4,215 research outputs found
Magneto-acoustic waves in sunspots from observations and numerical simulations
We study the propagation of waves from the photosphere to the chromosphere of
sunspots. From time series of cospatial Ca II H (including its line blends)
intensity spectra and polarimetric spectra of Si I 1082.7 nm and He I 1083.0 nm
we retrieve the line-of-sight velocity at several heights. The analysis of the
phase difference and amplification spectra shows standing waves for frequencies
below 4 mHz and propagating waves for higher frequencies, and allows us to
infer the temperature and height where the lines are formed. Using these
observational data, we have constructed a model of sunspot, and we have
introduced the velocity measured with the photospheric Si I 1082.7 nm line as a
driver. The numerically propagated wave pattern fits reasonably well with the
observed using the lines formed at higher layers, and the simulations reproduce
many of the observed features. The observed waves are slow MHD waves
propagating longitudinally along field lines.Comment: proceedings of GONG 2010/SOHO 24 meeting, June 27 - July 2, 2010,
Aix-en-Provence, Franc
Detection of emission in the Si i 1082.7 nm line core in sunspot umbrae
We analyze spectropolarimetric sunspot umbra observations taken in the
near-infrared Si i 1082.7 nm line taking NLTE effects into account. The data
were obtained with the GRIS instrument installed at the German GREGOR
telescope. A point spread function (PSF) was constructed using prior Mercury
observations with GRIS and the information provided by the adaptive optics
system of the GREGOR telescope. The data were then deconvolved from the PSF
using a principal component analysis deconvolution method and were analyzed via
the NICOLE inversion code. The Si i 1082.7 nm line seems to be in emission in
the umbra of the observed sunspot after the effects of scattered light are
removed. We show how the spectral line shape of umbral profiles changes
dramatically with the amount of scattered light. Indeed, the continuum levels
range, on average, from 44% of the quiet Sun continuum intensity to about 20%.
The inferred levels are in line with current model predictions and empirical
umbral models. Current umbral empirical models are not able to reproduce the
emission in the deconvolved umbral Stokes profiles. The results of the NLTE
inversions suggests that to obtain the emission in the Si i 1082.7 nm line, the
temperature stratification should first have a hump located at about log tau -2
and start rising at lower heights when moving into the transition region. This
is, to our knowledge, the first time the Si i 1082.7 nm line is seen in
emission in sunspot umbrae. The results show that the temperature
stratification of current umbral models may be more complex than expected with
the transition region located at lower heights above sunspot umbrae. Our
finding might provide insights into understanding why the sunspot umbra
emission in the millimeter spectral range is less than that predicted by
current empirical umbral models
Signatures of the impact of flare ejected plasma on the photosphere of a sunspot light-bridge
We investigate the properties of a sunspot light-bridge, focusing on the
changes produced by the impact of a plasma blob ejected from a C-class flare.
We observed a sunspot in active region NOAA 12544 using spectropolarimetric
raster maps of the four Fe I lines around 15655 \AA\ with the GREGOR Infrared
Spectrograph (GRIS), narrow-band intensity images sampling the Fe I 6173 \AA\
line with the GREGOR Fabry-P\'erot Interferometer (GFPI), and intensity broad
band images in G-band and Ca II H band with the High-resolution Fast Imager
(HiFI). All these instruments are located at the GREGOR telescope at the
Observatorio del Teide, Tenerife, Spain. The data cover the time before,
during, and after the flare event. The analysis is complemented with
Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI)
data from the Solar Dynamics Observatory (SDO). The physical parameters of the
atmosphere at differents heights were inferred using spectral-line inversion
techniques. We identify photospheric and chromospheric brightenings, heating
events, and changes in the Stokes profiles associated to the flare eruption and
the subsequent arrival of the plasma blob to the light bridge, after traveling
along an active region loop. The measurements suggest that these phenomena are
the result of reconnection events driven by the interaction of the plasma blob
with the magnetic field topology of the light bridge.Comment: Accepted for publication in A&
Aspects of thermal leptogenesis in braneworld cosmology
The mechanism of thermal leptogenesis is investigated in the high-energy
regime of braneworld cosmology. Within the simplest seesaw framework with
hierarchical heavy Majorana neutrinos, we study the implications of the
modified Friedmann equation on the realization of this mechanism. In contrast
with the usual leptogenesis scenario of standard cosmology, where low-energy
neutrino data favors a mildly strong washout regime, we find that leptogenesis
in the braneworld regime is successfully realized in a weak washout regime.
Furthermore, a quasi-degenerate light neutrino mass spectrum is found to be
compatible with this scenario. For an initially vanishing heavy Majorana
neutrino abundance, thermal leptogenesis in the brane requires the decaying
heavy Majorana neutrino mass to be M1 > 10^10 GeV and the fundamental
five-dimensional gravity scale 10^12 < M5 < 10^16 GeV, which corresponds to a
transition from brane to standard cosmology at temperatures 10^8 < Tt < 10^14
GeV.Comment: 7 pages, 3 figures, a few comments and references added. Final
version to appear in Phys. Rev.
Gravitational baryogenesis in Gauss-Bonnet braneworld cosmology
The mechanism of gravitational baryogenesis, based on the CPT-violating
gravitational interaction between the derivative of the Ricci scalar curvature
and the baryon-number current, is investigated in the context of the
Gauss-Bonnet braneworld cosmology. We study the constraints on the fundamental
five-dimensional gravity scale, the effective scale of B-violation and the
decoupling temperature, for the above mechanism to generate an acceptable
baryon asymmetry during the radiation-dominated era. The scenario of
gravitational leptogenesis, where the lepton-number violating interactions are
associated with the neutrino mass seesaw operator, is also considered.Comment: 8 pages, 3 figures, final version to appear in Phys. Rev.
Multi-layer study of wave propagation in sunspots
We analyze the propagation of waves in sunspots from the photosphere to the
chromosphere using time series of co-spatial Ca II H intensity spectra
(including its line blends) and polarimetric spectra of Si I 10827 and the He I
10830 multiplet. From the Doppler shifts of these lines we retrieve the
variation of the velocity along the line-of-sight at several heights. Phase
spectra are used to obtain the relation between the oscillatory signals. Our
analysis reveals standing waves at frequencies lower than 4 mHz and a
continuous propagation of waves at higher frequencies, which steepen into
shocks in the chromosphere when approaching the formation height of the Ca II H
core. The observed non-linearities are weaker in Ca II H than in He I lines.
Our analysis suggests that the Ca II H core forms at a lower height than the He
I 10830 line: a time delay of about 20 s is measured between the Doppler signal
detected at both wavelengths. We fit a model of linear slow magnetoacoustic
wave propagation in a stratified atmosphere with radiative losses according to
Newton's cooling law to the phase spectra and derive the difference in the
formation height of the spectral lines. We show that the linear model describes
well the wave propagation up to the formation height of Ca II H, where
non-linearities start to become very important.Comment: Accepted by The Astrophysical Journa
Bulk viscosity in a cold CFL superfluid
We compute one of the bulk viscosity coefficients of cold CFL quark matter in
the temperature regime where the contribution of mesons, quarks and gluons to
transport phenomena is Boltzmann suppressed. In that regime dissipation occurs
due to collisions of superfluid phonons, the Goldstone modes associated to the
spontaneous breaking of baryon symmetry. We first review the hydrodynamics of
relativistic superfluids, and remind that there are at least three bulk
viscosity coefficients in these systems. We then compute the bulk viscosity
coefficient associated to the normal fluid component of the superfluid. In our
analysis we use Son's effective field theory for the superfluid phonon, amended
to include scale breaking effects proportional to the square of the strange
quark mass m_s. We compute the bulk viscosity at leading order in the scale
breaking parameter, and find that it is dominated by collinear splitting and
joining processes. The resulting transport coefficient is zeta=0.011 m_s^4/T,
growing at low temperature T until the phonon fluid description stops making
sense. Our results are relevant to study the rotational properties of a compact
star formed by CFL quark matter.Comment: 19 pages, 2 figures; one reference added, version to be published in
JCA
Spatially Resolved Outflows in a Seyfert Galaxy at z = 2.39
We present the first spatially resolved analysis of rest-frame optical and UV
imaging and spectroscopy for a lensed galaxy at z = 2.39 hosting a Seyfert
active galactic nucleus (AGN). Proximity to a natural guide star has enabled
high signal-to-noise VLT SINFONI + adaptive optics observations of rest-frame
optical diagnostic emission lines, which exhibit an underlying broad component
with FWHM ~ 700 km/s in both the Balmer and forbidden lines. Measured line
ratios place the outflow robustly in the region of the ionization diagnostic
diagrams associated with AGN. This unique opportunity - combining gravitational
lensing, AO guiding, redshift, and AGN activity - allows for a magnified view
of two main tracers of the physical conditions and structure of the
interstellar medium in a star-forming galaxy hosting a weak AGN at cosmic noon.
By analyzing the spatial extent and morphology of the Ly-alpha and
dust-corrected H-alpha emission, disentangling the effects of star formation
and AGN ionization on each tracer, and comparing the AGN induced mass outflow
rate to the host star formation rate, we find that the AGN does not
significantly impact the star formation within its host galaxy.Comment: 16 pages, 5 figures, accepted for publication in Ap
Statistical Mechanics of Unbound Two Dimensional Self-Gravitating Systems
We study, using both theory and molecular dynamics simulations, the
relaxation dynamics of a microcanonical two dimensional self-gravitating
system. After a sufficiently large time, a gravitational cluster of N particles
relaxes to the Maxwell-Boltzmann distribution. The time to reach the
thermodynamic equilibrium, however, scales with the number of particles. In the
thermodynamic limit, at fixed total mass, equilibrium state is
never reached and the system becomes trapped in a non-ergodic stationary state.
An analytical theory is presented which allows us to quantitatively described
this final stationary state, without any adjustable parameters
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