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We present five far- and near-ultraviolet spectra of the Type II plateausupernova, SN 2022acko, obtained 5, 6, 7, 19, and 21 days after explosion, allobserved with the Hubble Space Telescope/Space Telescope Imaging Spectrograph.The first three epochs are earlier than any Type II plateau supernova has beenobserved in the far-ultraviolet revealing unprecedented characteristics. Thesethree spectra are dominated by strong lines, primarily from metals, whichcontrasts with the relatively featureless early optical spectra. The fluxdecreases over the initial time series as the ejecta cools and line-blanketingtakes effect. We model this unique dataset with the non-local thermodynamicequilibrium radiation transport code CMFGEN, finding a good match to theexplosion of a low mass red supergiant with energy Ekin = 6 x 10^50 erg. Withthese models we identify, for the first time, the ions that dominate the earlyUV spectra. We also present optical photometry and spectroscopy, showing thatSN 2022acko has a peak absolute magnitude of V = -15.4 mag and plateau lengthof ~115d. The spectra closely resemble those of SN 2005cs and SN 2012A. Usingthe combined optical and UV spectra, we report the fraction of flux redwards ofthe uvw2, U, B, and V filters on days 5, 7, and 19. We also create a spectraltime-series of Type II supernovae in the ultraviolet, demonstrating the rapiddecline of UV flux over the first few weeks of evolution. Future observationsof Type II supernovae will continue to explore the diversity seen in thelimited set of high-quality UV spectra.

SN 2022acko: the First Early Far-Ultraviolet Spectra of a Type IIP Supernova