Spectroscopic detection of Carbon Monoxide in the Young Supernova Remnant Cassiopeia A

TL;DR

This study reports the first spectroscopic detection of CO molecules in the supernova remnant Cassiopeia A, revealing that CO forms early and persists for centuries, affecting our understanding of dust formation and element locking in supernova ejecta.

Contribution

It provides the first spectroscopic evidence of CO in Cas A, showing molecules form early and survive long, impacting models of supernova chemistry and dust formation.

Findings

CO detected at 4.65 microns in Cas A

CO molecules formed early in ejecta

Significant CO mass persists after 330 years

Abstract

We report the detection of carbon monoxide (CO) emission from the young supernova remnant Cassiopeia A (Cas A) at wavelengths corresponding to the fundamental vibrational mode at 4.65 micron. We obtained AKARI Infrared Camera spectra towards 4 positions which unambiguously reveal the broad characteristic CO ro-vibrational band profile. The observed positions include unshocked ejecta at the center, indicating that CO molecules form in the ejecta at an early phase. We extracted a dozen spectra across Cas A along the long 1 arcmin slits, and compared these to simple CO emission models in Local Thermodynamic Equilibrium to obtain first-order estimates of the excitation temperatures and CO masses involved. Our observations suggest that significant amounts of carbon may have been locked up in CO since the explosion 330 years ago. Surprisingly, CO has not been efficiently destroyed by reactions with ionized He or the energetic electrons created by the decay of the radiative nuclei. Our CO detection thus implies that less carbon is available to form carbonaceous dust in supernovae than is currently thought and that molecular gas could lock up a significant amount of heavy elements in supernova ejecta.