Spectral modelling of Type IIb supernovae Comparison with SN 2011dh and the effect of macroscopic mixing

We use the new non-local-thermodynamical-equilibrium (NLTE) light curve and spectral synthesis code JEKYLL to evolve a macroscopically mixed ejecta model of a Type IIb supernova (SN) originating from a star with an initial mass of 12M(circle dot) through the photospheric and nebular phase. The ejecta model is adopted from earlier work and has a mass of 1.7 M-circle dot, has a kinetic energy of 0.7 foe, and contains 0.075 M-circle dot of Ni-56. The macroscopic mixing is simulated through a statistical representation of ejecta fragmented into small clumps but spherically symmetric on average. We compare our model with SN 2011dh and find that both the spectra and the light curves are well reproduced in both the photospheric and nebular phase, although there are also some differences. Our work further strengthens the evidence that this SN originated from a star with an initial mass of similar to 12M(circle dot) that had lost all but a tiny (<0.1 M-circle dot) fraction of its hydrogen envelope, strongly suggesting a binary origin. We also investigate the effects of the macroscopic mixing by comparing macroscopically and microscopically mixed models and by varying the clumping geometry. In the photospheric phase, we find strong effects on the effective opacity in the macroscopically mixed regions, which affects the model light curves. The diffusion peak is considerably narrower (rise time decreased by 11%) in the macroscopically mixed case and differs strongly (rise time decreased by 29%) if the radioactive material in the helium envelope is allowed to expand more than in our standard model. The effect is mainly geometrical and is driven by the expansion of the clumps that contain radioactive material, which tend to decrease the effective opacity. In the limit of optically thick clumps, the decrease is roughly given by the product of the (volume) expansion and filling factors for the radioactive material, and in our models values up to similar to 8 are explored. These findings have implications for light curve modelling of stripped-envelope SNe in general, and the effect would increase the estimated ejecta masses. In the nebular phase, we find strong effects on the collisional cooling rates in the macroscopically mixed regions, which affects lines driven by collisional cooling, in particular the [Ca II] 7291, 7323 angstrom and [OI] 6300, 6364 angstrom lines. The effect is mainly related to differences in composition between macroscopically and microscopically mixed ejecta. As these lines are often used for mass determinations, this highlights the importance of how and to what extent the calcium- and oxygen-rich material is mixed. As shown in this and earlier work, both NLTE and macroscopic mixing are essential ingredients for accurately modelling the light curves and spectra of Type IIb SNe throughout their evolution.