Indian astronomers from the Indian Institute of Astrophysics (IIA) and Gauhati University, along with international collaborators, have reported the rare discovery of an actively forming Blue Straggler Star (BSS) in binary system TIC 327546480 located in the old open star cluster Collinder 261 approximately 9,500 light-years away. The discovery provides direct observational evidence for how these peculiar stars, which appear hotter, brighter, and more massive than their peers, are created through mass transfer from companion stars.
The study combined high-precision light curves from NASA's TESS space mission with radial velocity measurements from the European Southern Observatory's Very Large Telescope in Chile. Researchers found the binary system to be semidetached, with one star overflowing its gravitational boundary (Roche lobe) and transferring matter to the BSS. The system shows an orbital period of 2.11 days, a low mass ratio of approximately 0.19, and the BSS rotates rapidly at about 69.55 km/s.
Detailed analysis revealed the BSS has a mass of about 1.67 times that of the Sun and is actively accreting matter from a companion with a mass of about 0.32 solar mass. Evolutionary modeling suggests mass transfer began around 5.46 billion years ago and continues to the present day, while the cluster itself is estimated to be about 7 billion years old. The researchers also detected strong X-ray emission from the system, providing independent evidence that mass transfer is currently ongoing.
This discovery offers one of the strongest pieces of direct observational evidence for the formation of BSSs through mass transfer in binary systems, helping resolve a decades-old mystery in stellar astrophysics. The system serves as a unique laboratory for testing theories of binary evolution and stellar rejuvenation. The research was published in the Astrophysical Journal Letters and involved collaborators from INAF Italy, IUCAA Pune, Observatoire de Paris France, and University of North Florida USA.
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