337 research outputs found
On the physical origin of the second solar spectrum of the Sc II line at 4247 A
The peculiar three-peak structure of the linear polarization profile shown in
the second solar spectrum by the Ba II line at 4554 A has been interpreted as
the result of the different contributions coming from the barium isotopes with
and without hyperfine structure (HFS). In the same spectrum, a triple peak
polarization signal is also observed in the Sc II line at 4247 A. Scandium has
a single stable isotope (^{45}Sc), which shows HFS due to a nuclear spin I=7/2.
We investigate the possibility of interpreting the linear polarization profile
shown in the second solar spectrum by this Sc II line in terms of HFS. A
two-level model atom with HFS is assumed. Adopting an optically thin slab
model, the role of atomic polarization and of HFS is investigated, avoiding the
complications caused by radiative transfer effects. The slab is assumed to be
illuminated from below by the photospheric continuum, and the polarization of
the radiation scattered at 90 degrees is investigated. The three-peak structure
of the scattering polarization profile observed in this Sc II line cannot be
fully explained in terms of HFS. Given the similarities between the Sc II line
at 4247 A and the Ba II line at 4554 A, it is not clear why, within the same
modeling assumptions, only the three-peak Q/I profile of the barium line can be
fully interpreted in terms of HFS. The failure to interpret this Sc II
polarization signal raises important questions, whose resolution might lead to
significant improvements in our understanding of the second solar spectrum. In
particular, if the three-peak structure of the Sc II signal is actually
produced by a physical mechanism neglected within the approach considered here,
it will be extremely interesting not only to identify this mechanism, but also
to understand why it seems to be less important in the case of the barium line.Comment: 8 pages, 8 figures, and 1 table. Accepted for publication in
Astronomy and Astrophysic
Theoretical formulation of Doppler redistribution in scattering polarization within the framework of the velocity-space density matrix formalism
Within the framework of the density matrix theory for the generation and
transfer of polarized radiation, velocity density matrix correlations represent
an important physical aspect that, however, is often neglected in practical
applications by adopting the simplifying approximation of complete
redistribution on velocity. In this paper, we present an application of the
Non-LTE problem for polarized radiation taking such correlations into account
through the velocity-space density matrix formalism. We consider a two-level
atom with infinitely sharp upper and lower levels, and we derive the
corresponding statistical equilibrium equations neglecting the contribution of
velocity-changing collisions. Coupling such equations with the radiative
transfer equations for polarized radiation, we derive a set of coupled
equations for the velocity-dependent source function. This set of equations is
then particularized to the case of a plane-parallel atmosphere. The equations
presented in this paper provide a complete and solid description of the physics
of pure Doppler redistribution, a phenomenon generally described within the
framework of the redistribution matrix formalism. The redistribution matrix
corresponding to this problem (generally referred to as R_I) is derived
starting from the statistical equilibrium equations for the velocity-space
density matrix and from the radiative transfer equations for polarized
radiation, thus showing the equivalence of the two approaches.Comment: Accepted for publication in Astronomy & Astrophysic
Isotropic inelastic and superelastic collisional rates in a multiterm atom
The spectral line polarization of the radiation emerging from a magnetized
astrophysical plasma depends on the state of the atoms within the medium, whose
determination requires considering the interactions between the atoms and the
magnetic field, between the atoms and photons (radiative transitions), and
between the atoms and other material particles (collisional transitions). In
applications within the framework of the multiterm model atom (which accounts
for quantum interference between magnetic sublevels pertaining either to the
same J-level or to different J-levels within the same term) collisional
processes are generally neglected when solving the master equation for the
atomic density matrix. This is partly due to the lack of experimental data
and/or of approximate theoretical expressions for calculating the collisional
transfer and relaxation rates (in particular the rates for interference between
sublevels pertaining to different J-levels, and the depolarizing rates due to
elastic collisions). In this paper we formally define and investigate the
transfer and relaxation rates due to isotropic inelastic and superelastic
collisions that enter the statistical equilibrium equations of a multiterm
atom. Under the hypothesis that the atom-collider interaction can be described
by a dipolar operator, we provide expressions that relate the collisional rates
for interference between different J-levels to the usual collisional rates for
J-level populations. Finally, we apply the general equations to the case of a
two-term atom with unpolarized lower term, illustrating the impact of inelastic
and superelastic collisions on scattering polarization through radiative
transfer calculations in a slab of stellar atmospheric plasma anisotropically
illuminated by the photospheric radiation field.Comment: Accepted for publication in Astronomy & Astrophysic
Fish oil-based emulsion for the treatment of parenteral nutrition associated liver disease in an adult patient
Background & aims: Reversal of parenteral nutrition associated liver disease with fish oil emulsion (FO) has been reported in infants. We report a similar case in an adult patient. Methods: A 58 year-old female on home parenteral nutrition for a short bowel syndrome due to Crohn's disease, showed a progressive worsening of liver steatosis, and a persistent increase of the plasma liver function tests (LFTs). LFTs, serum alpha-tochopherol, red blood cell membrane fatty acids and liver histology were evaluated before and after an 8 month treatment with FO. Results: The patient's LFT's improved. There was an increase of the n-3 and a decrease of the n-6 series of fatty acids in erythrocyte membrane. There was an approximate 30% increase in vitamin E status. Before FO, liver histology showed a non-alcoholic steatohepatitis with grade 2 steatosis and inflammation and stage 3 fibrosis. After the treatment, steatosis and inflammation were grade 1, whereas fibrosis remained at stage 3. Conclusions: Infusion of FO was associated with consistent changes of cell membrane fatty acid structure and with mild improvement of vitamin E status. A potential role of FO in decreasing liver steatosis and inflammation with no change of liver fibrosis might be suggested. © 2010 European Society for Clinical Nutrition and Metabolism
Observational indications of magneto-optical effects in the scattering polarization wings of the Ca I 4227 \AA\ line
Several strong resonance lines, such as H I Ly-, Mg II k, Ca II K, Ca
I 4227 \AA\, which are characterized by deep and broad absorption profiles in
the solar intensity spectrum, show conspicuous linear scattering polarization
signals when observed in quiet regions close to the solar limb. Such signals
show a characteristic triplet-peak structure, with a sharp peak in the line
core and extended wing lobes. The line core peak is sensitive to the presence
of magnetic fields through the Hanle effect, which however is known not to
operate in the line wings. Recent theoretical studies indicate that, contrary
to what was previously believed, the wing linear polarization signals are also
sensitive to the magnetic field through magneto-optical effects (MO). We search
for observational indications of this recently discovered physical mechanism in
the scattering polarization wings of the Ca I 4227 \AA\ line. We performed a
series of spectropolarimetric observations of this line using the Zurich
IMaging POLarimeter (ZIMPOL) camera at the Gregory-Coud\'e telescope of IRSOL
(Switzerland) and at the GREGOR telescope in Tenerife (Spain). Spatial
variations of the total linear polarization degree and of the linear
polarization angle are clearly appreciable in the wings of the observed line.
We provide a detailed discussion of our observational results, showing that the
detected variations always take place in regions where longitudinal magnetic
fields are present, thus supporting the theoretical prediction that they are
produced by MO effects.Comment: Accepted by A&
Spatial variations of the Sr i 4607 {\AA} scattering polarization peak
Context. The scattering polarization signal observed in the photospheric Sr i
4607 {\AA} line is expected to vary at granular spatial scales. This variation
can be due to changes in the magnetic field intensity and orientation (Hanle
effect), but also to spatial and temporal variations in the plasma properties.
Measuring the spatial variation of such polarization signal would allow us to
study the properties of the magnetic fields at subgranular scales, but
observations are challenging since both high spatial resolution and high
spectropolarimetric sensitivity are required.
Aims. We aim to provide observational evidence of the polarization peak
spatial variations, and to analyze the correlation they might have with
granulation.
Methods. Observations conjugating high spatial resolution and high
spectropolarimetric precision were performed with the Zurich IMaging
POLarimeter, ZIMPOL, at the GREGOR solar telescope, taking advantage of the
adaptive optics system and the newly installed image derotator.
Results. Spatial variations of the scattering polarization in the Sr i 4607
{\AA} line are clearly observed. The spatial scale of these variations is
comparable with the granular size. Small correlations between the polarization
signal amplitude and the continuum intensity indicate that the polarization is
higher at the center of granules than in the intergranular lanes.Comment: 5 pages, 4 figure
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