'Vampire' star caught in the act by Indian space observatory

India's first dedicated space observatory, ASTROSAT, has captured the rare phenomenon of a small, 6-billion-year-old "vampire" star "preying" on a bigger celestial body.

Scientists say the smaller star, also called a "blue straggler", feeds off its companion star by sucking out its mass and energy, causing its eventual death.

"The most popular explanation is that these are binary systems in which the smaller star sucks material out of the bigger companion star to become a blue straggler, and hence is called a vampire star.

"The small star becomes bigger, hotter and bluer, which gives it the appearance of being young, while the ageing companion burns out and collapses to a stellar remnant," said Annapurni Subramaniam, a Professor at the Indian Institute of Astrophysics.

Though this phenomenon is not unheard of, the observation of the entire process through the telescope will provide insights that will help scientists in studying the formation of 'blue straggler' stars.

This discovery also highlights the capabilities of the telescopes on ASTROSAT, a dedicated space observatory satellite launched in September 2015.

The study was recently published in Astrophysical Journal Letters by a team of scientists from IIA, Inter-University Centre of Astronomy and Astrophysics (IUCAA), Tata Institute of Fundamental Research (TIFR), Indian Space Research Organisation (ISRO) and the Canadian Space Agency (CSA).

Scientists are now looking to understand the chemical composition of the 'blue straggler' using high resolution spectroscopy, which could reveal more about the evolution of these peculiar celestial objects.

The stars are part of a "cluster" called NGC 188 formed some 6 billion years ago, and are much older than the sun, which is believed to have come into existence nearly 4.5 billion years ago.

"As the sucked up material from the ageing star will be polluted with material processed within the ageing star, this blue straggler will throw light on the kind of nuclear processing that happens in ageing stars", Subramaniam, also the Calibration Scientist of UVIT on board the ASTROSAT, said. 

"The data from six filters of UVIT were used to estimate the precise temperature of the companion star, its size and brightness. This has been possible only due to the excellent capability of the UVIT telescope," Subramaniam added.

The UVIT is made of twin telescopes with an effective diameter of 375mm each and records images in the Far UV, Near UV and visible frequencies. UVIT provides very sharp UV images over a field of view as large as the Moon.

"The companion (the bigger star) is still going through ageing and has not yet become a remnant. It is also hot and large. Therefore it appears very bright in ultraviolet image, but not so bright in an image taken with an optical telescope looking at visible light," she said.

This is why previous studies of the 'blue straggler' in the optical range could not detect the companion. "This pair thus becomes a rare sample to study the details of the formation of blue straggler stars," Subramaniam added.

The purpose of ASTROSAT is to understand high energy processes in binary star systems containing neutron stars and black holes, estimate magnetic fields of neutron stars, study star birth regions and high energy processes in star systems lying beyond our galaxy.

It is also tasked to detect new briefly bright X-ray sources in the sky and perform a limited deep field survey of the universe in the Ultraviolet region.

Apart from the UVIT, the ASTROSAT also includes a Large Area X-ray Proportional Counter (LAXPC), a Soft X-ray Telescope (SXT), a Cadmium Zinc Telluride Imager (CZTI) and a Scanning Sky Monitor (SSM).

These instruments observe the sky for electro-magnetic radiations in the visible, ultraviolet and X-ray frequency ranges coming from distant celestial sources.