The origin of cosmic rays: Role of Type II supernovae

This dissertation presents the results of a Very Large Array study of supernova remnants (SNRs) in the nearby spiral galaxy, NGC 6946. The question we address in this study is which type of supernovae (SNe) is responsible for the origin of cosmic rays. We present unique empirical evidence that Type II SNe are the precursors of cosmic ray producing supernova remnants. This evidence is based on a combined sample of 27 optically selected SNRs and 37 radio selected SNRs identified in a survey of NGC 6946. The majority of the SNRs without detectable radio emission are located in the interarm regions, whereas the majority of SNRs with detectable radio emission are located on the spiral arms. The spiral arms are sites of recent star formation, including the massive, short-lived stars that are the progenitors of Type II SNe. This empirical evidence leads to the conclusion that Type II SNe result in cosmic-ray producing SNRs. This is the first time that the progenitors of a population of cosmic ray producing SNRs have been identified empirically.; The above result was quantified by investigating the effects of adiabatic compression on the ambient medium by the remnant shock and active particle acceleration at the shock, in the arm and interarm regions of the galaxy. Based on predictions of radio emissivity from adiabatic compression, it is concluded that adiabatic shock compression cannot be solely responsible for the differences in radio emissivity among the SNRs in the sample. Diffusive shock acceleration theory, on the other hand, predicts flux densities that fit the observed range. Therefore, it is concluded that particle acceleration is required to explain the observed radio properties of the SNRs in NGC 6946. In particular, it is shown that SNRs arising from Type II SNe have much greater cosmic ray yields than SNe of Type Ia origin.