221 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
Multiline Zeeman Signatures Through Line Addition
In order to get a significant Zeeman signature in the polarised spectra of a
magnetic star, we usually 'add' the contributions of numerous spectral lines;
the ultimate goal is to recover the spectropolarimetric prints of the magnetic
field in these line additions. Here we want to clarify the meaning of these
techniques of line addition; in particular, we try to interpret the meaning of
the 'pseudo-line' formed during this process and to find out why and how its
Zeeman signature is still meaningful. We create a synthetic case of line
addition and apply well tested standard solar methods routinely used in the
research on magnetism in our nearest star. The results are convincing and the
Zeeman signatures well detected; Solar methods are found to be quite efficient
also for stellar observations. We statistically compare line addition with
least-squares deconvolution and demonstrate that they both give very similar
results as a consequence of the special statistical properties of the weights.
The Zeeman signatures are unequivocally detected in this multiline approach. We
may anticipate the outcome that magnetic field detection is reliable well
beyond the weak-field approximation. Linear polarisation in the spectra of
solar type stars can be detected when the spectral resolution is sufficiently
high.Comment: 8 pages, accepted for publication in A&
Radiative diffusion in stellar atmospheres: diffusion velocities
The present paper addresses some of the problems in the buildup of element
stratification in stellar magnetic atmospheres due to microscopic diffusion, in
particular the redistribution of momentum among the various ionisation stages
of a given element and the calculation of diffusion velocities in the presence
of inclined magnetic fields.
We have considerably modified and extended our CARAT code to provide
radiative accelerations, not only from bound-bound but also from bound-free
transitions. In addition, our code now computes ionisation and recombination
rates, both radiative and collisional. These rates are used in calculating the
redistribution of momentum among the various ionisation stages of the chemical
elements. A careful comparison shows that the two different theoretical
approaches to redistribution that are presently available lead to widely
discrepant results for some chemical elements, especially in the magnetic case.
In the absence of a fully satisfactory theory of redistribution, we propose to
use the geometrical mean of the radiative accelerations from both methods.
Diffusion velocities have been calculated for 28 chemical elements in a T_eff
= 12000K, log g = 4.00 stellar magnetic atmosphere with solar abundances.
Velocities and resulting element fluxes in magnetic fields are discussed; rates
of abundance changes are analysed for systematic trends with field strength and
field direction. Special consideration is given to the Si case and our results
are confronted in detail with well-known results derived more than two decades
ago.Comment: To be published in Astronomy & Astrophysics (accepted 02/03/2006
Modelling element distributions in the atmospheres of magnetic Ap stars
In recent papers convincing evidence has been presented for chemical
stratification in Ap star atmospheres, and surface abundance maps have been
shown to correlate with the magnetic field direction. Radiatively driven
diffusion in magnetic fields is among the processes responsible for these
inhomogeneities. Here we explore the hypothesis that equilibrium
stratifications can, in a number of cases, explain the observed abundance maps
and vertical distributions of the various elements. The investigation of
equilibrium stratifications in stellar atmospheres with temperatures from 8500K
to 12000K and fields up to 10 kG reveals considerable variations in the
vertical distribution of the 5 elements studied (Mg, Si, Ca, Ti, Fe), often
with zones of large over- or under-abundances and with indications of other
competing processes (such as mass loss). Horizontal magnetic fields can be very
efficient in helping the accumulation of elements in higher layers. A
comparison between our calculations and the vertical abundance profiles and
surface maps derived by magnetic Doppler imaging reveals that equilibrium
stratifications are in a number of cases consistent with the main trends
inferred from observed spectra. However, it is not clear whether such
equilibrium solutions will ever be reached during the evolution of an Ap star.Comment: 7 pages, 6 figures, the paper will be published in Astronomy &
Astrophysics, on November 200
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