Modelling the incomplete Paschen-Back effect in the spectra of magnetic Ap stars

TL;DR

This paper investigates the incomplete Paschen-Back effect in magnetic Ap stars, demonstrating how it influences spectral line profiles, shifts, and magnetic field measurements, highlighting the need for specialized modeling in strong magnetic fields.

Contribution

It provides the first systematic analysis of the incomplete Paschen-Back effect in Ap star spectra and discusses the implications for magnetic field measurements and spectral modeling.

Findings

Line shifts depend on multiplet and magnetic field strength.

Ghost components appear but are likely unobservable.

Magnetic field modulus measurements remain reliable.

Abstract

We present first results of a systematic investigation of the incomplete Paschen-Back effect in magnetic Ap stars. A short overview of the theory is followed by a demonstration of how level splittings and component strengths change with magnetic field strength for some lines of special astrophysical interest. Requirements are set out for a code which allows the calculation of full Stokes spectra in the Paschen-Back regime and the behaviour of Stokes I and V profiles of transitions in the multiplet 74 of FeII is discussed in some detail. It is shown that the incomplete Paschen-Back effect can lead to noticeable line shifts which strongly depend on total multiplet strength, magnetic field strength and field direction. Ghost components (which violate the normal selection rule on J) show up in strong magnetic fields but are probably unobservable. Finally it is shown that measurements of the integrated magnetic field modulus $H_s$ are not adversely affected by the Paschen-Back effect, and that there is a potential problem in (magnetic) Doppler mapping if lines in the Paschen-Back regime are treated in the Zeeman approximation.