SN 2023ixf in Messier 101: Photo-ionization of Dense, Close-in Circumstellar Material in a Nearby Type II Supernova

We present UV/optical observations and models of supernova (SN) 2023ixf, a type II SN located in Messier 101 at 6.9 Mpc. Early-time ("flash") spectroscopy of SN 2023ixf, obtained primarily at Lick Observatory, reveals emission lines of H I, He I/II, C IV, and N III/IV/V with a narrow core and broad, symmetric wings arising from the photo-ionization of dense, close-in circumstellar material (CSM) located around the progenitor star prior to shock breakout. These electron-scattering broadened line profiles persist for ~8 days with respect to first light, at which time Doppler broadened features from the fastest SN ejecta form, suggesting a reduction in CSM density at r 10¹⁵ cm. The early-time light curve of SN2023ixf shows peak absolute magnitudes (e.g., Mᵤ = -18.6 mag, Mg = -18.4 mag) that are 2 mag brighter than typical type II supernovae, this photometric boost also being consistent with the shock power supplied from CSM interaction. Comparison of SN 2023ixf to a grid of light curve and multi-epoch spectral models from the non-LTE radiative transfer code CMFGEN and the radiation-hydrodynamics code HERACLES suggests dense, solar-metallicity, CSM confined to r = (0.5-1) × 10¹⁵ cm and a progenitor mass-loss rate of Ṁ = 10⁻² M_yr⁻¹. For the assumed progenitor wind velocity of vw = 50 km s⁻¹, this corresponds to enhanced mass-loss (i.e., ``super-wind'' phase) during the last ~3-6 years before explosion.