530 research outputs found
Type Ia supernovae from exploding oxygen-neon white dwarfs
The progenitor problem of Type Ia supernovae (SNe Ia) is still unsolved. Most
of these events are thought to be explosions of carbon-oxygen (CO) white dwarfs
(WDs), but for many of the explosion scenarios, particularly those involving
the externally triggered detonation of a sub-Chandrasekhar mass WD (sub-M Ch
WD), there is also a possibility of having an oxygen-neon (ONe) WD as
progenitor. We simulate detonations of ONe WDs and calculate synthetic
observables from these models. The results are compared with detonations in CO
WDs of similar mass and observational data of SNe Ia. We perform hydrodynamic
explosion simulations of detonations in initially hydrostatic ONe WDs for a
range of masses below the Chandrasekhar mass (M Ch), followed by detailed
nucleosynthetic postprocessing with a 384-isotope nuclear reaction network. The
results are used to calculate synthetic spectra and light curves, which are
then compared with observations of SNe Ia. We also perform binary evolution
calculations to determine the number of SNe Ia involving ONe WDs relative to
the number of other promising progenitor channels. The ejecta structures of our
simulated detonations in sub-M Ch ONe WDs are similar to those from CO WDs.
There are, however, small systematic deviations in the mass fractions and the
ejecta velocities. These lead to spectral features that are systematically less
blueshifted. Nevertheless, the synthetic observables of our ONe WD explosions
are similar to those obtained from CO models. Our binary evolution calculations
show that a significant fraction (3-10%) of potential progenitor systems should
contain an ONe WD. The comparison of our ONe models with our CO models of
comparable mass (1.2 Msun) shows that the less blueshifted spectral features
fit the observations better, although they are too bright for normal SNe Ia.Comment: 6 pages, 5 figure
Domain Wall Resistance based on Landauer's Formula
The scattering of the electron by a domain wall in a nano-wire is calculated
perturbatively to the lowest order. The resistance is calculated by use of
Landauer's formula. The result is shown to agree with the result of the linear
response theory if the equilibrium is assumed in the four-terminal case
Arepo-MCRT: Monte Carlo Radiation Hydrodynamics on a Moving Mesh
We present Arepo-MCRT, a novel Monte Carlo radiative transfer (MCRT)
radiation-hydrodynamics (RHD) solver for the unstructured moving-mesh code
Arepo. Our method is designed for general multiple scattering problems in both
optically thin and thick conditions. We incorporate numerous efficiency
improvements and noise reduction schemes to help overcome efficiency barriers
that typically inhibit convergence. These include continuous absorption and
energy deposition, photon weighting and luminosity boosting, local packet
merging and splitting, path-based statistical estimators, conservative
(face-centered) momentum coupling, adaptive convergence between time steps,
implicit Monte Carlo algorithms for thermal emission, and discrete-diffusion
Monte Carlo techniques for unresolved scattering, including a novel advection
scheme. We primarily focus on the unique aspects of our implementation and
discussions of the advantages and drawbacks of our methods in various
astrophysical contexts. Finally, we consider several test applications
including the levitation of an optically thick layer of gas by trapped infrared
radiation. We find that the initial acceleration phase and revitalized second
wind are connected via self-regulation of the RHD coupling, such that the RHD
method accuracy and simulation resolution each leave important imprints on the
long-term behavior of the gas.Comment: 23 pages, 20 figures, ApJ, in pres
Three-dimensional simulations of gravitationally confined detonations compared to observations of SN 1991T
The gravitationally confined detonation (GCD) model has been proposed as a
possible explosion mechanism for Type Ia supernovae in the single-degenerate
evolution channel. Driven by buoyancy, a deflagration flame rises in a narrow
cone towards the surface. For the most part, the flow of the expanding ashes
remains radial, but upon reaching the outer, low-pressure layers of the white
dwarf, an additional lateral component develops. This makes the deflagration
ashes converge again at the opposite side, where the compression heats fuel and
a detonation may be launched. To test the GCD explosion model, we perform a 3D
simulation for a model with an ignition spot offset near the upper limit of
what is still justifiable, 200 km. This simulation meets our deliberately
optimistic detonation criteria and we initiate a detonation. The detonation
burns through the white dwarf and leads to its complete disruption. We
determine nucleosynthetic yields by post-processing 10^6 tracer particles with
a 384 nuclide reaction network and we present multi-band light curves and
time-dependent optical spectra. We find that our synthetic observables show a
prominent viewing-angle sensitivity in UV and blue bands, which is in tension
with observed SNe Ia. The strong dependence on viewing-angle is caused by the
asymmetric distribution of the deflagration ashes in the outer ejecta layers.
Finally, we perform a comparison of our model to SN 1991T. The overall
flux-level of the model is slightly too low and the model predicts pre-maximum
light spectral features due to Ca, S, and Si that are too strong. Furthermore,
the model chemical abundance stratification qualitatively disagrees with recent
abundance tomography results in two key areas: our model lacks low velocity
stable Fe and instead has copious amounts of high-velocity 56Ni and stable Fe.
We therefore do not find good agreement of the model with SN 1991T.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic
The Sun's Preferred Longitudes and the Coupling of Magnetic Dynamo Modes
Observations show that solar activity is distributed non-axisymmetrically,
concentrating at "preferred longitudes". This indicates the important role of
non-axisymmetric magnetic fields in the origin of solar activity. We
investigate the generation of the non-axisymmetric fields and their coupling
with axisymmetric solar magnetic field. Our kinematic generation (dynamo) model
operating in a sphere includes solar differential rotation, which approximates
the differential rotation obtained by inversion of helioseismic data, modelled
distributions of the turbulent resistivity, non-axisymmetric mean helicity, and
meridional circulation in the convection zone. We find that (1) the
non-axisymmetric modes are localised near the base of the convection zone,
where the formation of active regions starts, and at latitudes around
; (2) the coupling of non-axisymmetric and axisymmetric modes
causes the non-axisymmetric mode to follow the solar cycle; the phase relations
between the modes are found. (3) The rate of rotation of the first
non-axisymmetric mode is close to that determined in the interplanetary space.Comment: 22 pages, 18 figures. Accepted for publication in the Astrophysical
Journa
Melatonin: A Cutaneous Perspective on its Production, Metabolism, and Functions
Melatonin, an evolutionarily ancient derivative of serotonin with hormonal properties, is the main neuroendocrine secretory product of the pineal gland. Although melatonin is best known to regulate circadian rhythmicity and lower vertebrate skin pigmentation, the full spectrum of functional activities of this free radical-scavenging molecule, which also induces/promotes complex antioxidative and DNA repair systems, includes immunomodulatory, thermoregulatory, and antitumor properties. Because this plethora of functional melatonin properties still awaits to be fully appreciated by dermatologists, the current review synthesizes the main features that render melatonin a promising candidate for the management of several dermatoses associated with substantial oxidative damage. We also review why melatonin promises to be useful in skin cancer prevention, skin photo- and radioprotection, and as an inducer of repair mechanisms that facilitate the recovery of human skin from environmental damage. The fact that human skin and hair follicles not only express functional melatonin receptors but also engage in substantial, extrapineal melatonin synthesis further encourages one to systematically explore how the skin's melatonin system can be therapeutically targeted in future clinical dermatology and enrolled for preventive medicine strategies
- …