MODEL ATMOSPHERES FOR LOW-FIELD NEUTRON-STARS

We compute model atmospheres and emergent spectra for low-field (B les s than or similar to 10(10) G) neutron stars, using new opacity and eq uation of state data from the OPAL project. These computations, incorp orating improved treatments of flux transport and convective stability , provide spectra for hydrogen, solar abundance, and iron atmospheres. We compare our results to high field magnetic atmospheres, available only for hydrogen. An application to apparently thermal flux from the low-field millisecond pulsar PSR J0437-4715 shows that H atmospheres f it substantially better than Fe models. We comment on extension to hig h fields and the implication of these results for neutron star luminos ities and radii.