We construct models for strongly-magnetized neutron star atmospheres composed of mid-Z elements ( carbon , oxygen and neon ) with magnetic fields B = 10 ^ { 12 } – 10 ^ { 13 } G and effective temperatures T _ { eff } = ( 1 - 5 ) \times 10 ^ { 6 } K ; this is done by first addressing the physics relevant to strongly-magnetized plasmas and calculating the equation of state and polarization-dependent opacities .
We then obtain the atmosphere structure and spectrum by solving the radiative transfer equations in hydrostatic and radiative equilibrium .
In contrast to hydrogen opacities at the relevant temperatures , mid-Z element opacities are dominated by numerous bound-bound and bound-free transitions .
Consequently , temperature profiles are closer to grey profiles , and photosphere densities are lower than in the hydrogen case .
Mid-Z element atmosphere spectra are significantly softer than hydrogen atmosphere spectra and show numerous absorption lines and edges .
The atmosphere spectra depend strongly on surface composition and magnetic field but weakly on surface gravity .
Absorption lines are primarily broadened by motional Stark effects and the ( unknown ) surface magnetic field distribution .
When magnetic field variation is not severe , substructure in broad absorption features can be resolved by ( phase-resolved ) CCD spectroscopy from Chandra and XMM-Newton .
Given the multiple absorption features seen in several isolated neutron stars , it is possible to determine the surface composition , magnetic field , temperature , and gravitational redshift with existing X-ray data ; we present qualitative comparisons between our model spectra and the neutron stars 1E1207.4 - 5209 and RX J1605.3 + 3249 .
Future high-resolution X-ray missions such as Constellation-X will measure the gravitational redshift with high accuracy by resolving narrow absorption features ; when combined with radius measurements , it will be possible to uniquely determine the mass and radius of isolated neutron stars .