Abstract:
Early-time observations of the Type II supernovae (SNe) 2013cu and 2013fs
have revealed an interaction of ejecta with material near the star surface.
Unlike the Type IIn SN2010jl, which interacts with a dense wind for ~1yr,
the interaction ebbs after 2-3d, suggesting a dense and compact circumstellar envelope.
Here, we use multi-group radiation-hydrodynamics and non-local-thermodynamic-equilibrium
radiative transfer to explore the properties of red supergiant (RSG) star explosions embedded
in a variety of dense envelopes. We consider the cases of an extended static atmosphere or a
steady-state wind, adopting a range of mass loss rates.
The shock-breakout signal, the SN radiation up to 10d, and the ejecta dynamics are strongly
influenced by the properties of this nearby environment. This compromises the use of early-time
observations to constrain Rstar. The presence of narrow lines for
2-3d in 2013fs and 2013cu require a cocoon of material of ~0.01Msun out to 5-10Rstar.
Spectral lines evolve from electron-scattering to Doppler broadened, with a growing blueshift of
their emission peaks.
Recent studies propose a super-wind phase with a mass loss rate from 0.001 up
to 1Msun/yr in the last months/years of the RSG life, although there is no
observational constraint that this external material is a steady-state outflow.
Alternatively, observations may be explained by the explosion of a RSG star
inside its complex atmosphere. Indeed, spatially resolved observations
reveal that RSG stars have extended atmospheres, with the presence of downflows and
upflows out to several Rstar, even in a standard RSG like Betelgeuse. Mass loading
in the region intermediate between star and wind can accommodate the 0.01Msun
needed to explain the observations of 2013fs. Signatures of interaction in early-time spectra of RSG
star explosions may therefore be the norm, not the exception, and a puzzling super-wind phase
prior to core-collapse may be superfluous.
The full paper is available here
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