Finite-velocity effects on atoms in strong magnetic fields and implications for neutron star atmospheres

We consider the effects of a finite velocity on the properties of atom in the strong magnetic field characteristic of neutron stars. Whereas in the absence of significant center-of-mass velocities the atomic structure is determined by the cylindrical symmetry, the electric field induced by the finite motion breaks this symmetry and distorts the atomic structure. The resulting dependence of the total energy on a generalized momentum of the atom can be interpreted in terms of a mass anisotropy - the atom becomes "heavier" when it moves across the magnetic field, the transverse mass being higher for the more excited states. The field-dependent mass anisotropy, together with the field dependence of the binding energy of the atom, leads to a bending of the trajectories of neutral atoms in nonuniform magnetic fields, tending to channel and retain them in regions of high field. It also leads to a number of thermodynamic and spectrocopic effects. In particular, the mass anisotropy introduces both quantitative spectroscopic changes relative to the stationary magnetized atom, such as additional shifts and broadening of photoionization edges and lines, as well as qualitative changes, such as new selection rules for radiative processed and for the annihilation of magnetic positronium. The ionization balance of atoms and ions in pulsar atmospheres may also be strongly influenced, which together with the opacity changes could lead to effects of significant importance for the modeling of neutron star atmospheres in magnetic fields of strength B ≳ 10⁹ G.