Issue |
A&A
Volume 425, Number 1, October I 2004
|
|
---|---|---|
Page(s) | 271 - 280 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20047180 | |
Published online | 10 September 2004 |
The effect of the surface distribution of elements on measuring the magnetic field of chemically peculiar stars*
The case of the roAp star HD 24712
INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: [fleone;gcatanzaro]@ct.astro.it
Received:
2
February
2004
Accepted:
4
June
2004
A non-homogeneous distribution over the stellar surface of the
chemical elements and the presence of a large-scale magnetic field
are invoked in order to explain the periodic line strength,
photometric and magnetic variability of magnetic chemically peculiar
stars. In such a framework, the variability period is identical to
the rotational period.
In magnetic chemically peculiar stars, the so-called effective
magnetic field Heff, the average over the visible stellar
disk of the longitudinal field component weighted by the local line
strength, is routinely measured from Stokes I and V profiles of
selected spectral lines. In spite of evidence that the distribution
of the various chemical elements over the stellar surface can be
inhomogeneous and different from element to element, Heff
values obtained from lines of different elements are often statistically
combined to improve the accuracy of effective field measurements.
Similarly, mean high S/N profiles are obtained from the profiles
of lines of different elements.
We have established, by means of circular spectropolarimetry
of the magnetic chemically peculiar star HD 24712 in the 4700-7000 Å
range, the dependence of the measurements of Heff on the
atomic weight of 24 elements (from carbon to erbium, if possible at
different ionisation states). At all 3 rotational phases considered,
Heff values derived from different elements can differ by up
to 800 G. We find an overall increase in Heff with atomic
number and a maximum near
. The behaviour of sodium is quite
singular inasmuch as it always exhibits a negative value of the field,
peaking at -0.39 kG when the rare earths give
kG.
Under the assumption of a dipolar field, we conclude that the
elements giving the largest values of Heff are concentrated
near the positive polar region and that the other elements are more
homogeneously distributed over the stellar surface or concentrated in
belts around the magnetic equator. Sodium seems to be localised in
the negative magnetic hemisphere only.
This picture is corroborated by the equivalent width variability of
the lines: up to iron, equivalent widths changes out-of-phase
with respect to the Heff variability; elements heavier than
iron present equivalent widths that are variable in-phase. We do not
find any relation between the respective amplitudes of the equivalent
width variations and the atomic numbers of the elements. For iron,
the observed equivalent width variability does not seem to be simply
related to any non-homogeneous distribution over the stellar surface.
We conclude that measurements originating from different elements
cannot in general be combined to improve the precision of Heff
measurements. Indeed, any modelling attempt based on the periodic
variations in Heff is subject to the risk that the sampling
of the magnetic field over the stellar surface by the lines of a given
chemical element is uneven or incomplete.
Key words: stars: magnetic fields / stars: chemically peculiar / stars: individual: HD 24712
© ESO, 2004
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