Weak Mass Loss from the Red Supergiant Progenitor of the Type II SN 2021yja

TL;DR

This study analyzes early observations of SN 2021yja, revealing it had a low-density circumstellar environment and a progenitor likely less than 9 solar masses, challenging assumptions about red supergiant mass loss.

Contribution

It provides detailed early-time data and analysis of a Type II supernova, highlighting the importance of progenitor structure and CSM properties in supernova evolution.

Findings

SN 2021yja shows signs of CSM interaction without typical early light curve features.

Progenitor mass constrained to less than 9 solar masses, conflicting with some SN evolution indicators.

Progenitor likely an extended red supergiant with low-density circumstellar material.

Abstract

We present high-cadence optical, ultraviolet (UV), and near-infrared data of the nearby ($D\approx23$ Mpc) Type II supernova (SN) 2021yja. Many Type II SNe show signs of interaction with circumstellar material (CSM) during the first few days after explosion, implying that their red supergiant (RSG) progenitors experience episodic or eruptive mass loss. However, because it is difficult to discover SNe early, the diversity of CSM configurations in RSGs has not been fully mapped. SN 2021yja, first detected within ${\approx}5.4$ hours of explosion, shows some signatures of CSM interaction (high UV luminosity, radio and x-ray emission) but without the narrow emission lines or early light curve peak that can accompany CSM. Here we analyze the densely sampled early light curve and spectral series of this nearby SN to infer the properties of its progenitor and CSM. We find that the most likely progenitor was an RSG with an extended envelope, encompassed by low-density CSM. We also present archival Hubble Space Telescope imaging of the host galaxy of SN 2021yja, which allows us to place a stringent upper limit of ${\lesssim}9\ M_\odot$ on the progenitor mass. However, this is in tension with some aspects of the SN evolution, which point to a more massive progenitor. Our analysis highlights the need to consider progenitor structure when making inferences about CSM properties, and that a comprehensive view of CSM tracers should be made to give a fuller view of the last years of RSG evolution.