Atmospheres and Spectra of Strongly Magnetized Neutron Stars. III. Partially Ionized Hydrogen Models

We construct partially ionized hydrogen atmosphere models for magnetizedneutron stars in radiative equilibrium with surface fields B=10^12-5 \times10^14 G and effective temperatures T_eff \sim a few \times 10^5-10^6 K. Thesemodels are based on the latest equation of state and opacity results formagnetized, partially ionized hydrogen plasmas that take into account variousmagnetic and dense medium effects. The atmospheres directly determine thecharacteristics of thermal emission from isolated neutron stars. For the modelswith B=10^12-10^13 G, the spectral features due to neutral atoms lie at extremeUV and very soft X-ray energy bands and therefore are difficult to observe.However, the continuum flux is also different from the fully ionized case,especially at lower energies. For the superstrong field models (B\ga 10^14 G),we show that the vacuum polarization effect not only suppresses the protoncyclotron line as shown previously, but also suppresses spectral features dueto bound species; therefore spectral lines or features in thermal radiation aremore difficult to observe when the neutron star magnetic field is \ga 10^14 G.Comment: 12 pages, 10 figures; ApJ, accepted (v599: Dec 20, 2003