arXiv subjects

M. J. Stift

Publications and source records attributed to M. J. Stift.

15 recordsLinked to original sources

arXiv2025

Zeeman Doppler mapping deconstructed

Aims. Magnetic and abundance maps of chemically peculiar (CP) stars, derived with the help of Zeeman Doppler mapping, have invariably been used as arguments against theories, in particular atomic diffusion theory. We intend to expose the fallacy of these claims. Methods. We have identified in the literature those (5) CP stars for which multiple maps have been published, all based on the same Zeeman Doppler mapping strategy. For each of these stars we have then carried out inter comparisons between the recovered distributions of magnetic field and of abundances. Results. Agreement between maps often turns out to be quite poor in regard to both abundances, field topology and absolute field strengths. Maps based on the same set of observations can differ considerably, even when they are coming from the same authors. Conclusions. It becomes clear that Zeeman Doppler mapping cannot be guaranteed to yield unique results. When a number of physically impossible magnetic geometries all provide good fits to the observed Stokes $IQUV$ profiles, these solutions must necessarily be spurious and cannot be used as constraints to diffusion theory.

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arXiv2021

Zeeman Doppler maps. II: the perils of eschewing physics

For the observational modeling of horizontal abundance distributions and of magnetic geometries in chemically peculiar (CP) stars, Zeeman Doppler mapping (ZDM) has become the method of choice. Comparisons between abundance maps obtained for CP stars and predictions from numerical simulations of atomic diffusion have always proved unsatisfactory, with the blame routinely put on theory. Expanding a previous study aimed at clarifying the question of the uniqueness of ZDM maps, this paper inverts the roles between observational modeling and time-dependent diffusion results, casting a cold eye on essential assumptions and algorithms underlying ZDM, in particular the Tikhonov-style regularization functionals, from 1D to 3D. We show that these have been established solely for mathematical convenience, but that they in no way reflect the physical reality in the atmospheres of magnetic CP stars. Recognizing that the observed strong magnetic fields in most well-mapped stars require the field geometry to be force-free, we demonstrate that many published maps do not meet this condition. There follows a discussion of the frequent changes in magnetic and abundance maps of well observed stars and a caveat concerning the use of least squares deconvolution in ZDM analyses. It emerges that because of the complexity and non-linearity of the field-dependent chemical stratifications, Tikhonov based ZDM inversions cannot recover the true abundance and magnetic geometries. As our findings additionally show, there is no way to define a physically meaningful 3D regularization functional instead. ZDM remains dysfunctional and does not provide any observational constraints for the modeling of atomic diffusion.

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arXiv2018

Time-dependent atomic diffusion in the atmospheres of CP stars. A big step forward: introducing numerical models including a stellar mass loss

Calculating abundance stratifications in ApBp/HgMn star atmospheres, we are considering mass-loss in addition to atomic diffusion in our numerical code in order to achieve more realistic models. These numerical simulations with mass-loss solve the time dependent continuity equation for plane-parallel atmospheres; the procedure is iterated until stationary concentrations of the diffusing elements are obtained throughout a large part of the stellar atmosphere. We find that Mg stratifications in HgMn star atmospheres are particularly sensitive to the presence of a mass-loss. For main-sequence stars with $T_{\rm{eff}}\approx 12000$ K, the observed systematic mild underabundances of this element can be explained only if a mass-loss rate of around $4.2\,10^{-14}$ solar mass per year is assumed in our models. Numerical simulations also reveal that the abundance stratification of P observed in the HgMn star HD53929 may be understood if a weak horizontal magnetic field of about 75G is present in this star. However, for a better comparison of our results with observations, it will be necessary to carry out 3D modelling, especially when magnetic fields and stellar winds -- which render the atmosphere anisotropic -- are considered together.

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arXiv2017

The multi-line slope method for the measure of the effective magnetic field of the cool stars: an application to the solar like cycle of ε Eri

A method for the determination of integrated longitudinal stellar fields from low-resolution spectra is the so-called slope method, which is based on the regression of the Stokes V signal against the first derivative of Stokes I. Here we investigate the possibility to extend this technique to measure the magnetic fields of cool stars from high resolution spectra. For this purpose we developed a multi-line modification to the slope method, called multi-line slope method. We tested this technique by analysing synthetic spectra computed with the COSSAM code and real observations obtained with the high resolution spectropolarimeters Narval, HARPSpol and Catania Astrophysical Observatory Spectropolarimeter (CAOS). We show that the multi-line slope method is a fast alternative to the Least Squares Deconvolution (LSD) technique for the measurement of the effective magnetic fields of cool stars. Using a Fourier transform on the effective magnetic field variations of the star $ε$ Eri, we find that the long term periodicity of the field corresponds to the 2.95 yr period of the stellar dynamo, revealed by the variation of the activity index.

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arXiv2017

Three-dimensional abundance distributions in ApBp star atmospheres: non-axisymmetric magnetic geometry

Numerical models for the atmospheres of magnetic ApBp stars have in the past dealt only with centred dipole magnetic field geometries. These models include atomic diffusion that stratifies the abundances of metals according to the local magnetic field strength and the direction with respect to the surface normal. The magnetic variations with rotational phase of most well observed stars however reveal that this assumption is far too simplistic. In this work we establish for the first time a three-dimensional (3D) model with abundance stratifications arising from atomic diffusion of 16 metals, adopting a non-axisymmetric magnetic field geometry inspired by the configuration derived for a real ApBp star. We find that the chemical elements are distributed in complex patterns in all three dimensions, far from the simple rings that have been proposed as the dominant abundance structures from calculations that assume a perfectly centred dipolar magnetic geometry

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arXiv2016

A spectroscopic study of the open cluster NGC 6250

We present the chemical abundance analysis of 19 upper main-sequence stars of the young open cluster NGC 6250 (log t ~ 7.42 yr). This work is part of a project aimed at setting observational constraints on the theory of atomic diffusion in stellar photospheres, by means of a systematic study of the abundances of the chemical elements of early F-, A- and late B-type stars of well-determined age. Our data set consists of low-, medium- and high-resolution spectra obtained with the Fibre Large Array Multi Element Spectrograph (FLAMES) instrument of the ESO Very Large Telescope (VLT). To perform our analysis, we have developed a new suite of software tools for the chemical abundance analysis of stellar photospheres in local thermodynamical equilibrium. Together with the chemical composition of the stellar photospheres, we have provided new estimates of the cluster mean radial velocity, proper motion, refined the cluster membership, and we have given the stellar parameters including masses and fractional age. We find no evidence of statistically significant correlation between any of the parameters, including abundance and cluster age, except perhaps for an increase in Ba abundance with cluster age. We have proven that our new software tool may be successfully used for the chemical abundance analysis of large data sets of stellar spectra.

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arXiv2016

Stellar longitudinal magnetic field determination through multi-Zeeman signatures

Context. A lot of effort has been put into the detection and determination of stellar magnetic fields using the spectral signal obtained from the combination of hundreds or thousands of individual lines, an approach known as multi-line techniques. However, so far most of multi-line techniques developed that retrieve stellar mean longitudinal magnetic fields recourse to sometimes heavy simplifications concerning line shapes and Zeeman splittings. Aims. To determine stellar longitudinal magnetic fields by means of the Principal Components Analysis and Zeeman Doppler Imaging (PCA-ZDI) multi-line technique, based on accurate polarised spectral line synthesis. Methods. In this paper we present the methodology to perform inversions of profiles obtained with PCA-ZDI. Results. Inversions with various magnetic geometries, field strengths and rotational velocities show that we can correctly determine the effective longitudinal magnetic field in stars using the PCA-ZDI method.

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arXiv2011

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.

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arXiv2009

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.

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arXiv2009

Empirical chemical stratifications in magnetic Ap stars: questions of uniqueness

Over the last decades, modelling of the inhomogeneous vertical abundance distributions of various chemical elements in magnetic peculiar A-type has largely relied on simple step-function approximations. In contrast, the recently introduced regularised vertical inverse problem (VIP) is not based on parametrised stratification profiles and has been claimed to yield unique solutions without a priori assumptions as to the profile shapes. It is the question of uniqueness of empirical stratifications which is at the centre of this article. An error analysis establishes confidence intervals about the abundance profiles and it is shown that many different step-functions of sometimes widely different amplitudes give fits to the observed spectra which equal the VIP fits in quality. Theoretical arguments are advanced in favour of abundance profiles that depend on magnetic latitude, even in moderately strong magnetic fields. Including cloud, cap and ring models in the discussion, it is shown that uniqueness of solutions cannot be achieved without phase resolved high signal-to-noise ratio (S/N) and high spectral resolution (R) spectropolarimetry in all 4 Stokes parameters.

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arXiv2008

Multiline Zeeman signatures as demonstrated through the Pseudo-line

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 lines 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. The Zeeman signatures are unequivocally detected in this multiline approach. We may anticipate the outcome magnetic fields to be 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.

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arXiv2008

Paschen is Partially Back

We present a discussion of the partial Paschen-Back (PB) effect in magnetic Ap stars. An overview of the theory is illustrated with examples of how splittings deviate non-linearly from the simple Zeeman picture; normally forbidden ``ghost lines'' appear in strong fields. Resulting asymmetric stellar Stokes profiles for a dipolar magnetic geometry are shown for the FeII 6149 line and it is established that PB lines may be subject to wavelength shifts. Modelling of Stokes profiles in the PB regime opens exciting new diagnostics.

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arXiv2008

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 $H_s$ 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.

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arXiv2007

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.

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arXiv2006

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.

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