Sci & Tech

ISRO’s AstroSat Space-based Observatory discovers UV Wings on the Butterfly Nebula


Indian astrophysicists have discovered large ultraviolet lobes and jets, hurled out from a dying star, using data from AstroSat, the space observatory launched by the Indian Space Research Organisation (ISRO) in 2015. The discovery has been featured as the AstroSat Picture of the Month (APOM) for October.

Kameswara Rao of the Indian Institute of Astrophysics and his collaborators used the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat to study a planetary nebula called NGC 6302, popularly called the Butterfly Nebula. A planetary nebula is formed when a star like our Sun – or a few times heavier – is in its dying days. The term, a misnomer now, was coined by astronomers in the 19th century since the nebula looked like planets through their telescopes.

“When hydrogen and helium fuel that kept the star shining gets exhausted, the star expands in size and becomes a red giant star,” Rao explained. “Such stars shed most of their outer layers which expands outwards, and the inner core, made of carbon and oxygen, shrinks further and becomes hotter. This hot core shines brightly in the ultraviolet, and ionises the expanding gas. This glowing ionised gas is what is seen as a planetary nebula.”

Sriram Krishna, a student of Rao, spent many hours analysing the data from the Butterfly Nebula. “Its central star is one of the hottest that we know, at 220,000 degrees celsius. The name itself comes from the shape of the two lobes of expanding gas that look like the wings of a butterfly,” he said.

One might expect a planetary nebula to be spherical, but it actually exhibits a range of complicated structures. “We used the UVIT on AstroSat to make four images of the nebula, each in different ultraviolet ‘colours’, or filters. The image made with the filter centred at 160.8 nm, called F169M, had a surprise in store for us,” said Sriram.

Astronomers have studied the two lobes of the nebula for many years through visible light images. They expect that the more energetic ultraviolet light would be emitted closer to the central star, where the hot stellar wind hits the slowly expanding gas. “However, we discovered that the lobes imaged with the F169M filter in ultraviolet were about three times larger than the size of the lobes imaged in visible light,” according to Sriram. After careful analysis, their study concluded that this ultraviolet emission must be due to cold molecular hydrogen gas outside the visible lobes, which had gone undetected so far.

“Our discovery points to an unseen companion star in an orbit with the central star,” said Firoza Sutaria, one of the coauthors. In addition, researchers also discovered two faint jets blasting out from the centre at almost right angles to the new ultraviolet lobes.

The team led by Rao recently discovered a large ultraviolet halo in yet another planetary nebula using AstroSat, and will be looking at more such objects in the future. They hope that such discoveries may provide the answer to the age-old puzzle of the missing mass problem in planetary nebulae.

This discovery was made possible because of the uniqueness of UVIT. “Of all the ultraviolet telescopes in space, UVIT is special in its ability to image a large field of view with a very high resolution, or detail”, said V. Girish of ISRO.

“This ability, coupled with a novel image analysis software that we had developed, led us to this discovery”, explained Jayant Murthy, a coauthor of the paper and director of the Indian Institute of Astrophysics.

These results were accepted for publication in the journal Astronomy and Astrophysics on October 3, 2018.

Source – The Wire

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