The circumstellar material around the Type IIP SN 2021yja

TL;DR

This paper models the circumstellar environment of SN 2021yja, revealing a complex, asymmetric CSM around a red supergiant progenitor, which influences the supernova's early light curve and flux excess.

Contribution

It provides a detailed fit of SN 2021yja's light curves incorporating asymmetric circumstellar matter, highlighting the role of CSM in early supernova behavior.

Findings

Presence of 0.55 Msun CSM within ~2x10^14 cm

Early flux excess indicates a compact CSM component

Asymmetry in CSM suggested by light curve fitting difficulties

Abstract

The majority of Type II-plateau supernovae (SNe IIP) have light curves that are not compatible with the explosions of stars in a vacuum; instead, the light curves require the progenitors to be embedded in circumstellar matter (CSM). We report on the successful fitting of the well-observed SN IIP 2021yja as a core-collapse explosion of a massive star with an initial mass of ~15 Msun and a pre-explosion radius of 631 Rsun. To explain the early-time behaviour of the broad-band light curves, the presence of 0.55 Msun CSM within ~2x10^14 cm is needed. Like many other SNe IIP, SN 2021yja exhibits an early-time flux excess including ultraviolet wavelengths. This, together with the short rise time (<2 days) in the gri bands, indicates the presence of a compact component in the CSM, essentially adjacent to the progenitor. We discuss the origin of the pre-existing CSM, which is most likely a common property of highly convective red supergiant envelopes. We argue that the difficulty in fitting the entire light curve with one spherical distribution indicates that the CSM around the SN 2021yja progenitor was asymmetric.