185 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
Zeeman Doppler Maps: Always Unique, Never Spurious?
Numerical models of atomic diffusion in magnetic atmospheres of ApBp stars predict abundance structures that differ from the empirical maps derived with (Zeeman) Doppler mapping (ZDM). An in-depth analysis of this apparent disagreement investigates the detectability by means of ZDM of a variety of abundance structures, including (warped) rings predicted by theory, but also complex spot-like structures. Even when spectra of high signal-to-noise ratio are available, it can prove difficult or altogether impossible to correctly recover shapes, positions, and abundances of a mere handful of spots, notwithstanding the use of all four Stokes parameters and an exactly known field geometry; the recovery of (warped) rings can be equally challenging. Inversions of complex abundance maps that are based on just one or two spectral lines usually permit multiple solutions. It turns out that it can by no means be guaranteed that any of the regularization functions in general use for ZDM (maximum entropy or Tikhonov) will lead to a true abundance map instead of some spurious one. Attention is drawn to the need for a study that would elucidate the relation between the stratified, field-dependent abundance structures predicted by diffusion theory on the one hand, and empirical maps obtained by means of "canonical" ZDM, i.e., with mean atmospheres and unstratified abundances, on the other hand. Finally, we point out difficulties arising from the three-dimensional nature of the atomic diffusion process in magnetic ApBp star atmospheres
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&
Rebuilding the Cepheid Distance Scale I: A Global Analysis of Cepheid Mean Magnitudes
We develop a statistical method for using multicolor photometry to determine
distances using Cepheid variables including the effects of temperature,
extinction, and metallicity and apply it to UBVRIJHK photometry of 694 Cepheids
in 17 galaxies. We derive homogeneous distance, extinction and uncertainty
estimates for four models, starting from the standard extragalactic method and
then adding the physical effects of temperature distributions, extinction
distributions, requiring positive definite extinctions, and metallicity. While
we find general agreement with published distances when we make similar
systematic assumptions, there is a clear problem in the standard distances
because they require Cepheids with negative extinctions, particularly in low
metallicity galaxies, unless the mean LMC extinction exceeds E(B-V) > 0.25. The
problem can be explained by the physically expected metallicity dependence of
the Cepheid distance scale, where metal-poor Cepheids are hotter and fainter
than metal-rich Cepheids. For V and I we found that the mean magnitude change
is -0.14 +/- 0.14 mag/dex and the mean color change is 0.13 +/- 0.04 mag/dex,
with the change in color dominating the change in distance. The effect on Type
Ia supernova estimates of the Hubble constant is dramatic because most were
found in the metal poor galaxies with the bluest Cepheids. The Type Ia
Multi-color Light Curve Shape (MLCS) method estimate for H_0 formally rises
from 69 +/- 8 km/s Mpc to 80 +/- 6 km/s Mpc with the metallicity correction.Comment: 54 pages, 7 figures, 4 tables, submitted to Ap
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