HI absorption measurements of seven low-latitude pulsars

HI absorption spectra have been measured for seven low-latitude pulsars with the Arecibo 305-m telescope. Distance limits based on the Schmidt (1965) galactic rotation model are: pulsars 1919 + 21 and 1929 + 10, d <= 1.5 kpc; and pulsars 2016 + 28 and 2020 + 28, d >= 1.3 kpc. An attempt is made to apply the TASS galactic rotation model (Roberts, 1972) to absorption spectra of pulsars 0525 + 21 and 0540 + 23, but no evidence is found for the predicted Perseus arm shock. Similarities of absorption spectra, dispersion and rotation measures, and Stroemgren spheres along their lines of sight indicate that PSR 0525 + 21 is approximately as distant as the Crab Nebula pulsar (approx. 2 kpc), in accord with the findings of Gomez-Gonzalez and Guelin (1974). Similar arguments further suggest strongly that PSR 0540 + 23 is more distant than the Crab Nebula pulsar since its dispersion measure is approx. 30% greater. Finally, absorption in PSR 0611 + 22 is similar to possible absorption seen in SNR IC 443 (De Noyer, 1977), but the resemblances may follow merely from the small value of dv/dr in the anticenter direction. (orig./WL)