| |
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
| |
