator for not integer.
For integer the commutators become temperature-dependent operator
valued distributions. The -point functions do not factorize as for quasifree
states.Comment: 14 pages, LaTeX (misprints corrected, a reference added
The magnetic fields of hot subdwarf stars
Detection of magnetic fields has been reported in several sdO and sdB stars.
Recent literature has cast doubts on the reliability of most of these
detections. We revisit data previously published in the literature, and we
present new observations to clarify the question of how common magnetic fields
are in subdwarf stars. We consider a sample of about 40 hot subdwarf stars.
About 30 of them have been observed with the FORS1 and FORS2 instruments of the
ESO VLT. Here we present new FORS1 field measurements for 17 stars, 14 of which
have never been observed for magnetic fields before. We also critically review
the measurements already published in the literature, and in particular we try
to explain why previous papers based on the same FORS1 data have reported
contradictory results. All new and re-reduced measurements obtained with FORS1
are shown to be consistent with non-detection of magnetic fields. We explain
previous spurious field detections from data obtained with FORS1 as due to a
non-optimal method of wavelength calibration. Field detections in other surveys
are found to be uncertain or doubtful, and certainly in need of confirmation.
There is presently no strong evidence for the occurrence of a magnetic field in
any sdB or sdO star, with typical longitudinal field uncertainties of the order
of 2-400 G. It appears that globally simple fields of more than about 1 or 2 kG
in strength occur in at most a few percent of hot subdwarfs, and may be
completely absent at this strength. Further high-precision surveys, both with
high-resolution spectropolarimeters and with instruments similar to FORS1 on
large telescopes, would be very valuable
Combining MODIS LAI with ICESat-Based Canopy Heights Improves Spaceborne Estimates of Vegetation Roughness Length for Momentum
Most land-surface models require parameterization of vertical wind profiles within the atmospheric boundary layer. For vegetated surfaces, it is common to assume a logarithmic profile in the surface layer, which includes estimates of vegetation roughness length for momentum (z0) and zero-plane displacement height (d0). This study finds that remotely-sensed forest canopy heights improve estimates of aerodynamic roughness length for momentum using a previously-developed representation of the roughness sublayer (Raupach 1992; Jasinski et al. 2005). Resulting roughness products consist of two datasets: 1) 14 years of 8-day snapshots of the global land surface at a nominal spatial resolution of 500-meters for users who wish to retain full temporal resolution and interannual variability; and 2) multiyear averages of the 8-day snapshots, here referred to as "climatologies" of roughness, which retain underlying seasonality. Both products are suitable for use in data assimilation and reanalyses such as the National Climate Assessment Land Data Assimilation System (NCA-LDAS), for which these products were initially developed
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