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Supernovae of Type IIb (SNe IIb) contain large fractions of helium and traces of hydrogen, which can be observed in the early and late spectra. Estimates of the hydrogen and helium mass and distribution are mainly based on early‐time spectroscopy and are uncertain since the respective lines are usually observed in absorption. Constraining the mass and distribution of H and He is important to gain insight into the progenitor systems of these SNe.  We implement a non‐local thermodynamic equilibrium treatment of hydrogen and helium in a three‐dimensional nebular code. Ionization, recombination, (non‐)thermal electron excitation and Hα line scattering are taken into account to compute the formation of Hα, which is by far the strongest H line observed in the nebular spectra of SNe IIb. Other lines of H and He are also computed but are rarely identified in the nebular phase. Nebular models are computed for the Type IIb SNe 1993J, 2001ig, 2003bg and 2008ax as well as for SN 2007Y, which shows Hα absorption features at early times and strong Hα emission in its late phase, but has been classified as a SN Ib. We suggest classifying SN 2007Y as a SN IIb. Optical spectra exist for all SNe of our sample, and there is one infrared nebular observation of SN 2008ax, which allows an exploration of its helium mass and distribution.  We develop a three‐dimensional model for SN 2008ax. We obtain estimates for the total mass and kinetic energy in good agreement with the results from light‐curve modelling found in the literature. We further derive abundances of He, C, O, Ca and  56 Ni. Estimates of the H mass are difficult but some constraints are derived. We demonstrate that Hα absorption is probably responsible for the double‐peaked profile of the [O  i ] λλ 6300, 6363 doublet in several SNe IIb and present an alternative mechanism to shock interaction for generating late‐time Hα emission of SNe IIb.

Hydrogen and helium in the late phase of supernovae of Type IIb