Modeling the signatures of interaction in Type II supernovae: UV emission, high-velocity features, broad-boxy profiles

Because mass loss is a fundamental phenomenon in massive stars, interaction with circumstellar material (CSM) should be universal in core-collapse supernovae (SNe). Leaving aside the extreme CSM density, extent, or mass typically encountered in Type IIn SNe, we investigate the diverse long-term radiative signatures of interaction between a Type II SN ejecta and CSM corresponding to mass loss rates up to 10$^{-3}$ $M_{\odot}$ yr$^{-1}$. Because these CSM are relatively tenuous and optically-thin to electron-scattering beyond a few stellar radii, radiation hydrodynamics is not essential and one may treat the interaction directly as an additional power source in the non-local thermodynamic equilibrium radiative transfer problem. The CSM accumulated since shock breakout forms a dense shell in the outer ejecta and leads to high-velocity absorption features in spectral lines, even for negligible shock power. Besides Balmer lines, such features may appear in NaID, HeI lines etc. A stronger interaction strengthens the continuum flux (preferentially in the UV), quenches the absorption of P-Cygni profiles, boosts the MgII $\lambda\lambda$ $2795,2802$ doublet, and fosters the production of a broad boxy H$\alpha$ emission component. The rise in ionization in the outer ejecta may quench some lines (e.g., the CaII near-infrared triplet). The interaction power emerges preferentially in the UV, in particular at later times, shifting the optical color to the blue, but increasing modestly the optical luminosity. Strong thermalization and clumping seem to be required to make an interaction superluminous in the optical. The UV range contains essential signatures that provide critical constraints to infer the mass loss history and inner workings of core-collapse SN progenitors at death.