M. J. Stift

LTE spectrum synthesis in magnetic stellar atmospheres - The interagreement of three independent polarised radiative transfer codes

LTE spectrum synthesis in magnetic stellar atmospheres. : The interagreement of three independent polarised radiative transfer codes

Bi-dimensional element stratifications computed for magnetic Ap star atmospheres

Context. Theoretical modelling of abundance stratifications and surface distributions of chemical elements in Ap stars constitutes a major challenge. The atomic diffusion model provides the most appropriate framework in which to understand these abundance anomalies. Aims. We present theoretical 2D stratifications of 16 metals in upper main sequence chemically peculiar stars, with and without magnetic fields to provide a reference point for further theoretical and observational studies. Methods. We used our code CaratStrat to compute a large grid of stratifications (equilibrium solutions in LTE) for plane-parallel Teff = 8500, 10 000, 12 000, and 14 000 K stellar atmospheres. By interpolation, we constructed bi-dimensional cuts through these stellar atmospheres, which are permeated by a dipolar magnetic field of strength 20 kG at the magnetic pole. We also provide vertical (1D) stratifications of metals in non-magnetic stars (HgMn). Results. We present a large number of 2D and 1D stratifications, mostly as online material. We discuss in detail the case of Fe for the Teff = 8500 K model in the printed version, and compare it with stratifications derived from observed spectra.

Modelling element distributions in the atmospheres of magnetic Ap stars

Context. 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, which is known to be sensitive to the magnetic field strength and direction, is among the processes responsible for these inhomogeneities. Aims. Here we explore the hypothesis that equilibrium stratifications – such that the diffusive particle flux is close to zero throughout the atmosphere – can, in a number of cases, explain the observed abundance maps and vertical distributions of the various elements. Methods. An iterative scheme adjusts the abundances in such a way as to achieve either zero particle flux or zero effective acceleration throughout the atmosphere, taking strength and direction of the magnetic field into account. Results. The investigation of equilibrium stratifications in stellar atmospheres with temperatures from 8500 to 12 000 K 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. Conclusions. 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.

Radiative accelerations in stars: The effects of magnetic polarisation revisited

We present a revision of the results obtained by Alecian & Stift (2002) on the amplification, by Zeeman splitting in strong stellar magnetic fields, of radiative accelerations of chemical elements. These results had been obtained for blended spectra and were based on the Zeeman Feautrier method as presented by Rees et al. (1989) which however requires perfect symmetry of the line profiles. The use of this method in an inappropriate context led to the incorrect identification of those line absorption terms which change sign for the incoming radiation. The question of magnetic amplification of radiative accelera- tions had to be revisited. Following the formulation of an alternative Zeeman Feautrier scheme which remains valid for blends (albeit only when macroscopic velocity fields are excluded), the resulting radiative accelerations are now less amplified than what had been found in Alecian & Stift (2002). In a 12 000 K, log g = 4.0 Kurucz atmosphere with solar abundances, ampli- fications at a field strength of 4 T peak at about 0.4 dex, and there is very little dependence on the field inclination. Depending on the Zeeman pattern, individual lines may exhibit amplifications of more than 1.1 dex. Blending is found to greatly affect radiative accelerations but not amplifications; only in exceptional cases such as for Ag can strong blending lead to an inversion of the amplification, i.e. accelerations actually decrease with increasing field strength. Finally magneto-optical effects continue to be non-negligible, horizontal accelerations remain small.

Measuring stellar magnetic fields from high resolution spectroscopy of near-infrared lines

Zeeman splitting of otherwise degenerate levels provides a straight-forward method of measuring stellar magnetic fields. In the optical, the relative displacements of the Zeeman components are quite small compared to the rotational line broad- ening, and therefore observations of Zeeman splitting are usually possible only for rather strong magnetic fields in very slowly rotating stars. However, the magnitude of the Zeeman splitting is proportional to the square of the wavelength, whereas rota- tional line broadening mechanisms are linear in wavelength; therefore, there is a clear advantage in using near-infrared spectral lines to measure surface stellar magnetic fields. We have obtained high resolution ( R 25 000) spectra in the 15 625-15 665 A region for two magnetic chemically peculiar stars, viz. HD 176232 and HD 201601, and for the suspected magnetic chemi- cally peculiar star HD 180583, as part of a pilot study aimed at determining the accuracy with which we can measure stellar magnetic fields using the Zeeman splitting of near-infrared lines. We confirm that in principle the magnetic field strength can be estimated from the magnetic intensification of spectral lines, i.e. the increase in equivalent width of a line over the zero- field value. However, due to line blending as well as the dependence of this intensification on abundance and field geometry, accurate estimates of the magnetic field strengths can be obtained only by modelling the line profiles by means of spectral synthesis techniques. Using this approach, we find a 1.4 kG magnetic field modulus in HD 176132 and an upper limit of 0.2 kG in HD 180583. The very weak infrared lines in the spectrum of HD 201601 are consistent with a 3.9 kG field modulus estimated from the splitting of the Feii 6149.258 A line seen in an optical spectrum. Finally, we would like to draw attention to the fact that there are no suciently detailed and reliable atomic line lists available for the near-infrared region that can be used in high resolution work; a large fraction of the features observed in our spectra remains to be identified.

New Experiments with Zeeman Doppler Mapping

We present recent experiments using a Levenberg-Marquardt algorithm and the polarised radiative transfer code COSSAM to produce a new ZDM code. Currently the code is able to recover the magnetic parameters of model stars with either a decentred dipole morphology or a morphology consisting of a centred dipole and a quadrupole, while simultaneously calculating multiple chemical abundances (including a basic stratification model). The ZDM code has been tested using both synthetic spectra and real, well studied stars. Additional features are currently being added such as a multipole morphology of arbitrary order and more sophisticated chemical stratification models.

Broadband Linear Polarisation of CP Stars

In recent years, an interesting new technique for the diagnosis of stellar magnetic fields has been developed, which employs the frequency integrated linear polarisation signal over a spectral interval. The form of the temporal variations in frequency integrated Stokes Q and U reflects the geometry of the magnetic field. Here we investigate the dependence of this broadband linear polarisation upon the detailed features of the stellar spectrum and present a campaign of new observations of chemically peculiar stars at the South African Astronomical Observatory.