ACCRETING, ISOLATED NEUTRON-STARS .3. PREHEATING OF INFALLING GAS ANDCOMETARY H-II REGIONS

Nearby, isolated, old neutron stars (IONSs) accreting interstellar mat erial might be detectable as sources of UV and soft X-ray radiation. W e investigate the interaction between this radiation field and the sur rounding medium, assumed to consist of purely hydrogen and helium. Thi s interaction results in a cometary H II region around the star which we model in detail by including all the relevant time-dependent ioniza tion, recombination, heating, and cooling processes. We find that preh eating of the ambient gas to temperatures higher than 10(4) K may sign ificantly quench the accretion rate onto these neutron stars, thereby reducing the IONS number counts in the EUV and soft X-ray bands predic ted by previous investigations. However, the reprocessing of hard radi ation by the surrounding H II regions may enhance the detectability of IONSs at optical wavelengths. Searches for nearby accreting IONSs are currently being undertaken.