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Expansion of the mini supernova GK Per

posted Mar 17, 2015, 1:55 PM by Jeremy Drake   [ updated Mar 18, 2015, 7:59 AM ]
The year was 1901. The world had just celebrated the turn of the new century, Queen Victoria was laid to rest, and six colonies were federated to form the new nation of the Commonwealth of Australia.  In Edinburgh, Scotland, at about 2:40 in the morning of February 22, keen amateur astronomer Thomas Anderson, "the most assiduous watcher of the skies" according to contemporaries, was about to retire to rest for the night.  Throwing a last glance upward, he saw a brilliant star in the constellation Perseus.  It turned out to be the brightest nova of modern times, until unseated by Nova Aquilae 1918.  Photographs of the nova, like this one taken with the Two-Foot Reflector at Yerkes Observatory in 1901 September, showed the central star to be surrounded by a shell-like nebulosity. 

Earlier posts have detailed how novae are now understood as thermonuclear blasts on a white dwarf star that has accreted gas from a very close companion. The gas ignites when it has reached a critical temperature and pressure. The resulting blast wave can shock any surrounding gas to X-ray emitting temperatures, as well as fling off million degree debris in the process, much like a miniature supernova.  Novae offer the distinct advantage that we can watch them evolve over a period of years, whereas supernova remnants evolve much more slowly, over hundreds to thousands of years.

Chandra observed the Nova Per 1901 remnant in 2000 February, finding highly asymmetric X-ray emission that was bright toward the southwest as illustrated by the blue colour in the accompanying image made up from Chandra, Hubble (orange) and Very Large Array radio (magenta) data. This is the same quadrant in which the brightest nebulosity seen in the 1901 photographs was located. The nebulosity is thought to be material thrown off from the central star - now know as the variable GK Per - in a recent planetary nebula phase. Chandra re-observed the remnant in 2013 November, and was able to measure the expansion rate, temperature and brightness. While the remnant has dimmed through expansion, it has not cooled appreciably. Dimming has been greatest in the fainter areas to the north and east (upper left in these images), indicating a faster expansion rate in those directions.  This is just what is expected based on the old 1901 images of the distribution of gas around the star when it exploded.  The blastwave expands faster into lower density gas, and is slowed by the more dense gas that forms the X-ray bright southwest rim.  This study was published in the 2015 March 10 edition of the Astrophysical Journal and was lead by ex-SAO postdoc Dai Takei, now a scientist at RIKEN Center, Japan.  It also forms the subject of a Chandra image release. 
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