Dr Loretta Dunne and her team have found new evidence of huge dust production in the Cassiopeia A supernova remnant, the remains of a star that exploded about 300 years ago. The paper is set to be published in the Monthly Notices of the Royal Astronomical Society.
Interstellar dust is found throughout the cosmos. It is responsible for the dark patches seen in the Milky Way on a moonless night. It consists of carbon and silicate particles, about the size of those in cigarette smoke. The dust helps stars like the Sun to form and subsequently coagulates to form planets like Earth and the cores of giant gas planets like Jupiter. It is found in huge quantities in galaxies, even very early in the history of the universe.
A multi-color 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 temperature of this dust is around -250°C. The scale bar represents 30% polarised emission. The direction of the lines indicates the orientation of the magnetic field in Cassiopeia A. The underlying image is a composite of data from the Chandra X-ray observatory, the Hubble Space Telescope and the Spitzer Space Telescope. The red colours are infrared light from hot dust at 10°C, yellow is optical light from gas at 10,000°C and the blue/green colours show X-rays from gas at 10 million °C. Image credit: Submm:Loretta Dunne, University of Nottingham; X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech.
But the origin of all this dust is a mystery. Does it condense like snowflakes in the winds of red giant stars or is it produced in supernovae — the violent death-throes of massive stars? Supernovae are an efficient way of producing dust in a blink of the cosmic eye, as massive stars evolve relatively quickly, taking a few million years to reach their supernova stage. In contrast lower-mass stars like our Sun take billions of years to reach their dust-forming red giant phase. Despite many decades of research, 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, the scientists searched for a signal from dust grains spinning in the strong magnetic field of the supernova remnant. If the dust grains are slightly elongated (like little cigars) 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.
The polarisation signal from the supernova dust is the strongest ever measured anywhere in the Milky Way, marking it out as unusual. It emits more radiation per gram than regular interstellar dust and the alignment of the grains must be very orderly to produce such highly polarised emission.
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, with the strength marked by the length of each line. The scale bar represents 30% polarised emission. The direction of the lines indicates the orientation of the magnetic field in Cassiopeia A. Image: Loretta Dunne, University of Nottingham.
"It is like nothing we've ever seen" said Dr Dunne, who is based in the Centre for Astronomy and Particle Physics at The University of Nottingham. "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 UK Astronomy Technology Centre at the Institute for Astronomy, University of Edinburgh comments further. "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".
Alternatively, the grains could be a more pristine version of the dust found elsewhere in the Galaxy, the same composition but able to produce more radiation due to the nuances of its 3-D structure. A final verdict requires further observations using the Herschel Space Observatory, to be launched this year by the European Space Agency.