501 research outputs found
The recondite intricacies of Zeeman Doppler mapping
We present a detailed analysis of the reliability of abundance and magnetic
maps of Ap stars obtained by Zeeman Doppler mapping (ZDM). It is shown how they
can be adversely affected by the assumption of a mean stellar atmosphere
instead of appropriate "local" atmospheres corresponding to the actual
abundances in a given region. The essenceof the difficulties was already shown
by Chandrasekhar's picket-fence model. The results obtained with a suite of
Stokes codes written in the Ada programming language and based on modern
line-blanketed atmospheres are described in detail. We demonstrate that the
high metallicity values claimed to have been found in chemically inhomogeneous
Ap star atmospheres would lead to local temperature structures, continuum and
line intensities, and line shapes that differ significantly from those
predicted by a mean stellar atmosphere. Unfortunately, past applications of ZDM
have consistently overlooked the intricate aspects of metallicity with their
all-pervading effects. The erroneous assumption of a mean atmosphere for a
spotted star can lead to phase-dependent errors of uncomfortably large
proportions at varying wavelengths both in the Stokes I and V profiles, making
precise mapping of abundances and magnetic field vectors largely impossible.
The relation between core and wings of the H_beta line changes, too, with
possible repercussions on the determination of gravity and effective
temperature. Finally, a ZDM analysis of the synthetic Stokes spectra of a
spotted star reveals the disturbing differences between the respective
abundance maps based on a mean atmosphere on the one hand, and on appropriate
"local" atmospheres on the other. We then discuss what this all means for
published ZDMresults. Our discussion makes it clear that realistic local
atmospheres must be used, especially if credible small-scale structures are to
be obtained.Comment: Accepted for publication in MNRA
Modelling the incomplete Paschen-Back effect in the spectra of magnetic Ap stars
We present first results of a systematic investigation of the incomplete
Paschen-Back effect in magnetic Ap stars. A short overview of the theory is
followed by a demonstration of how level splittings and component strengths
change with magnetic field strength for some lines of special astrophysical
interest. Requirements are set out for a code which allows the calculation of
full Stokes spectra in the Paschen-Back regime and the behaviour of Stokes I
and V profiles of transitions in the multiplet 74 of FeII is discussed in some
detail. It is shown that the incomplete Paschen-Back effect can lead to
noticeable line shifts which strongly depend on total multiplet strength,
magnetic field strength and field direction. Ghost components (which violate
the normal selection rule on J) show up in strong magnetic fields but are
probably unobservable. Finally it is shown that measurements of the integrated
magnetic field modulus are not adversely affected by the Paschen-Back
effect, and that there is a potential problem in (magnetic) Doppler mapping if
lines in the Paschen-Back regime are treated in the Zeeman approximation.Comment: 8 pages, 10 figures, to appear in MNRA
Magnetic deformation of the white dwarf surface structure
The influence of strong, large-scale magnetic fields on the structure and
temperature distribution in white dwarf atmospheres is investigated. Magnetic
fields may provide an additional component of pressure support, thus possibly
inflating the atmosphere compared to the non-magnetic case. Since the magnetic
forces are not isotropic, atmospheric properties may significantly deviate from
spherical symmetry. In this paper the magnetohydrostatic equilibrium is
calculated numerically in the radial direction for either for small deviations
from different assumptions for the poloidal current distribution. We generally
find indication that the scale height of the magnetic white dwarf atmosphere
enlarges with magnetic field strength and/or poloidal current strength. This is
in qualitative agreement with recent spectropolarimetric observations of
Grw+10\degr8247. Quantitatively, we find for e.g. a mean surface poloidal
field strength of 100 MG and a toroidal field strength of 2-10 MG an increase
of scale height by a factor of 10. This is indicating that already a small
deviation from the initial force-free dipolar magnetic field may lead to
observable effects. We further propose the method of finite elements for the
solution of the two-dimensional magnetohydrostatic equilibrium including
radiation transport in the diffusive approximation. We present and discuss
preliminary solutions, again indicating on an expansion of the magnetized
atmosphere.Comment: 14 pages with 14 figure
Data of German speech minorities in the Archive for Spoken German: an overview
Speech islands are historically and developmentally unique and will inevitably disappear within the next decades. We urgently need to preserve their remains and exploit what is left in order to make research on language-in-contact and historical as well as current comparative language research possible.
The Archive for Spoken German (AGD) at the Institute for German Language collects, fosters and archives data from completed research projects and makes them available to the wider research community.
Besides large variation corpora and corpora of conversational speech, the archive already contains a range of collections of data on German speech minorities. The latter will be outlined in this chapter. Some speech island data is already made available through the personal service of the AGD, or the database of spoken German (DGD), e.g. data on Australian German, Unserdeutsch, or German in North America. Some corpora are still being prepared for publication, but still important to document for potentially interested research projects. We therefore also explain the current problems and efforts related to the curation of speech island data, from the digitization of recordings and the collection of metadata, to the integration of transcriptions, annotations and other ways of accessing and sharing data
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