Freshly Formed Dust in the Cassiopeia A Supernova Remnant as Revealed by the Spitzer Space Telescope

TL;DR

This study used Spitzer infrared observations to identify and characterize three distinct populations of freshly formed dust in the Cassiopeia A supernova remnant, estimating a total dust mass that supports supernovae as significant dust sources in early galaxies.

Contribution

It provides the first detailed spectral analysis of multiple dust populations in Cas A, revealing their compositions and estimating their total mass, thus linking supernova dust formation to cosmic dust budgets.

Findings

Identified three distinct dust populations with different compositions.

Estimated total dust mass in Cas A between 0.02 and 0.054 solar masses.

Confirmed that supernovae can produce enough dust to account for early universe observations.

Abstract

We performed Spitzer Infrared Spectrograph mapping observations covering nearly the entire extent of the Cassiopeia A supernova remnant (SNR), producing mid-infrared (5.5-35 micron) spectra every 5-10". Gas lines of Ar, Ne, O, Si, S and Fe, and dust continua were strong for most positions. We identify three distinct ejecta dust populations based on their continuum shapes. The dominant dust continuum shape exhibits a strong peak at 21 micron. A line-free map of 21 micron-peak dust made from the 19-23 micron range closely resembles the [Ar II], [O IV], and [Ne II] ejecta-line maps implying that dust is freshly formed in the ejecta. Spectral fitting implies the presence of SiO2, Mg protosilicates, and FeO grains in these regions. The second dust type exhibits a rising continuum up to 21 micron and then flattens thereafter. This ``weak 21 micron'' dust is likely composed of Al2O3 and C grains. The third dust continuum shape is featureless with a gently rising spectrum and is likely composed of MgSiO3 and either Al2O3 or Fe grains. Using the least massive composition for each of the three dust classes yields a total mass of 0.02 Msun. Using the most-massive composition yields a total mass of 0.054 Msun. The primary uncertainty in the total dust mass stems from the selection of the dust composition necessary for fitting the featureless dust as well as 70 micron flux. The freshly formed dust mass derived from Cas A is sufficient from SNe to explain the lower limit on the dust masses in high redshift galaxies.