The nebular spectra of SN 2023ixf: a lower mass, partially stripped progenitor may be the result of binary interaction

ABSTRACT SN 2023ixf is one of the brightest core collapse supernovae of the 21st century and offers a rare opportunity to investigate the late stage of a supernova through nebular phase spectroscopy. We present four nebular phase spectra from day +291 to +413 after explosion. This is supplemented with high-cadence early phase spectroscopic observations and photometry covering the first 500 d to investigate explosion parameters. The narrow and blueshifted nebular oxygen emission lines are used to infer an ejected oxygen mass of $<0.65$ M$_\odot$, consistent with models of a relatively low mass ($M_{\mathrm{ ZAMS}} < 15$ M$_\odot$) progenitor. An energy of 0.3–$1.4 \times 10^{51}$ erg and a light curve powered by an initial $^{56}$Ni mass of $0.049 \pm 0.005$ M$_\odot$ appear consistent with a relatively standard Type II explosion, while an incomplete $\gamma$-ray trapping (with time-scale of $240\pm 4$ d) suggests a lower ejecta mass. Assuming a typical explosion, the broad hydrogen and calcium profiles suggest a common origin within a lower mass, partially stripped envelope. Hydrogen emission broadens with time, indicating contribution from an additional power source at an extended distance; while the emergence of high-velocity ($\sim$6000 km s$^{-1}$) hydrogen emission features (beginning around day +200) may be explained by shock interaction with a dense hydrogen-rich region located at $\sim 1.5 \times 10^{16}$ cm. Such envelope mass-loss for a low-mass progenitor may be explained through theoretical models of binary interaction.