Hydrogen photoionization in a magnetized medium: The rigid-wavefunction approach revisited

¹ Instituto de Ciencias Astronómicas, de la Tierra y del Espacio (ICATE, CONICET-UNSJ), C.C. 467, 5400 San Juan, Argentina
² Facultad de Ciencias Exactas, Físicas y Naturales (FCEFN, UNSJ), Av. Ignacio de la Roza 590 (O), J5402DCS San Juan, Argentina

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 January 2026
Accepted: 7 March 2026

Abstract

Realistic modeling of stellar spectra requires accurate radiative opacity coefficients. Owing to the fragmentary nature of existing data from rigorous quantum-mechanical calculations, photoionization coefficients based on the rigid-wavefunction approximation remain the only practical option for studies of magnetic white dwarfs. Although variants of this approach have been widely used in spectral analyses for decades, a complete and explicit treatment of degeneracy-level breaking has not previously been presented. In this work, we provide a comprehensive description of this procedure, including explicit expressions for the photoionization probability of individual bound–free transitions as functions of magnetic field strength and radiation polarization. We also evaluate the occupation numbers of bound states in a magnetized gas under ionization equilibrium, enabling the calculation of absolute photoionization opacities. Because high-lying atomic states are strongly perturbed by the magnetic field and ultimately dissolved, substantial modifications of the monochromatic absorption are found even for fields below 10 MG – a regime where fully rigorous quantum calculations are numerically demanding and have not yet been applied. Over a wide range of magnetic field strengths, pronounced dichroic features appear in the hydrogen continuum absorption.