Chemical and Temperature Inhomogeneities on Stellar Surfaces as a Result of an Instability

Extract

Observations show that chemical anomalies are distributed inhomogeneously on Ap star surfaces. The most elaborated explanation of the observations is based on the fact that different ions and atoms are affected by radiative forces of different strengths and, hence, have different diffusion velocities. The diffusion across the magnetic field is a factor of (l + ν_H^2/νC²)⁻¹ slower than along the field (ν_H is the gyrofrequency and ν_C is the collision frequency of the ions). It leads to the increasing of the heavy ion number density in regions occupied by magnetic traps.

However, such an explanation meets a number of difficulties: a) the conditions holds for the most of ion species only in regions where the optical depth is less than 10⁻² if the field H exceeds 10⁵ gs. Although little is known on the depth of the region occupied by the chemical anomalies, there are some indirect indications that it is larger than 10⁻²; b) The observed dipole field has a value 10³−10⁴ gs and does not form traps corresponding to the observed chemical spots which have very complicated configurations; c) the magnetic trap is imperfect. The separation process in the field is assumed to be produced by the diffusion which needs a long time. However, ions can escape from the trap together with the surrounding hydrogen plasma because of various plasma instablished which take much shorter time; d) observed space distribution of rare elements and also of Fe, Cr, Ti contradicts the predictions of the magnetic separation hypothesis (cf. V. Khohlova IAU Coll. No. 90, this volume).