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Mapping the Surface Distribution of Elements on Ap Stars Using the Maximum Entropy Method

Published online by Cambridge University Press:  12 April 2016

Artie P. Hatzes
Artie P. Hatzes*
Affiliation:
McDonald Observatory, The University of Texas at Austin, Austin, TX, 78712

Abstract

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A technique for deriving the distribution of elements on the surface of Ap stars using maximum entropy reconstruction principles is described. The technique is applied to deriving the silicon distribution on 56 Ari, CU Vir, 11 Ori and the chromium distribution on ϒ2 Ari. Silicon on these stars is depleted at the magnetic poles and is enhanced in regions between the magnetic equator and poles. The chromium distribution on ϒ2 Ari is markedly different than the chromium distribution seen on other Ap stars. It shows depletions at one of the magnetic poles (as do other Ap stars) but it does not show the depleted band at the equator as has been seen on θ Aur, 45 Her, and ω Her. The silicon distribution on 11 Ori also differs from that found on other stars in that it shows evidence for a depleted band, similar to what has been seen in the chromium distribution is some stars. Characteristic features in the abundance maps such as spots or bands appear to mark the location of the magnetic poles or equator so that these maps can be used to infer the magnetic field geometries on these stars. Dipole decentering parameters derived from the abundance maps yield decentering parameters of about 0.2 stellar radii. The amount of decentering seems to be correlated with rotation period (longer period Ap stars have less decentering). Horizontal diffusion can complicate the use of abundance maps to determine the field geometry. The effects of horizontal diffusion can only be understood by a proper theoretical study of its effects or by mapping the elemental distribution on Ap stars of known age.

Type
II. Magnetic Fields: Observations and Theories
Information
International Astronomical Union Colloquium , Volume 138: Peculiar versus Normal Phenomena in A-Type and Related Stars , 1993 , pp. 258 - 271
Copyright
Copyright © Astronomical Society of the Pacific 1993

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