Dust Production by Supernovae: Spitzer Observations of SN 2008S

The Universe contains dust in huge quantities, but the origin of this dust remains mysterious.  Dust formation by all the mechanisms we know of does not seem to be enough to explain the quantities observed in many galaxies, particularly those in the very distant Universe.  Research carried out at UCL has attempted to quantify the role that core-collapse supernovae play in producing dust.  These supernova explosions occur at the end of the lives of massive stars and briefly shine as brightly as entire galaxies.

Face on Spiral NGC6946

A fruitful hunting ground for supernovae has been the face-on spiral galaxy NGC 6946, shown here on the right. Located about 20 million light years from Earth, this galaxy has been the site of nine supernova explosions in the last hundred years, much more than most spiral galaxies produce.  Three of these have been discovered since the millennium.  While observing one of these, SN 2002hh, with the Spitzer Space Telescope, we fortuitously caught another, SN 2008S, just a few days after it exploded.  We obtained further images a few months after the outburst.

Spitzer observes the Universe at infrared (IR) wavelengths, and our very early IR observations of SN2008S (see figure below) allowed us to detect emission from dust near the supernova that had been heated up by the flash.  The existence of this infrared "echo" allowed us to infer that there was a large amount of dust surrounding the star which exploded. Images taken of the galaxy before the explosion occurred showed that the star had previously been hidden beneath a thick veil of dust, some of which was evaporated by the supernova flash.

Spitzer Montage of SN2008S
Spitzer Space Telescope images of the galaxy NGC 6946, showing the region of SN 2008S before, 18 days after, and six months after the explosion

The explosion was 30 million times as bright as the Sun, but still faint by normal supernova standards, and it is not yet known whether it was really a supernova or in fact just a giant outburst of a star which still survives.  However, we do now know that the progenitor star produced a few thousand earth-masses of dust before its outburst.  Most supernovae don't produce as much dust as this, but even if objects like SN 2008S are common, their dust production still does not seem to be sufficient to account for the quantities observed in distant galaxies.

Reference: Wesson et al, 2010, MNRAS, 403, 474

For further information, contact Mike Barlow (mjb AT star.ucl.ac.uk)

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