Explosion of red-supergiant stars: Influence of the atmospheric structure on shock breakout and early-time supernova radiation

Luc Dessart, D. John Hillier, and Edouard Audit, 2017, A&A, 605, 83

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.

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