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The weather report from IRC+10216: evolving irregular clouds envelop carbon star


10 December 2015


photo credit: The changing face of CW Leo over eight years reveals an impossibly large shift in the apparent position of the star. Paul Stewart and Peter Tuthill, University of Sydney
photo credit: The changing face of CW Leo over eight years reveals an impossibly large shift in the apparent position of the star. Paul Stewart and Peter Tuthill, University of Sydney

Astronomers have been misinterpreting the shapes of a dying star for decades, believing billowing dust was actually permanent structural features. However, a new study led by Sydney University has found that astronomers have been daydreaming at their work, over-interpreting clumps and clouds for something more fundamental. The conclusion was reached with the help of possibly the coolest instrument for stellar astronomy of all time: the rings of Saturn acting as a filter for the Cassini spacecraft.

CW Leo, also known as IRC+10216 is a star at the very end of its life. With a mass less than the Sun's, it has blown out to an estimated 700 times our star's size and shrouded itself in a gigantic cloud of dust. Exceptionally faint in visible light, it is the brightest star in the sky at certain infrared wavelengths, thanks to the heat imparted to the dust cloud.

All this makes it hard to study, which prompted to the University of Sydney's Professor Peter Tuthill and his PhD student Paul Stewart to seek a radical solution. "If you are trying to work out whether a star is a single point or has a companion you can watch it go behind another object," Tuthill explained. "If it is a single star it will just wink out. But if it is a binary it will disappear in steps."

In the space of a couple of years, it has completely shed its familiar identity and adopted an entirely new visage - Mr Paul Stewart.

Professor Tuthill noted this technique has been used when the Moon passes in front of a star to study objects, but he and Mr Stewart took it up several levels. They used the way CW Leo appeared and disappeared behind Saturn's rings, as seen from the Cassini spacecraft, to reconstruct a map of its light.

Cassini originally took some of the images of CW Leo for other reasons. As Professor Tuthill explained, "One way to study the rings is to watch the way they extinguish stars." However, the idea of using Cassini for stellar astronomy sufficiently impressed NASA that Stewart got to briefly fly Cassini to position it for additional photos.

The most fun I have had professionally - Mr Paul Stewart.

Professor Tuthill and Mr Stewart combined the results obtained with those from the more conventional sources of the Keck Observatory and Very Large Telescope to produce maps of CW Leo's recent changes. They found that what astronomers have been interpreting as structural jets are actually a case of parediolia. Professor Tuthill, who is co-author of the paper, reflected: "This is one of those humble moments when nature reminds us all who is boss. For the past 20 years, many astronomers - and I count as one - have tried to put a skeleton underneath the clumpy images we see."

Professor Tuthill said that a structural feature of a dying star would be something that was effectively permanent, but the clouds around CW Leo are more similar to the random movements of rain clouds. Previous observations had revealed bright spots, which astronomers interpreted as being where the star itself was located. Professor Tuthill said this was a case of astronomers fooling themselves into believing they had found what they expected to see. The bright spots have moved while we have been watching and the distances of the apparent shifts are several times the diameter of the Solar System.

Instead, he thinks there are times where windows open up that allow us to see deeper into the clouds, viewing spots that are further in and therefore more lit up.

The paper, The weather report from IRC+10216: evolving irregular clouds envelop carbon star was published in Monthly Notices of the Royal Astronomical Society on 3 December.


Contact: Vivienne Reiner

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