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Type Ib SN 1999dn as an example of the thoroughly mixed ejecta of Ib supernovae

We present the results of modelling archival observations of type Ib SN 1999dn. In the spectra, two He I absorption features are seen: a slower component with larger opacity, and a more rapid He I component with smaller opacity. Complementary results are obtained from modelling the bolometric light curve of SN 1999dn, where a two-zone model (dense inner region, and less dense outer region) provides a much better fit than a one-zone model. A key result we find is that roughly equal amounts of radioactive material are found in both regions. The two-zone analytical model provides a more realistic representation of the structure of the ejecta, including mixing and asymmetries, which offers a physical explanation for how the radioactive material is propelled to, and mixed within, the outer regions. Our result supports the theoretical expectation that the radioactive content in the outflow of a type Ib supernova (SN) is thoroughly mixed. We fit our model to six additional SNe Ibc, of which the majority of the SNe Ib are best described by the two-zone model, and the majority of the SNe Ic by the one-zone model. Of the SNe Ic, only SN 2007gr was best fit by the two-zone model, indicating that the lack of helium spectral features for this event cannot be attributed to poor mixing.