Cassiopeia A: dust factory revealed via submillimetre polarimetry

TL;DR

This study provides new evidence that supernova remnants, specifically Cassiopeia A, rapidly produce significant quantities of dust, with high polarisation indicating efficient grain alignment or unique dust properties.

Contribution

The paper presents the first detection of highly polarised submillimetre emission from Cas A, confirming the presence of cold dust formed rapidly in a supernova remnant.

Findings

Polarised submm emission exceeds synchrotron levels in Cas A.

Dust polarisation fraction is around 30%, much higher than typical Galactic sources.

Evidence suggests rapid dust formation and efficient grain alignment in supernova remnants.

Abstract

If Type-II supernovae - the evolutionary end points of short-lived, massive stars - produce a significant quantity of dust (>0.1 M_sun) then they can explain the rest-frame far-infrared emission seen in galaxies and quasars in the first Gyr of the Universe. Submillimetre observations of the Galactic supernova remnant, Cas A, provided the first observational evidence for the formation of significant quantities of dust in Type-II supernovae. In this paper we present new data which show that the submm emission from Cas A is polarised at a level significantly higher than that of its synchrotron emission. The orientation is consistent with that of the magnetic field in Cas A, implying that the polarised submm emission is associated with the remnant. No known mechanism would vary the synchrotron polarisation in this way and so we attribute the excess polarised submm flux to cold dust within the remnant, providing fresh evidence that cosmic dust can form rapidly. This is supported by the presence of both polarised and unpolarised dust emission in the north of the remnant, where there is no contamination from foreground molecular clouds. The inferred dust polarisation fraction is unprecedented (f_pol ~ 30%) which, coupled with the brief timescale available for grain alignment (<300 yr), suggests that supernova dust differs from that seen in other Galactic sources (where f_pol=2-7%), or that a highly efficient grain alignment process must operate in the environment of a supernova remnant.