Nebular Spectra of the Extremely Metal-Poor SN II 2023ufx Over a Year After Explosion: A Massive Progenitor and Unique Circumstellar Environment

Abstract

We present new observations of the metal-poor ($<0.1$ ${\mathrm{Z}}_{\odot }$) Type II supernova 2023ufx and its host galaxy. The deep nebular spectrum, obtained roughly a year after explosion, has a triple-peaked [O I] emission profile suggesting an asymmetric explosion. Comparison to nebular spectra models suggests a zero-age main sequence mass of $M_{\mathrm{ZAMS}}\sim 25-35$ ${\mathrm{M}}_{\odot }$, which is supported by the low [Ca II]/[O I] emission-line ratio of $\approx 0.4$. The diminishing, broad, boxy H$\alpha$ emission and flattening of the light curve in the late ($\sim 2$ yr) photometry suggest a complex mass-loss history in the centuries to millennia before explosion. New optical and near-infrared imaging of the host galaxy confirms that it is a dwarf, with a stellar mass of $10^{6.6\pm 0.1}$ ${\mathrm{M}}_{\odot }$ and a SFR of $10^{-2.5\pm 0.1}$ ${\mathrm{M}}_{\odot }$/yr. Results from both SED fitting and galaxy stellar mass-metallicity scaling relations all lead to an environmental metallicity estimate of $0.02-0.07$ ${\mathrm{Z}}_{\odot }$. Taken together, these observations confirm that SN 2023ufx is the explosion of a very metal-poor, heavily stripped red supergiant.

10 pages, 10 figures, 4 tables, to be submitted to OJA