H-I ABSORPTION-MEASUREMENTS TOWARD 15 PULSARS AND THE RADIAL-DISTRIBUTION OF DIFFUSE IONIZED-GAS IN THE GALAXY

We report neutral hydrogen (H I) absorption measurements at 21 cm toward 15 pulsars with the Arecibo 305 m telescope and the VLA. The distances to these pulsars derived from the kinematic method have allowed us to rule out one potential pulsar-supernova remnant association while giving support to another. In addition, the secular change in the absorption spectrum toward one pulsar (PSR 1821 + 05) may provide independent confirmation of the recent discovery of AU-sized H I structures in the ISM. A study of the radial distribution of the diffuse ionized medium has been carried out using mean electron density measurements made toward pulsars. With our new measurements and those from the literature, there are a total of 18 lines of sight that sample the electron density at small galactocentric radii. The data show a clear rise in the electron density in the inner Galaxy, to about 3 times the local average. From this and other evidence we argue that O stars are the principal heating agent of the warm ionized medium.