Late-time Hubble Space Telescope Ultraviolet Spectra of SN 2023ixf and SN 2024ggi Show Ongoing Interaction with Circumstellar Material

We present far- and near-ultraviolet (UV) spectra of the Type II supernovae (SNe) SN~2023ixf from days 199 to 722 and SN~2024ggi at days 41 and 232. Both supernovae show broad, blueshifted, and asymmetric UV emission lines with an initial maximum velocity of $\sim9000\,km\,s^{-1}$ and narrow unresolved emission in CIV. We compare the optical and UV emission-line profiles, showing that they evolve from two distinct velocity profiles to a single profile tracing the UV emission. We interpret this as shock power from interaction with circumstellar material coming to dominate over the radioactive-decay power from the inner ejecta. Comparing our observations to radiative transfer models with injected shock power, we find SN~2024ggi is best matched by $P_{\mathrm{shock, abs}}=1\times10^{41}\,erg\,s^{-1}$ at day 40, SN~2023ixf at day 300 and SN~2024ggi at day 200 are best matched by $P_{\mathrm{shock,abs}}=1\times10^{40}\,erg\,s^{-1}$, and SN~2023ixf at day 600 is best matched by $P_{\mathrm{shock,abs}}=5\times10^{39}\,erg\,s^{-1}$. From these models, we find the mass-loss rate of both supernovae increased just before explosion. For SN~2023ixf our mass-loss rates go from $4\times10^{-5}\,M_{\odot}\,yr^{-1}$ at 600 yr before explosion to $2\times10^{-2}\,M_{\odot}\,yr^{-1}$ at 15 yr prior to explosion. For SN~2024ggi, we find a mass-loss rate of $9\times10^{-5}\,M_{\odot}\,yr^{-1}$ at 150 yr before explosion and $1\times10^{-3}\,M_{\odot}\,yr^{-1}$ at 30 yr before explosion.