The dust content of the Cassiopeia A supernova remnant

Cassiopeia A as viewed by the Herschel PACS instrument at 70 micron (red to green colours) and the Hubble Space Telescope (HST) (white to purple colours)
Fig. 1 - Cassiopeia A as viewed by the Herschel PACS instrument at 70 micron (red to green colours) and the Hubble Space Telescope (HST) (white to purple colours)

The large reservoirs of dust observed in some high redshift galaxies have been hypothesised to originate from dust produced by supernovae from massive stars. Some theoretical studies have supported such a high efficiency of dust production (0.1-1.0M) in core-collapse supernovae which would suffice to account for the dust mass budget observed in dusty high-redshift sources. However, the dust reservoirs (<10-2M) that were detected at mid-IR wavelengths during the first 1000 days after explosion remained several orders of magnitude below these theoretical predictions.

With the recent advent of far-infrared and submillimetre observatories (e.g., Herschel, ALMA), the ability to also detect the emission from colder dust in supernova remnants opened up and resulted in the detection of dust masses on the order of 0.1-1.0M in some nearby supernova remnants (SN 1987A, Crab Nebula, Cassiopeia A).

Due to its location in the Perseus arm of the Milky Way, Cassiopeia A (Cas A, see Figure 1) is embedded in dense clouds of interstellar material which made the explosion of its massive progenitor star 330 years ago hardly visible to the naked eye. Studying the condensation of dust in Cas A thus requires infrared and submillimetre observations of the thermal dust emission, but at the same time the contamination by dust emission from foreground and background interstellar dust and the remnant's synchrotron radiation in those wavebands should be corrected for.

The Spitzer, WISE and Herschel Space observatories obtained photometric and spectroscopic observations of Cas A in the 3.4-500 micron wavelength range which allowed for studies of the dust continuum emission in this Galactic supernova remnant. In a study based on the Herschel data of Cas A, Barlow et al. (2010) found a cool (~35 K) dust component that is located interior to the reverse shock region with a mass estimated around 0.075 solar masses.

Example of a four-component fit to the dust emission spectrum of a spatially resolved region in the reverse shock of the supernova remnant. The interstellar dust and three (cold, warm, hot) supernova dust components are constrained by the observed 17-500 micron photometry.
Fig. 2 - Example of a four-component fit to the dust emission spectrum of a spatially resolved region in the reverse shock of the supernova remnant. The interstellar dust and three (cold, warm, hot) supernova dust components are constrained by the observed 17-500 micron photometry.

De Looze et al. (2017) present the first spatially resolved analysis of Cas A based on Spitzer and Herschel data at a common resolution of ~0.6 arcmin for this 5arcmin diameter remnant. They fit the dust continuum from 17 to 500 micron with a four-component interstellar medium and supernova dust model (see Figure 2) following a careful removal of contaminating line emission and synchrotron radiation. They identified a concentration of cold dust in the unshocked ejecta of Cas A with an estimated mass of 0.3-0.5M of silicate grains or 0.4-0.6M for a mixture of 50% of silicate-type grains and 50% of carbonaceous grains.

Their spatially resolved spectral energy distribution fitting analysis reveals the presence of a cold SN dust component that is mainly distributed interior to the reverse shock of Cas A (see Figure 3), suggesting that some part of the newly formed dust has been destroyed by the reverse shock. Their interstellar dust model predicts an average interstellar extinction of AV = 6-8mag towards Cas A, which explains the difficulty to detect a possible binary companion from optical observations.

Map of the total mass of dust grains condensed in Cas A and that have not yet been destroyed by the reverse shock. The total dust mass includes the mass of the cold (T~30K), warm (T~80K) and hot (T~100K) supernova dust components.
Fig. 3 - Map of the total mass of dust grains condensed in Cas A and that have not yet been destroyed by the reverse shock. The total dust mass includes the mass of the cold (T~30K), warm (T~80K) and hot (T~100K) supernova dust components.

More details can be found in the papers by Barlow et al. (2010) and De Looze et al. (2017). Please contact Dr. Ilse De Looze or Prof. Mike Barlow for more information.

last updated 08 November 2017