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We present the first non‐LTE time‐dependent radiative‐transfer simulations of supernovae (SNe) II‐Plateau (II‐P) covering both the photospheric and nebular phases, from  ∼ 10 to  ≳ 1000 d after the explosion, and based on 1.2 B piston‐driven ejecta produced from a 15 M  ⊙   and a 25 M  ⊙   non‐rotating solar‐metallicity star. The radial expansion of the gradually cooling photosphere gives rise to a near‐constant luminosity up to  ≳ 100 d after the explosion. The photosphere remains in the outer 0.5 M  ⊙   of the ejecta for up to  ∼ 50 d after the explosion. As the photosphere reaches the edge of the helium core, the SN luminosity drops by an amount mitigated by the progenitor radius and the   56  Ni mass. Synthetic light curves exhibit a bell‐shaped morphology, evolving faster for more compact progenitors, and with an earlier peak and narrower width in bluer filters. UV and   U  ‐band fluxes are very sensitive to line blanketing, the metallicity and the adopted model atoms. During the recombination epoch synthetic spectra are dominated by H  i  and metal lines, and are largely insensitive to the differing H/He/C/N/O composition of our two progenitor stars. In contrast, synthetic nebular‐phase spectra reveal a broader/stronger O  i  doublet line in the higher‐mass progenitor model, reflecting the larger masses of oxygen and nickel that are ejected. Our simulations overestimate the typical luminosity and the visual rise time of standard SNe II‐P, most likely a consequence of our progenitor stars being too big and/or too hydrogen rich. Comparison of our simulations with photospheric‐phase observations of SN1999em of the same colour is satisfactory. Our neglect of non‐thermal excitation/ionization leads to a fast disappearance of continuum radiation and Balmer‐line emission at the end of the plateau phase. With the exception of H  i  lines, our nebular spectra show a striking similarity to contemporaneous observations of SN1999em.

monthly notices of the royal astronomical society

Non‐LTE time‐dependent spectroscopic modelling of Type II‐plateau supernovae from the photospheric to the nebular phase: case study for 15 and 25 M ⊙  progenitor stars