Astronomers at the Indian Institute of Astrophysics have long been chasing the formation path of vampire stars that are thought to rejuvenate their youth by sucking up material from their companion. They have now discovered a vampire star that bears the chemical imprint of recently sucked barium-rich material from its binary companion and unambiguously detected emission from the dead-remnant of its companion. The key to this detection was data from the UltraViolet Imaging Telescope, on board AstroSat, India’s first dedicated space observatory. This finding is an important missing link in the rejuvenation of these stars.
Vampire stars, known to astronomers as blue straggler stars (BSS), are identified easily in star clusters. These stars defy simple models of stellar evolution and show many characteristics of younger stars. This anomalous youth is explained theoretically as due to rejuvenation by eating up material from a binary stellar companion. Star clusters are useful test-beds to test this theory as they host a large number of binary stars, some of which can lead to the formation of vampire stars. Once rejuvenated, these stars follow a different path of evolution when compared to Sun-like single stars. So far, the detection of sucked-up material along with the sighting of their remnant binary companion was elusive.
Star clusters, being born from the same molecular cloud, can contain hundreds to thousands of stars with a wide range of masses but all contain very similar surface chemistry, making them ideal laboratories to understand how single and binary stars live and die. One such intriguing star cluster is M67, located in the constellation Cancer. Recently, a team of astronomers from the Indian Institute of Astrophysics (IIA), an autonomous institute of the Department of Science & Technology, Government of India made a ground-breaking discovery of a vampire star in M67, that sheds light on a complex rejuvenation process, known as mass-transfer in a binary system. The research paper, to be published in The Astrophysical Journal Letters, provides rare insights into the binary star evolution process.
The scientists studied the surface composition of the vampire star in M67, called WOCS 9005, using spectroscopy, a technique where the light of the star is dispersed into its colors like the rainbow. The spectra of stars are bar codes that decipher their surface/atmosphere chemistry. The team used the archival spectral data from the GALAH survey (GALactic Archeology using Hermes) that uses the Two-Degree Field fibre positioner with the HERMES spectrograph at the Anglo-Australian Telescope. “This star is expected to show chemistry very similar to our Sun, but we found that its atmosphere is rich in heavy elements such as barium, yttrium, and lanthanum”, said Harshit Pal, the lead author of the paper. Harshit Pal is a former BS-MS student of IISER Berhampur and he carried out this work as a part of his MS thesis project.
These heavy elements are rare and are found in a class of stars called ‘asymptotic giant branch (AGB) stars’, where abundant neutrons for a slowly occurring neutron capture process (s-process) are available to produce these heavy elements from lighter ones. This process is responsible for creating approximately half the atomic nuclei heavier than Iron. However, these AGB stars shed their outer layers enriched with heavy elements into their surroundings before ending their lives as white dwarfs (WDs). However, these AGB stars are more massive and evolved than WOCS 9005, leading to a puzzle. “The presence of heavy elements in the spectrum pointed to a polluted atmosphere of the vampire star and the source of pollution being an external source. The external source is likely to be its binary companion, which must made the heavy elements when it passed through its AGB phase, and later became a white dwarf star”, said Prof. Annapurni Subramaniam, co-author of the paper and Director IIA. “The blue straggler star that we see now must have eaten up most of this barium-rich material due to its gravitational pull, and is now presenting itself as a rejuvenated star”, she added. When the enhancement of barium (and other s-process elements) is detected in stars earlier than the AGB evolutionary phase, such as the main sequence (MS), subgiant (SG), or red giant branch (RGB), these stars are called barium stars. “The presence of significant barium in this vampire star makes it the first barium blue straggler star discovered in the cluster M67”, said Dr. Bala Sudhakara Reddy, a co-author of the paper. The mass transfer from a companion AGB star has been extensively studied, though only a few chemically enriched post-mass transfer binaries have been identified in star clusters. Having established that the mass transfer took place, the team started their search for the unseen companion.
This vampire star was already known to have a very small unseen companion of half the mass of the Sun. White dwarf stars are hot and small and are bright in the ultraviolet, but very faint in the visible range of the electromagnetic spectrum. The team made use of the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat, India’s first multi-wavelength satellite launched in 2015, for their study. Using the UVIT on AstroSat, the scientists took images of the vampire star and estimated its UV brightness. As the vampire star has a temperature similar to the Sun, it is not expected to be bright in the UV. “Instead, we detected considerable UV brightness for this star, which on analysis proved that it indeed originated from its hot and small companion”, said Dr. Jadhav, who is a co-author and a Humbolt fellow at University of Bonn, Germany. The scientists then theoretically calculated and validated that this is indeed the remnant of the star that produced heavy elements and that the two stars are close enough to transfer the matter from the donor star through the wind.
“This is for the first time the white dwarf remnant of the donor is sighted in the case of the polluted blue straggler star”, said Pal. This discovery experimentally confirms the theoretical prediction that vampire stars are formed by acquiring polluted matter through transfer from their companion, leaving behind a remnant white dwarf. The rarity of such chemically polluted systems is still a mystery and the team thinks that it may be due to the quick settling of the pollutants in the atmosphere of the vampire stars.
