Dust factory found

around Cas A

DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 24 February, 2009

A team of astronomers have found that copious amounts of dust are being produced within the dead remains of the Cassiopeia A supernova remnant.

Cassiopeia A exploded about 300 years ago in a dramatic supernova event that marks the end of a star’s life. In a new study, a team of astronomers led by Loretta Dunne from the University of Nottingham, have found some very unusual stardust within the Cas A remnant.

A multi-colour image of the Cassiopeia A supernova remnant. The overlaid lines indicate the polarised signal from cold dust within the remnant with the strength marked by the length of each line. The direction of the lines indicates the orientation of the magnetic field in Cassiopeia A. Image: Submm:Loretta Dunne, University of Nottingham; X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech.

Interstellar dust is a prolific component of the cosmos, consisting of carbon and silicate particles that are responsible for dark patches seen in the Milky Way on a dark night. This dust also aids the formation of stars and planets, and is found in vast quantities in galaxies dating back to the Universe’s early years.

The origin of the dust remains a mystery, however. Some theories for its genesis include it condensing like snowflakes in the winds of red giant stars, or alternatively, produced in supernovae events resulting from the death of massive stars. Supernovae produce dust relatively fast, since massive stars evolve relatively quickly, taking a few million years to reach their supernova stage. For comparison, lower mass stars like our Sun take billions of years to reach their dust-forming red giant phase. Despite many decades of research, however, astronomers have still not found conclusive evidence that supernovae can produce dust in the quantities required to account for the dust they see in the early Universe.
 
Using the SCUBA polarimeter on the James Clerk Maxwell Telescope in Hawaii, Dunne and colleagues studied the dust in the Cas A supernova remnant. In particular they searched for a signal from dust grains spinning in the remnant’s strong magnetic field. If the dust grains are slightly elongated (cigar-shaped) they tend to line up the same way and produce a polarised signal. When the polarimeter detector is rotated, the strength of the signal changes – much the same as if you look at the sky with polaroid sunglasses held at different angles.

An image of the sub-mm radiation emission from dust in and around Cassiopeia A. The overlaid black lines indicate the polarised signal from the dust within Cassiopeia A. The scale bar represents 30% polarised emission. Image: Loretta Dunne, University of Nottingham.
 
The scientists found that the polarisation signal from the supernova dust is the strongest ever measured anywhere in the Milky Way, emitting more radiation per gram than regular interstellar dust. Such an observation requires an orderly alignment of the grains. “It is like nothing we’ve ever seen,” says Dunne. “It could be that the extreme conditions inside the supernova remnant are responsible for the strong polarised signal, or it could be that the dust grains themselves are highly unusual”.
 
Team member Professor Rob Ivison of the Science and Technology Facilities Council’s Astronomy Technology Centre in Edinburgh comments: “It could be that the material we're seeing is in the form of iron needles – exotic, slender, metallic whiskers. If these grains are distributed throughout the Universe they may be re-radiating microwaves. This has major consequences for our understanding of the cosmic microwave background – one of the most important building blocks of the Big Bang model of our Universe”.
 
Another suggestion could be that the grains are a more pristine version of the same dust found elsewhere in the Galaxy, with the same composition but able to produce more radiation due to subtle difference in its structure. The final verdict will fall to the Herschel Space Observatory, scheduled to launch by the European Space Agency on 16 April.