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 s in the strong magnetic field characteristic of neutron stars. Wherea s in the absence of significant center-of-mass velocities the atomic s tructure is determined by the cylindrical symmetry, the electric field induced by the finite motion breaks this symmetry and distorts the at omic structure. The resulting dependence of the total energy on a gene ralized momentum of the atom can be interpreted in terms of a mass ani sotropy-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 dependenc e of the binding energy of the atom, leads to a bending of the traject ories of neutral atoms in nonuniform magnetic fields, tending to chann el and retain them in regions of high field. It also leads to a number of thermodynamic and spectroscopic effects. In particular, the mass a nisotropy introduces both quantitative spectroscopic changes relative to the stationary magnetized atom, such as additional shifts and broad ening of photoionization edges and lines, as well as qualitative chang es, such as new selection rules for radiative processes and for the an nihilation of magnetic positronium. The ionization balance of atoms an d ions in pulsar atmospheres may also be strongly influenced, which to gether with the opacity changes could lead to effects of significant i mportance for the modeling of neutron star atmospheres in magnetic fie lds of strength B greater than or similar to 10(9) G.