Detection of Pristine Circumstellar Material of the Cassiopeia A Supernova

TL;DR

This study presents high-resolution near-infrared spectroscopy of Cassiopeia A, revealing pristine circumstellar material with unprocessed iron, supporting a core-origin scenario and providing insights into the supernova progenitor's nature.

Contribution

First detection of unprocessed circumstellar material in Cassiopeia A using high-resolution spectroscopy, indicating a progenitor near the He core and supporting a blue supergiant explosion model.

Findings

Detected unprocessed iron in circumstellar clump

Most iron is in gas phase, not depleted onto dust

Supports a blue supergiant progenitor with a thin hydrogen envelope

Abstract

Cassiopeia A is a nearby young supernova remnant that provides a unique laboratory for the study of core-collapse supernova explosions. Cassiopeia A is known to be a Type IIb supernova from the optical spectrum of its light echo, but the immediate progenitor of the supernova remains uncertain. Here we report results of near-infrared, high-resolution spectroscopic observations of Cassiopeia A where we detected the pristine circumstellar material of the supernova progenitor. Our observations revealed a strong emission line of iron (Fe) from a circumstellar clump that has not yet been processed by the supernova shock wave. A comprehensive analysis of the observed spectra, together with an HST image, indicates that the majority of Fe in this unprocessed circumstellar material is in the gas phase, not depleted onto dust grains as in the general interstellar medium. This result is consistent with a theoretical model of dust condensation in material that is heavily enriched with CNO-cycle products, supporting the idea that the clump originated near the He core of the progenitor. It has been recently found that Type IIb supernovae can result from the explosion of a blue supergiant with a thin hydrogen envelope, and our results support such a scenario for Cassiopeia A.