SN 2023ixf: interaction signatures in the spectrum at 445 days

TL;DR

SN 2023ixf, a nearby Type II supernova, shows late-time spectral signs of ejecta-CSM interaction at 445 days, revealing details about its progenitor's mass loss history and environment.

Contribution

This study provides the first detailed nebular spectrum analysis of SN 2023ixf at 445 days, highlighting interaction signatures and refining progenitor mass estimates.

Findings

Detection of ejecta-CSM interaction at 445 days.

Identification of a CSM shell at ~10^16 cm from the progenitor.

Progenitor mass estimated between 10-15 solar masses.

Abstract

SN 2023ixf is one of the most neaby and brightest Type II supernovae (SNe) of the past decades. A rich set of pre-explosion data provided important insight on the properties of the progenitor star. There has been a wide range of estimated initial masses of 9 - 22 $M{_\odot}$. Early monitoring of the SN also showed the presence of a dense CSM structure near the star ($10^{15}$ cm) that was probably expelled in the last years prior to the explosion. These extended CSM structure can be further probed with late-time observations during the nebular phase. This study is based on a nebular spectrum obtained with GMOS at the Gemini North Telescope 445 days after explosion. The SN evolution is analyzed in comparison with a previous spectrum at an age of 259 days, and compared with those of similar SNe II and with synthetic radiation-transfer nebular spectra. The 445-d spectrum exhibits a dramatic evolution with clear signs of ejecta-CSM interaction. The H${\alpha}$ profile shows a complex profile that can be separated into a boxy component arising from the interaction with a CSM shell and a central peaked component that may be due to the radioactive-powered SN ejecta. The CSM shell would be located at a distance of $\approx10^{16}$ cm from the progenitor and it may be associated with mass loss occurring up until $\approx 500 - 1000$ years before the explosion. Similar interaction signatures have been detected in other SNe II, although for events with standard plateau durations this happened at times later than 600 - 700 days. SN 2023ixf appears to belong to a group of SNe II with short plateaus or linear light curves that develop interaction features before $\approx 500$ days. Other lines, such as those from [O I] and [Ca II] appear to be unaffected by the CSM interaction. This allowed us to estimate an initial progenitor mass, which resulted in the relatively low range of 10 - 15 $M{_\odot}$.