Astron. Astrophys. 322, 489-492 (1997)

1. Introduction

One method to study the evolution of the properties of radio pulsars is radio pulsar population synthesis. In such a synthesis, neutron stars are given initial properties, such as position, velocity, rotation period and magnetic field, and these properties are allowed to evolve according to given prescriptions. Observations of the resulting population are simulated, and the simulated detected pulsars are compared with the real detected pulsars.

A recent example of such a study is the synthesis by Hartman et al. (1997). Whereas the overall results of the synthesis compare well with observations, a detailed comparison of the simulated dispersion measures DM with the observed values for real pulsars shows systematic differences. It is suggested by Hartman et al. (1997) that this is due to small scale structure in the electron distribution in the Galaxy. In the model for this distribution by Taylor & Cordes (1993) several components are present (a thin layer, a thick layer, spiral arms, the Gum nebula), but each of these components is modelled with a smooth distribution. In this paper we investigate a highly simplified model for small scale variations in the electron density, in which all electrons are in uniform clouds. Based on this model, we propose a method to describe such fluctuations in population synthesis (Sect. 2). From the observed distribution of high values of (where b is the galactic latitude) we derive the characteristic dispersion measure of one cloud (Sect. 3). We use this to implement the method in the population synthesis (Sect. 4). A discussion of our results is given in Sect. 5.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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