The final months of massive star evolution from the circumstellar environment around SN Ic 2020oi

TL;DR

This study uses ALMA observations to reveal complex, non-smooth circumstellar material around SN 2020oi, indicating recent, rapid changes in the progenitor star's mass-loss just before explosion.

Contribution

It provides the first detailed radio evidence of sub-year scale fluctuations in the circumstellar environment of a type Ic supernova, linked to late-stage stellar nuclear burning.

Findings

Detected non-smooth CSM structure around SN 2020oi

Linked CSM fluctuations to rapid progenitor mass-loss changes

Highlighted importance of radio follow-up for supernova progenitor studies

Abstract

We present the results of ALMA band 3 observations of a nearby type Ic supernova (SN) 2020oi. Under the standard assumptions on the SN-circumstellar medium (CSM) interaction and the synchrotron emission, the data indicate that the CSM structure deviates from a smooth distribution expected from the steady-state mass loss in the very vicinity of the SN (~10^{15} cm), which is then connected to the outer smooth distribution (~10^{16} cm). This structure is further confirmed through the light curve modeling of the whole radio data set as combined with data at lower frequency previously reported. Being an explosion of a bare carbon-oxygen (C+O) star having a fast wind, we can trace the mass-loss history of the progenitor of SN 2020oi in the final year. The inferred non-smooth CSM distribution corresponds to fluctuations on the sub-year time scale in the mass-loss history toward the SN explosion. Our finding suggests that the pre-SN activity is likely driven by the accelerated change in the nuclear burning stage in the last moments just before the massive star's demise. The structure of the CSM derived in this study is beyond the applicability of the other methods at optical wavelengths, highlighting an importance and uniqueness of quick follow-up observations of SNe by ALMA and other radio facilities.