6,073 research outputs found
Scattering Polarization of Hydrogen Lines in Weakly Magnetized Stellar Atmospheres I. Formulation and Application to Isothermal Models
Although the spectral lines of hydrogen contain valuable information on the
physical properties of a variety of astrophysical plasmas, including the upper
solar chromosphere, relatively little is known about their scattering
polarization signals whose modification via the Hanle effect may be exploited
for magnetic field diagnostics. Here we report on a basic theoretical
investigation of the linear polarization produced by scattering processes and
the Hanle effect in Ly-a, Ly-b and H-a taking into account multilevel radiative
transfer effects in an isothermal stellar atmosphere model, the fine-structure
of the hydrogen levels, as well as the impact of collisions with electrons and
protons. The main aim of this first paper is to elucidate the physical
mechanisms that control the linear polarization in the three lines, as well as
its sensitivity to the perturbers density and to the strength and structure of
micro-structured and deterministic magnetic fields. To this end, we apply an
efficient radiative transfer code we have developed for performing numerical
simulations of the Hanle effect in multilevel systems with overlapping line
transitions. For low density plasmas such as that of the upper solar
chromosphere collisional depolarization is caused mainly by collisional
transitions between the fine-structure levels of n=3, so that it is virtually
insignificant for Ly-a but important for Ly-b and H-a. We show the impact of
the Hanle effect on the three lines taking into account the radiative transfer
coupling between the different hydrogen line transitions. For example, we
demonstrate that the linear polarization profile of the H-a line is sensitive
to the presence of magnetic field gradients in the line core formation region
and that in solar-like chromospheres selective absorption of polarization
components does not play any significant role on the emergent scattering
polarization.Comment: 24 pages, 16 figures, 2 tables, accepted for publication in Ap
Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects
A big challenge in solar and stellar physics in the coming years will be to
decipher the magnetism of the solar outer atmosphere (chromosphere and corona)
along with its dynamic coupling with the magnetic fields of the underlying
photosphere. To this end, it is important to develop rigorous diagnostic tools
for the physical interpretation of spectropolarimetric observations in suitably
chosen spectral lines. Here we present a computer program for the synthesis and
inversion of Stokes profiles caused by the joint action of atomic level
polarization and the Hanle and Zeeman effects in some spectral lines of
diagnostic interest, such as those of the He I 10830 A and D_3 multiplets. It
is based on the quantum theory of spectral line polarization, which takes into
account all the relevant physical mechanisms and ingredients (optical pumping,
atomic level polarization, Zeeman, Paschen-Back and Hanle effects). The
influence of radiative transfer on the emergent spectral line radiation is
taken into account through a suitable slab model. The user can either calculate
the emergent intensity and polarization for any given magnetic field vector or
infer the dynamical and magnetic properties from the observed Stokes profiles
via an efficient inversion algorithm based on global optimization methods. The
reliability of the forward modeling and inversion code presented here is
demonstrated through several applications, which range from the inference of
the magnetic field vector in solar active regions to determining whether or not
it is canopy-like in quiet chromospheric regions. This user-friendly diagnostic
tool called "HAZEL" (from HAnle and ZEeman Light) is offered to the
astrophysical community, with the hope that it will facilitate new advances in
solar and stellar physics.Comment: 62 pages, 19 figures, 3 tables. Accepted for publication in Ap
Depolarizing collisions with hydrogen: neutral and singly ionized alkaline earths
Depolarizing collisions are elastic or quasielastic collisions that equalize
the populations and destroy the coherence between the magnetic sublevels of
atomic levels. In astrophysical plasmas, the main depolarizing collider is
neutral hydrogen. We consider depolarizing rates on the lowest levels of
neutral and singly ionized alkaly-earths Mg I, Sr I, Ba I, Mg II, Ca II, and Ba
II, due to collisions with H. We compute ab initio potential curves of the
atom-H system and solve the quantum mechanical dynamics. From the scattering
amplitudes we calculate the depolarizing rates for Maxwellian distributions of
colliders at temperatures T <10000 K. A comparative analysis of our results and
previous calculations in the literature is done. We discuss the effect of these
rates on the formation of scattering polarization patterns of resonant lines of
alkali-earths in the solar atmosphere, and their effect on Hanle effect
diagnostics of solar magnetic fields.Comment: 18 pages, 3 figures. Summitted to ApJ (2014
The scattering polarization of the Ly-alpha lines of H I and He II taking into account PRD and J-state interference effects
Recent theoretical investigations have pointed out that the cores of the
Ly-alpha lines of H I and He II should show measurable scattering polarization
signals when observing the solar disk, and that the magnetic sensitivity,
through the Hanle effect, of such linear polarization signals is suitable for
exploring the magnetism of the solar transition region. Such investigations
were carried out in the limit of complete frequency redistribution (CRD) and
neglecting quantum interference between the two upper J-levels of each line.
Here we relax both approximations and show that the joint action of partial
frequency redistribution (PRD) and J-state interference produces much more
complex fractional linear polarization (Q/I) profiles, with large amplitudes in
their wings. Such wing polarization signals turn out to be very sensitive to
the temperature structure of the atmospheric model, so that they can be
exploited for constraining the thermal properties of the solar chromosphere.
Finally, we show that the approximation of CRD without J-state interference is
however suitable for estimating the amplitude of the linear polarization
signals in the core of the lines, where the Hanle effect operates.Comment: Accepted for publication in The Astrophysical Journal Letter
Tensorial depolarization of alkali atoms by isotropic collisions with neutral hydrogen
Results. We consider the problem of isotropic collisions between an alkali
atom and neutral hydrogen. We calculate the collisional tensorial components of
general p and s-states, characterized by their effective principal quantum
number . It is found that the behaviour of the tensorial components obey
simple power laws allowing quick calculations of the depolarizing collisional
rates. As application, our results should allow a rigorous treatment of the
atomic polarization profiles of the D1 -D2 lines of alkali atoms.
Conclusions. Close coupling treatments of atomic collisions are needed to
decipher the information encoded in the polarized radiation from the Sun.
Important problems remain unresolved like the role of collisions in the
Paschen-Back conditions.Comment: Accepted for publication in A&
Dichroic Masers due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization
The theory of the generation and transfer of polarized radiation, mainly
developed for interpreting solar spectropolarimetric observations, allows to
reconsider, in a more rigorous and elegant way, a physical mechanism that has
been suggested some years ago to interpret the high degree of polarization
often observed in astronomical masers. This mechanism, for which the name of
'dichroic maser' is proposed, can operate when a low density molecular cloud is
illuminated by an anisotropic source of radiation (like for instance a nearby
star). Here we investigate completely unsaturated masers and show that
selective stimulated emission processes are capable of producing highly
polarized maser radiation in a non-magnetic environment. The polarization of
the maser radiation is linear and is directed tangentially to a ring
equidistant to the central star. We show that the Hanle effect due to the
presence of a magnetic field can produce a rotation (from the tangential
direction) of the polarization by more that 45 degrees for some selected
combinations of the strength, inclination and azimuth of the magnetic field
vector. However, these very same conditions produce a drastic inhibition of the
maser effect. The rotations of about 90 degrees observed in SiO masers in the
evolved stars TX Cam by Kemball & Diamond (1997) and IRC+10011 by Desmurs et al
(2000) may then be explainedby a local modification of the anisotropy of the
radiation field, being transformed from mainly radial to mainly tangential.Comment: Accepted for publication on Ap
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