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We present panchromatic observations and modeling of supernova (SN) 2020tlf,the first normal type II-P/L SN with confirmed precursor emission, as detectedby the Young Supernova Experiment transient survey with the Pan-STARRS1telescope. Pre-explosion emission was detected in $riz-$bands at 130 days priorto SN 2020tlf and persisted at relatively constant flux until first light. Soonafter discovery, "flash" spectroscopy of SN 2020tlf revealed prominent narrowsymmetric emission lines ($v_w < 300$ km s$^{-1}$) that resulted from thephoto-ionization of unshocked circumstellar material (CSM) shedded inprogenitor mass loss episodes in the final weeks to months before explosion.Surprisingly, this novel display of pre-SN emission and associated mass lossoccurred in a RSG progenitor with ZAMS mass of only 10-12 M$_{\odot}$, asinferred from nebular spectra. Modeling of the light curve and multi-epochspectra with the non-LTE radiative transfer code CMFGEN andradiation-hydrodynamical (RHD) code HERACLES suggests a dense CSM limited to $r\approx 10^{15}$ cm, and mass loss rate of $10^{-2}$ M$_{\odot}$ yr$^{-1}$. Thesubsequent luminous light-curve plateau and persistent blue excess indicates anextended progenitor, compatible with a RSG model with $R_{\star} = 1100$R$_{\odot}$. Inferences from the limits on the shock-powered X-ray and radioluminosity are consistent with these conclusions and suggest a CSM density of$\rho < 2 \times 10^{-16}$ g cm$^{-3}$ for distances of $r \approx 5 \times10^{15}$ cm, as well as a mass loss rate of $\dot M<1.3 \times10^{-5}\,\rm{M_{\odot}\,yr^{-1}}$ at larger distances. A promising power sourcefor the observed precursor emission is the ejection of stellar materialfollowing energy disposition into the stellar envelope as a result of gravitywaves emitted during either neon/oxygen burning or a nuclear flash from siliconcombustion.

Final Moments I: Precursor Emission, Envelope Inflation, and Enhanced Mass loss Preceding the Luminous Type II Supernova 2020tlf