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A large fraction of red-supergiant stars seem to be enshrouded bycircumstellar material (CSM) at the time of explosion. Relative to explosionsin a vacuum, this CSM causes both a luminosity boost at early times as well asthe presence of symmetric emission lines with a narrow core andelectron-scattering wings typical of type IIn supernovae (SNe). For this study,we performed radiation-hydrodynamics and radiative transfer calculations for avariety of CSM configurations (i.e., compact, extended, and detached) anddocumented the resulting ejecta and radiation properties. We find that modelswith a dense, compact, and massive CSM of ~0.5Msun can match the earlyluminosity boost of type II-P SNe but fail to produce type IIn-like spectralsignatures (aka ``flash features''). These only arise if the photon mean freepath in the CSM is large enough (i.e, if the density is low enough) to allowfor a radiative precursor through a long-lived (i.e., a day to a week),radially extended unshocked optically thick CSM. The greater radiative lossesand kinetic-energy extraction in this case boost the luminosity even for modestCSM masses -- this boost is delayed for a detached CSM. The inadequateassumption of high CSM density, in which the shock travels quasi adiabatically,overestimates the CSM mass and associated mass-loss rate. Our simulations alsoindicate that type IIn-like spectral signatures last as long as there isoptically-thick unshocked CSM. Constraining the CSM structure thereforerequires a combination of light curves and spectra, rather than photometryalone. We emphasize that for a given total energy, the radiation excessfostered by the presence of CSM comes at the expense of kinetic energy, asevidenced by the disappearance of the fastest ejecta material and theaccumulation of mass in a dense shell. Both effects can be constrained fromspectra well after the interaction phase.

Using spectral modeling to break light-curve degeneracies of type II supernovae interacting with circumstellar material