A team of European astronomers have found an aged star surrounded by a disc, which is a reservoir of trapped dust that surrounds an elderly star. This discovery provides exciting new clues about the shaping of the one of the most beautiful sights in the universe—the planetary nebulae.
In the last phases of their life, aged stars evolve from an enormous red giant to a small white dwarf, which are not much larger than the Earth. The transition is accomplished by the shedding of a huge envelope of gas and dust that sparkles in many colors, producing a spectacular planetary nebula. This “celestial chrysalis” found will quickly, relatively speaking, unfold into a colorful, cosmic butterfly.
This metamorphosis is rapid in terms of the star's lifetime. But the actual process is rather complex and poorly understood. In particular, astronomers wonder how a spherical star can produce a great variety of planetary nebulae, some with very odd, asymmetrical shapes.
A team of scientists therefore embarked upon the study of a star which is currently on its way to becoming such a cosmic butterfly. The star, V390 Velorum, is 5000 times as bright as our Sun and is located 2,600 light-years away. It is also known to have a companion star that accomplishes its ballet in 500 days.
Astronomers postulate that elderly stars with companions possess a reservoir of dust that is thought to play a lead role in the final chapters of their lives. The shape and structure of these reservoirs remain, however, largely unknown.
To study the object with great precision, the astronomers linked observations taken with ESO's powerful interferometric instruments, AMBER and MIDI, at the Very Large Telescope Interferometer. In particular, they combined, using AMBER, the near-infrared light of three of VLT's 8.2-m Unit Telescopes. "Only this triple combination of powerful telescopes allows us to pinpoint the position and the shape of the dusty reservoir on a milli-arcsecond scale," explains Pieter Deroo, lead-author of the paper that presents these results in the research journal Astronomy and Astrophysics.
These observations clearly demonstrate that the dust present around the star cannot be distributed in a spherical shell. "This shows that whatever mechanism is shaping asymmetric planetary nebulae is already present prior to the metamorphosis taking place," says Hans Van Winckel, member of the team.
"This disc is found around a star that is in a very brief phase of its life - just a blink of an eye over the star's lifespan of billions of years - but this phase is very important," says Deroo. "It is in this period that a huge morphological change occurs, leading to the creation of a planetary nebula.”
The very high spatial resolution measurements allowed the astronomers to study the very inner structure of the disc as well as its orientation. The observations probed the physical nature of the disc and revealed that the dust in the inner rim is extremely hot and puffed up. The disc is circumbinary, meaning it surrounds both stars.
"The combination of MIDI and AMBER on ESO's VLTI is an extremely powerful and perhaps unique tool to study the geometry of the material around stars," concludes Van Winckel.
It looks like it is the season for disc 'hunting' and astronomers are so far delighted with their success. The detection of a dusty disc in the notable Ant Nebula was also just announced. Using VLTI and its unique ability to see small details, astronomers have uncovered a flat, nearly edge-on disc of silicates in the heart of the magnificent Ant Nebula. The disc seems, however, too 'skinny' to explain how the nebula got its intriguing ant-like shape.
The Ant Nebula is one of the most striking and planetary nebulae known. The term planetary nebulae arose because most are spherical and looked like planets when they were first discovered through older, less powerful telescopes. In fact, planetary nebulae are glowing structures of gas cast off by solar-like stars at the ends of their lives. The morphology of the Ant Nebula is a bright core, three nested pairs of bipolar lobes and a ring-like outflow, which is incredibly unique in shape. The disc portion had been previously unnoticed.
"The challenge is to actually detect these discs," explains team leader Olivier Chesneau, from the Observatoire de la Côte d'Azur, France. "Most astronomical instruments do not have a sharp enough view to find, let alone study them. The Very Large Telescope Interferometer however, with its exceptionally high spatial resolution, is a powerful disc-hunter."
The disc of the Ant Nebula extends from about 9 times the mean distance between the Earth and the Sun (9 Astronomical Units or 9 AU*) to more than 500 AU. At the distance of the Ant Nebula, this corresponds to having detected structures that subtend an angle of only 6 milli-arcseconds, which is similar to being able to distinguish a two-storey building on the Moon. The team says that the disc is too light to have a significant impact on the outflowing material and cannot explain the exotic shape of the Ant Nebula, which for now remains somewhat of a mystery.
Posted by Rebecca Sato
* One Astronomical Unit (AU) is the mean distance between the Earth and the Sun. It corresponds to 149.6 million kilometres. For comparison, Saturn is 10 AU away from the Sun.
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