Progenitor and Close-In Circumstellar Medium of Type II Supernova\n 2020fqv from High-Cadence Photometry and Ultra-Rapid UV Spectroscopy

Abstract

We present observations of SN 2020fqv, a Virgo-cluster Type II core-collapse\nsupernova (CCSN) with a high temporal resolution light curve from the\nTransiting Exoplanet Survey Satellite (TESS) covering the time of explosion;\nultraviolet (UV) spectroscopy from the Hubble Space Telescope (HST) starting\n3.3 days post-explosion; ground-based spectroscopic observations starting\n1.1~days post-explosion; along with extensive photometric observations. Massive\nstars have complicated mass-loss histories leading up to their death as CCSNe,\ncreating circumstellar medium (CSM) with which the SNe interact. Observations\nduring the first few days post-explosion can provide important information\nabout the mass-loss rate during the late stages of stellar evolution. Model\nfits to the quasi-bolometric light curve of SN 2020fqv reveal ~0.23 $M_{\\odot}$\nof CSM confined within ~1450 $R_{\\odot}$ ($10^{14}$ cm) from its progenitor\nstar. Early spectra (<4 days post-explosion), both from HST and ground-based\nobservatories, show emission features from high-ionization metal species from\nthe outer, optically thin part of this CSM. We find that the CSM is consistent\nwith an eruption caused by the injection of $\\sim$$5\\times 10^{46}$ erg into\nthe stellar envelope $\\sim$300 days pre-explosion, potentially from a nuclear\nburning instability at the onset of oxygen burning. Light-curve fitting,\nnebular spectroscopy, and pre-explosion \\textit{HST} imaging consistently point\nto a red supergiant (RSG) progenitor with $M_{\\rm ZAMS}$$\\approx$$13.5$--$15 \\,\nM_{\\odot}$, typical for SN~II progenitor stars. This finding demonstrates that\na typical RSG, like the progenitor of SN 2020fqv, has a complicated mass-loss\nhistory immediately before core collapse.\n