Vela X and the evolution of plerions

Based on the radio, optical, and X-ray observations available for the supernova remnant consisting of Vela X, Y, and Z, the suggestion is made that Vela X in fact resembles the Crab Nebula more closely that it resembles the vast majority of shell-type remnants present in radio supernova remnant surveys. As is the Crab Nebula, Vela X is dependent for its energy on the rotational energy losses of the pulsar rather than on the energy release in the original supernova explosion. It is thus included in the class of 'plerions' or 'filled center' supernova remnants containing, at present, about a dozen established or suggested members. Since the Crab Nebula and Vela X are of very different ages, for the first time a comparison of an old and a young plerion with known ages is possible. To do this, an evolutionary model for the plerions is developed and used to describe the size, expansion velocity, magnetic field, flux density, and electromagnetic spectrum and their changes with time. A formula to describe the development of the intrinsic flux density with time is derived and applied to obtain ages for the known plerions. Also, a distance dependent relation is obtained to estimate their distances. (orig.)