PULSAR DISTANCES AND THE GALACTIC DISTRIBUTION OF FREE-ELECTRONS

We describe a quantitative model for the distribution of free electron s in the Galaxy, with particular emphasis on its utility for estimatin g pulsar distances from dispersion measures. Contrary to past practice , we abandon the assumption of an axisymmetric Galaxy. Instead, we exp licitly incorporate spiral arms, the shapes and locations of which are derived from existing radio and optical observations of H II regions. Additional parameters of the model include the electron densities of ''outer'' and ''inner'' axisymmetric components, as well as of the spi ral arms; scale lengths for the r- and z-dependences of the axisymmetr ic features and the width and scale height of the arms; and ''fluctuat ion parameters'' used to relate the dispersion and scattering contribu tions of the outer. inner, and spiral arm components of the model. Bec ause of the large angular size and close proximity of the Gum Nebula, we also explicitly model its contribution to dispersion measures. Valu es of some of the model parameters have been fixed by appeal to indepe ndent astrophysical data of various sorts. The remaining adjustable qu antities have been calibrated by reference to three distinct types of information: (1) independently measured distance limits and dispersion measures for 74 pulsars; (2) interstellar scattering measurements for 223 Galactic and extragalactic radio sources, together with their dis tances or dispersion measures; and (3) the distribution of 553 pulsar dispersion measures with respect to Galactic longitude. We believe tha t for most known pulsars the new model provides distance estimates acc urate to approximately 25% or better. In an Appendix we describe sever al FORTRAN subroutines that implement the model, and we give instructi ons for obtaining copies of the code via Internet.