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HII Regions and the Distances to Pulsars

Naturevolume 222pages964965 (1969) | Download Citation



IT is well known that estimates of pulsar distances are based on their dispersion which is a direct measure of ∫nedl, where ne is the density of electrons cm−3 and where the integration is along the line of sight from the pulsar to the Earth. Usually, as a rough estimate, the value 0.1 cm−3 has been used for ne, but there have been indications that this may be much too high in most cases. It should be possible to do better, however, for the positions of pulsars in the sky are quite well known and it is suspected that most of them lie relatively close to us, within perhaps 1 kpc. We can therefore use our knowledge about stars along the line of sight and about the general distribution of hydrogen in our galaxy to give an approximate calculation of ne along the line of sight and thus to see at what point the observed ∫nedl runs out of distance. It is clear that the position of a hot star or association will materially affect the resultant distance l, estimated in this way, for in HII regions the value of ne will be at least an order of magnitude higher than 0.1 cm−3.

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  1. Department of Theoretical Physics, University of Oxford

    • A. J. R. PRENTICE
    •  & D. TER HAAR


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