Probing red-supergiant atmospheres and winds with early-time high-cadence high-resolution type II supernova spectra

Abstract

High-cadence high-resolution spectroscopic observations of infant Type II supernovae (SNe) represent an exquisite probe of the atmospheres and winds of exploding red-supergiant (RSG) stars. Using radiation hydrodynamics and radiative transfer, we study the gas and radiation properties during and after the phase of shock breakout, considering RSG progenitors enshrouded within a circumstellar material (CSM) that varies in extent, density, and velocity profile. In all cases, the original, unadulterated CSM structure is probed only at the onset of shock breakout, visible in high-resolution spectra as narrow, often blueshifted emission, possibly with an absorption trough. As the SN luminosity rises during breakout, radiative acceleration of the unshocked CSM starts, leading to a broadening of the ``narrow'' lines by ~100 and up to ~1000km/s, depending on CSM properties. This acceleration is maximum close to the shock, where the radiative flux is greater, and thus typically masked by optical-depth effects. Generally, narrow-line broadening is greater for more compact, tenuous CSM because of the proximity to the shock where the flux is born, and smaller in denser and more extended CSM. Narrow-line emission should show a broadening that slowly increases first (the line forms further out in the original wind), then sharply rises (the line forms in a region that is radiatively accelerated), before decreasing until late times (the line forms further away in regions more weakly accelerated). Radiative acceleration should inhibit X-ray emission during the early, IIn phase. Although high spectral resolution is critical at the earliest times to probe the original slow wind, radiative acceleration and associated line broadening may be captured with medium resolution allowing a simultaneous view of narrow, Doppler-broadened as well as extended, electron-scattering broadened line emission.