What’s the mystery of the ‘Vampire Star’ feeding on its companion? Indian astronomers reveal surprising details

Vampire stars, or blue stragglers, don’t follow typical star evolution models and appear much younger than they are. (Image: ISRO)

Astronomers from the Indian Institute of Astrophysics (IIA) have made an exciting discovery about a 'vampire star' in the star cluster M67. Using data from AstroSat’s UltraViolet Imaging Telescope, India’s first space observatory, they uncovered how this star is feeding on its companion.

Overview of M67 and the Vampire Star

M67 is a loose collection of more than 500 stars in the Cancer constellation, called an open cluster. The star, known as WOCS 9005, is a "vampire star" that refreshes itself by pulling material from a nearby companion star. This discovery, soon to be featured in The Astrophysical Journal Letters, reveals intriguing details about how stars transfer mass in binary systems.

Using spectroscopy, a method that disperses a star’s light into its color spectrum, the IIA team studied the surface composition of WOCS 9005. “This star should show chemistry similar to our Sun, but its atmosphere is rich in heavy elements like barium, yttrium, and lanthanum,” said lead author Harshit Pal, who conducted the study as part of his MS thesis at IISER Berhampur.

Mystery of Heavy Elements

These heavy elements are rare and usually found in a class of stars called asymptotic giant branch (AGB) stars. These stars end their lives as white dwarfs after shedding their outer layers enriched with heavy elements. However, AGB stars are generally more massive and evolved than WOCS 9005, presenting a mystery.

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"The presence of heavy elements in the spectrum indicates a polluted atmosphere, with the pollution likely coming from an external source," said Prof. Annapurni Subramaniam, co-author of the paper and Director of IIA. She explained that the external source is likely the star's binary companion, which would have produced heavy elements during its AGB phase before becoming a white dwarf.

UV Brightness and Theoretical Calculations

The team estimated the UV brightness of the vampire star using the UVIT on AstroSat. Despite having a temperature similar to the Sun, the vampire star showed significant UV brightness. This analysis confirmed that the UV light originated from its hot, small companion.

Theoretical calculations validated that this companion is indeed the remnant of a star that produced heavy elements. This discovery sheds light on the fascinating process of mass transfer in binary star systems and the evolution of such unique stars.

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