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Institute for Astronomy
04 August 2005 (09:52)
roAp Lambda Bootis Solar type stars Doppler Imaging

Doppler Imaging of Ap Stars

The important scientific impact of resolving the surface abundance structure of Ap stars, based on the modern tool of Doppler Imaging, is to produce observational constrains to the diffusion processes in the presence of a magnetic field. The diffusion theory predicts that stars with a known magnetic field variation explained by the oblique rotator model, should reveal characteristic abundance patterns for each element and their ions.

The past work in that need to be extended and improved in two directions, which are in the focus of this project. The first aspect is to increase the number of elements which are mapped on surface of the same star in order to derive a more complete picure on the particular diffusion of extremely different elements like, He, Mg, Si, Fe, Cr, Ti and if possible some present rare earth's. The second task is to get maps of a significant number of 'similar ' Ap stars, which may allow to derive some statistical conclusions of the 'general' behavior of an element compared to the others. Both points rely on the observation of a vast amount of high S/N spectras.

Up to now, we obtained data on 12 Ap stars with a sufficiently large wavelength coverage to map at least 5 different elements. Furthermore, the maps of elements like Fe and Si are based on many lines at different wavelength regions, which improves the reliability of the resulting abundance features.

Present Status

Presently we are working on the well known B9p Si star CU Virginis. Maps of Helium (Hiesberger et al 1995, A&A 296, 473) and Silicon (Proc. IAU Coll. 138) have been published already in the past. Also, the magnetic field geometry was recently modeled as a decentered dipol, by Hatzes (Proc. IAU Symp. 176, 314, Fig. 7). Based on new observations, (1994 and 1995, OHP) we are now able to present the abundance maps of Helium, Magnesium, Silicon, Iron and Chromium on the surface of CU Vir. The resolved high contrast spot feature of the Helium map (Fig. 1) centered at 200 degree longitude and +30 degree latitude is most probably the location of the strong magnetic pole. The overall He deficiency compared to the solar abundance and the spot concentration (at magnetic poles) is in agreement with the studies of Vauclair et al (1991,A&A,252,618). In contrast the abundance structures of Silicon, Iron and Chromium (Fig.2-4) have a depleted region in common, which is at the same place as the Helium spot. This picture confirms the theory which predicts that this elements are enhanced at region where the magnetic field lines are horizontal.The Magnesium distribution (Fig.5) is definitely different to all others which still has to be explained by the diffusion theory. Further details will be discussed in a publication which is in preparation.

Some nice color maps:

Fig.1: The Helium map of CU Vir derived from the He I 4471 A blend.
Fig.2: The Silicon distribution based on the Si II 4201 A line.
Fig.3: The Iron map based on the Fe II 4233 A blend.
Fig.4: The Chromium abundance structure based on the CR II 4588 line.
Fig.5: The Magnesium map of CU Vir derived from the MG 4481 A blend.

In charge of this page is: Theresa Lüftinger