On H II Regions and Pulsar Distances*

In this paper we first of all give a list of all clusters and associations within 1 kpc of the Sun (Table III) and all hot O-stars and supergiants which do not belong to clusters (Table IV). We also calculate the radius of the H II regions surrounding these objects. We then find statistically the distribution of stars of a given spectral class which have not been covered so far and we evaluate statistically the contributions from these stars to H II regions. We briefly discuss the level of ionization due to cosmic rays to be found in H I regions. We combine the results to derive from the observed dispersions the distances of pulsars from the Sun. In this evaluation we look first for the possibility that the line-of-sight to a pulsar passes through the H II region of one of the objects in Tables III and IV; this is the case certainly for six pulsars and possibly for another seven. Of the remaining 24 pulsars, we did not have dispersion data for one, and the others were discussed, using the expressions for the statistical contributions from H II and H I regions, taking into account the fall-off in density with height above the galactic plane. We find that, indeed, most pulsars lie within 1 kpc from the Sun. In fact, of the 36 pulsar distances given in our Table VII, 18 are less than 1 kpc, seven less than 2 kpc (of which two may well lie within 1 kpc), and of the remaining 11, eight may well lie within 1 kpc. The uncertainty in some of our distance determinations lies mainly in our lack of knowledge of the galactic structure at a height of more than about 500 pc from the galactic plane. It is felt that our discussion of possible contributions to dispersion of radio waves may also be useful for investigations other than pulsar distances. As a byproduct of our calculations we found a value of 0.04 cm –3 as the median value of the electron density in the galactic plane in the neighbourhood of the Sun.