4,462 research outputs found
First high-resolution images of the Sun in the 2796 \AA{} Mg II k line
We present the first high-resolution solar images in the Mg II k 2796 \AA{}
line. The images, taken through a 4.8 \AA{} broad interference filter, were
obtained during the second science flight of SUNRISE in June 2013 by the SuFI
instrument. The Mg II k images display structures that look qualitatively very
similar to images taken in the core of Ca II H. The Mg II images exhibit
reversed granulation (or shock waves) in the internetwork regions of the quiet
Sun, at intensity contrasts that are similar to those found in Ca II H. Very
prominent in Mg II are bright points, both in the quiet Sun and in plage
regions, particularly near disk center. These are much brighter than at other
wavelengths sampled at similar resolution. Furthermore, Mg II k images also
show fibril structures associated with plage regions. Again, the fibrils are
similar to those seen in Ca II H images, but tend to be more pronounced,
particularly in weak plage.Comment: Accepted for publication in The Astrophysical Journal Letter
Binary neutron-star mergers with Whisky and SACRA: First quantitative comparison of results from independent general-relativistic hydrodynamics codes
We present the first quantitative comparison of two independent
general-relativistic hydrodynamics codes, the Whisky code and the SACRA code.
We compare the output of simulations starting from the same initial data and
carried out with the configuration (numerical methods, grid setup, resolution,
gauges) which for each code has been found to give consistent and sufficiently
accurate results, in particular in terms of cleanness of gravitational
waveforms. We focus on the quantities that should be conserved during the
evolution (rest mass, total mass energy, and total angular momentum) and on the
gravitational-wave amplitude and frequency. We find that the results produced
by the two codes agree at a reasonable level, with variations in the different
quantities but always at better than about 10%.Comment: Published on Phys. Rev.
Moving Magnetic Features around a Pore
Spectropolarimetric observations from Sunrise II/IMaX obtained in June 2013
are used for a statistical analysis to determine the physical properties of
moving magnetic features (MMFs) observed near a pore. MMFs of the same and
opposite polarity with respect to the pore are found to stream from its border
at an average speed of 1.3 km s and 1.2 km s respectively, with
mainly same-polarity MMFs found further away from the pore. MMFs of both
polarities are found to harbor rather weak, inclined magnetic fields.
Opposite-polarity MMFs are blue-shifted, while same-polarity MMFs do not show
any preference for up- or downflows. Most of the MMFs are found to be of
sub-arcsecond size and carry a mean flux of 1.2 Mx.Comment: 8 pages, 4 figures, accepted for publication in ApJ
Accurate evolutions of inspiralling neutron-star binaries: prompt and delayed collapse to black hole
Binary neutron-star (BNS) systems represent primary sources for the
gravitational-wave (GW) detectors. We present a systematic investigation in
full GR of the dynamics and GW emission from BNS which inspiral and merge,
producing a black hole (BH) surrounded by a torus. Our results represent the
state of the art from several points of view: (i) We use HRSC methods for the
hydrodynamics equations and high-order finite-differencing techniques for the
Einstein equations; (ii) We employ AMR techniques with "moving boxes"; (iii) We
use as initial data BNSs in irrotational quasi-circular orbits; (iv) We exploit
the isolated-horizon formalism to measure the properties of the BHs produced in
the merger; (v) Finally, we use two approaches, based either on gauge-invariant
perturbations or on Weyl scalars, to calculate the GWs. These techniques allow
us to perform accurate evolutions on timescales never reported before (ie ~30
ms) and to provide the first complete description of the inspiral and merger of
a BNS leading to the prompt or delayed formation of a BH and to its ringdown.
We consider either a polytropic or an ideal fluid EOS and show that already
with this idealized EOSs a very interesting phenomenology emerges. In
particular, we show that while high-mass binaries lead to the prompt formation
of a rapidly rotating BH surrounded by a dense torus, lower-mass binaries give
rise to a differentially rotating NS, which undergoes large oscillations and
emits large amounts of GWs. Eventually, also the NS collapses to a rotating BH
surrounded by a torus. Finally, we also show that the use of a non-isentropic
EOS leads to significantly different evolutions, giving rise to a delayed
collapse also with high-mass binaries, as well as to a more intense emission of
GWs and to a geometrically thicker torus.Comment: 35 pages, 29 figures, corrected few typos to match the published
version. High-resolution figures and animations can be found at
http://numrel.aei.mpg.de/Visualisations/Archive/BinaryNeutronStars/Relativistic_Meudon/index.htm
Factors affecting metal mobilisation during oxidation of sulphidic, sandy wetland substrates
Most metals accumulate as sulphides under anoxic conditions in wetland substrates, reducing their bioavailability due to the solubility of metal sulphides. However, upon oxidation of these sulphides when the substrate is occasionally oxidised, metals can be released from the solid phase to the pore water or overlaying surface water. This release can be affected by the presence of carbonates, organic matter and clay. We compared changes of Cd, Cu and Zn mobility (CaCl2 extraction) during oxidation of a carbonate-rich and a carbonate-poor sulphidic, sandy wetland substrate. In addition, we studied how clay with low and high cation sorption capacity (bentonite and kaolinite, respectively) and organic matter (peat) can counteract Cd, Cu and Zn release during oxidation of both carbonate-rich and carbonate-poor sulphidic sediments. CaCl2-extractability of Cu, a measure for its availability, is low in both carbonate-poor and carbonate-rich substrates, whereas its variability is high. The availability of Cd and Zn is much higher and increases when peat is supplied to carbonate-poor substrates. A strong reduction of Cd and Zn extractability is observed when clay is added to carbonate-poor substrates. This reduction depends on the clay type. Most observations could be explained taking into account pH differences between treatments, with kaolinite resulting in a lower pH in comparison to bentonite. These pH differences affect the presence and characteristics of dissolved organic carbon and the metal speciation, which in turns affects the interaction of metals with the solid soil phase. In carbonate-rich substrates, Cd and Zn availability is lower and the effects of peat and clay amendment are less clear. The latter can also be attributed to the high pH and lack of pH differences between treatments
Morphological properties of slender Ca II H fibrils observed by SUNRISE II
We use seeing-free high spatial resolution Ca II H data obtained by the
SUNRISE observatory to determine properties of slender fibrils in the lower
solar chromosphere. In this work we use intensity images taken with the SUFI
instrument in the Ca II H line during the second scientific flight of the
SUNRISE observatory to identify and track elongated bright structures. After
the identification, we analyze theses structures in order to extract their
morphological properties. We identify 598 slender Ca II H fibrils (SCFs) with
an average width of around 180 km, a length between 500 km and 4000 km, an
average lifetime of ~400 s, and an average curvature of 0.002 arcsec^-1. The
maximum lifetime of the SCFs within our time series of 57 minutes is ~2000 s.
We discuss similarities and differences of the SCFs with other small-scale,
chromospheric structures such as spicules of type I and II, or Ca II K fibrils.Comment: Accepted for publication in The Astrophysical Journal Supplement
Serie
Maximum Entropy Limit of Small-scale Magnetic Field Fluctuations in the Quiet Sun
The observed magnetic field on the solar surface is characterized by a very
complex spatial and temporal behavior. Although feature-tracking algorithms
have allowed us to deepen our understanding of this behavior, subjectivity
plays an important role in the identification and tracking of such features. In
this paper, we continue studies Gorobets, A. Y., Borrero, J. M., & Berdyugina,
S. 2016, ApJL, 825, L18 of the temporal stochasticity of the magnetic field on
the solar surface without relying either on the concept of magnetic features or
on subjective assumptions about their identification and interaction. We
propose a data analysis method to quantify fluctuations of the line-of-sight
magnetic field by means of reducing the temporal field's evolution to the
regular Markov process. We build a representative model of fluctuations
converging to the unique stationary (equilibrium) distribution in the long time
limit with maximum entropy. We obtained different rates of convergence to the
equilibrium at fixed noise cutoff for two sets of data. This indicates a strong
influence of the data spatial resolution and mixing-polarity fluctuations on
the relaxation process. The analysis is applied to observations of magnetic
fields of the relatively quiet areas around an active region carried out during
the second flight of the Sunrise/IMaX and quiet Sun areas at the disk center
from the Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory satellite.Comment: 11 pages, 5 figures, The Astrophysical Journal Supplement Series
(accepted
WhiskyMHD: a new numerical code for general relativistic magnetohydrodynamics
The accurate modelling of astrophysical scenarios involving compact objects
and magnetic fields, such as the collapse of rotating magnetized stars to black
holes or the phenomenology of gamma-ray bursts, requires the solution of the
Einstein equations together with those of general-relativistic
magnetohydrodynamics. We present a new numerical code developed to solve the
full set of general-relativistic magnetohydrodynamics equations in a dynamical
and arbitrary spacetime with high-resolution shock-capturing techniques on
domains with adaptive mesh refinements. After a discussion of the equations
solved and of the techniques employed, we present a series of testbeds carried
out to validate the code and assess its accuracy. Such tests range from the
solution of relativistic Riemann problems in flat spacetime, over to the
stationary accretion onto a Schwarzschild black hole and up to the evolution of
oscillating magnetized stars in equilibrium and constructed as consistent
solutions of the coupled Einstein-Maxwell equations.Comment: minor changes to match the published versio
Vector spectropolarimetry of dark-cored penumbral filaments with Hinode
We present spectropolarimetric measurements of dark-cored penumbral filaments
taken with Hinode at a resolution of 0.3". Our observations demonstrate that
dark-cored filaments are more prominent in polarized light than in continuum
intensity. Far from disk center, the Stokes profiles emerging from these
structures are very asymmetric and show evidence for magnetic fields of
different inclinations along the line of sight, together with strong Evershed
flows of at least 6-7 km/s. In sunspots closer to disk center, dark-cored
penumbral filaments exhibit regular Stokes profiles with little asymmetries due
to the vanishing line-of-sight component of the horizontal Evershed flow. An
inversion of the observed spectra indicates that the magnetic field is weaker
and more inclined in the dark cores as compared with the surrounding bright
structures. This is compatible with the idea that dark-cored filaments are the
manifestation of flux tubes carrying hot Evershed flows.Comment: Accepted for publication in ApJ Letters. Use the Postscript version
for high quality figure
Boundary regularity for the Poisson equation in reifenberg-flat domains
This paper is devoted to the investigation of the boundary regularity for the
Poisson equation {{cc} -\Delta u = f & \text{in} \Omega u= 0 & \text{on}
\partial \Omega where belongs to some and is a
Reifenberg-flat domain of More precisely, we prove that given an
exponent , there exists an such that the
solution to the previous system is locally H\"older continuous provided
that is -Reifenberg-flat. The proof is based on
Alt-Caffarelli-Friedman's monotonicity formula and Morrey-Campanato theorem
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