It's a well-known astronomical fact that young stars often grow through their adolescent years sporting a dusty planet-forming ring called a protoplanetary disk. But in new observations it seems that even old stars, churning in the violent throes of stellar death, possess strikingly similar features. This finding has led astronomers to an intriguing, yet counter-intuitive thought: do dying stars also produce planets?
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Dusty Death of a Star The observation above was made by the European Southern Observatory's Very Large Telescope Interferometer (VLTI), located in the Atacama Desert in Chile, producing the sharpest ever view of a dusty disk surrounding a star that is reaching the end of its life.
When stars like our sun run out of hydrogen fuel, they begin to fuse heavier and heavier elements in their cores. This results in an old star becoming very unstable, puffing up in size, turning into a red giant and erupting with powerful stellar winds. These stellar winds culminate in a so-called "planetary nebula" surrounding the red giant. In addition, a dusty ring may form and that is what has piqued the interest of astrophysicists.
Until now the dusty disks surrounding dying stars have been hard to come by. Sure, we've found plenty of young stars with protoplanetary disks, but it would be nice if they can be compared with their older counterparts.
Using the VLTI, astronomers have found one of these disks surrounding an old red giant around 4,000 light-years from Earth. The star, which is one of a binary pair called IRAS 08544-4431, was located in the constellation of Vela. Its smaller binary partner orbits the larger star very closely within the main disk.
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"By combining light from several telescopes of the Very Large Telescope Interferometer, we obtained an image of stunning sharpness - equivalent to what a telescope with a diameter of 150 meters would see," said astronomer Jacques Kluska, of the University of Exeter (UK), in an ESO press release. "The resolution is so high that, for comparison, we could determine the size and shape of a one euro coin seen from a distance of two thousand kilometers."
Interferometry uses several separate telescopes that are positioned far apart. Their spacing creates a "virtual" telescope that is bigger than the sum of its parts. In the case of the VLTI, it is composed of four individual telescopes, each with a primary mirror over 8 meters wide. When used together, these four telescopes (plus four Auxiliary Telescopes) combine as one to simulate an optical telescope that is 150 meters wide. When it comes to telescopes, the bigger the better, making the VLTI an extremely high-performance observatory capable of capturing incredible detail in the dusty disk around IRAS 08544-4431.
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Using a new technique to block out the glare of the central star, the researchers were able to see the inner edge of the disk surrounding the large star, as theoretical models predict. The inner edges of stellar disks will always be free of dust as the star's intense heating will vaporize the innermost material. As an added bonus, the VLTI also detected a smaller dusty ring around the second, smaller star in the binary system.
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"We were also surprised to find a fainter glow that is probably coming from a small accretion disc around the companion star," added Michel Hillen, of the Instituut voor Sterrenkunde in Leuven, Belgium, and lead author of a paper published in the Astronomy & Astrophysics. "We knew the star was double, but weren't expecting to see the companion directly. It is really thanks to the jump in performance now provided by the new detector in PIONIER (Precision Integrated-Optics Near-infrared Imaging ExpeRiment, attached to the VLT), that we are able to view the very inner regions of this distant system."
This observation has led to speculation that these old star disks, like the protoplanetary disks surrounding baby stars, may give rise to a second generation of planetary formation, but more observations are needed.
"Our observations and modelling open a new window to study the physics of these discs, as well as stellar evolution in double stars. For the first time the complex interactions between close binary systems and their dusty environments can now be resolved in space and time," said co-author Hans Van Winckel, also of the Instituut voor Sterrenkunde in Leuven.