Time-dependent effects in photospheric-phase Type II supernova spectra

Spectroscopic modelling of Type II supernovae (SNe) generally assumes steady state. Following the recent suggestion of Utrobin & Chugai, but using the 1D non-local thermodynamic equilibrium line-blanketed model atmosphere code cmfgen, we investigate the effects of including time-dependent terms, which are generally neglected, that appear in the statistical and radiative equilibrium equations. We base our discussion on the ejecta properties and the spectroscopic signatures obtained from time-dependent simulations, investigating different ejecta configurations (slow, standard and fast), and covering their evolution from one day to six weeks after shock breakout. Compared to equivalent steady-state models, our time-dependent models produce SN ejecta that are systematically overionized, affecting helium at one week after explosion, but ultimately affecting all ions after a few weeks. While the continuum remains essentially unchanged, time-dependence effects on observed spectral lines are large. At the recombination epoch, H i lines and Na i D are considerably stronger and broader than in equivalent steady-state models, while Ca ii 8500 Å is weakened. If time dependence is allowed for, the He i lines at 5875 and 10 830 Å appear approximately three times stronger at one week, and He i 10 830 Å persists as a blueshifted absorption feature even at six weeks after explosion. Time dependence operates through the energy gain from changes in ionization and excitation and, perhaps more universally across SN types, from the competition between recombination and expansion which, in turn, can be affected by optical depth effects. Our time-dependent models compare well with observations of the low-luminosity low-velocity SN 1999br and the more standard SN 1999em, reproducing the Hα line strength at the recombination epoch, and without the need for setting unphysical requirements on the magnitude of nickel mixing.