Researchers from the Indian Institute of Astrophysics (IIA), part of the Department of Science and Technology, have made a significant discovery involving a "vampire star" in the star cluster M67. This finding, based on data from the UltraViolet Imaging Telescope aboard AstroSat, India's premier space observatory, has revealed new insights into the star's rejuvenation process.
M67, an open cluster consisting of over 500 loosely bound stars, contains the vampire star known as WOCS 9005, located in the constellation Cancer. This star has been rejuvenating by siphoning material from a companion star.
The upcoming publication in The Astrophysical Journal Letters explores the complex mass transfer process in this binary system. By employing spectroscopy to analyze the star's light, the IIA team examined WOCS 9005's surface composition.
Lead author Harshit Pal, from IIA and IISER Berhampur, noted that while the star's chemistry was expected to be similar to the Sun's, it was found to be rich in heavy elements like barium, yttrium, and lanthanum. These elements, typically found in asymptotic giant branch (AGB) stars, are produced through a neutron capture process. AGB stars usually end as white dwarfs, shedding their outer layers enriched with heavy elements.
The presence of these heavy elements in WOCS 9005, however, posed a puzzle since AGB stars are more evolved and massive. Co-author Prof. Annapurni Subramaniam, Director of IIA, suggested that the heavy elements likely came from the star's binary companion, which went through an AGB phase before becoming a white dwarf.
Using UV imaging from AstroSat, the team found that WOCS 9005 exhibited unexpected UV brightness, indicating the presence of material from its hot, small companion. This analysis confirmed that the heavy elements originated from the companion star.