Signatures of the Shock Interaction as an Additional Power Source in the Nebular Spectra of SN 2023ixf

TL;DR

This study analyzes the nebular spectrum of SN 2023ixf, revealing shock interaction signatures that suggest additional energy sources and providing insights into the progenitor's properties and circumstellar environment.

Contribution

First nebular spectrum of SN 2023ixf obtained with WEAVE, identifying shock interaction features and constraining progenitor characteristics through spectral analysis.

Findings

Detection of broad Hα components indicating shock interaction.

Estimated shock power of at least 5×10^40 erg/s at +363 days.

Constraints on oxygen mass, He-core mass, and progenitor ZAMS mass.

Abstract

Red supergiants may lose significant mass during the final 100-1000 years before core collapse, shaping their circumstellar environment. The supernova (SN) shockwave propagating through this environment forms a shock-swept dense shell that interacts with the surrounding circumstellar material (CSM), generating secondary shocks that energise the ejecta and may power the SN during the nebular phase. In the present work, we investigate the nebular spectrum of SN 2023ixf, observed one-year post-explosion (at +363 d) with the recently commissioned WEAVE instrument on the 4.2m William Herschel Telescope. This marks the first supernova spectrum captured with WEAVE. In this spectrum, H$\alpha$ exhibits a peculiar evolution, flanked by blueward and redward broad components centred at $\sim\pm 5650\,\mathrm{km\,s^{-1}}$, features that have been observed in only a few SNe as early as one-year post-explosion. These features may indicate energy deposition from shock generated by the interaction of shock-swept dense shell with CSM expelled a few hundred years prior to the explosion. Comparisons of the +363 d spectrum with model spectra from the literature suggest a shock power of at least $\sim5 \times 10 ^{40}\,\mathrm{erg\,s^{-1}}$ at this epoch. Additionally, analysis of the [O I] doublet and other emission lines helped to constrain the oxygen mass ($\lesssim 0.07-0.30 M_\odot$), He-core mass ($\lesssim 3 M_\odot$), and zero-age main sequence mass ($\lesssim 12 M_\odot$) for SN~2023ixf. The comparison with other Type II SNe highlights SN 2023ixf's unique shock interaction signatures and evidence of dust formation, setting it apart in terms of evolution and dynamics.