The mystery of vampire star rejuvenation solved

A team of astronomers from the Indian Institute of Astrophysics (IIA), an autonomous institute of the Department of Science & Technology, Government of India, discovered a vampire star in M67. This discovery offers significant clues on a complex rejuvenation process known as mass transfer in a binary system.

Vampire stars, also called blue straggler stars (BSS), can easily be found in star clusters. They show several characteristics of younger stars and defy simple models of stellar evolution.

This unusual young star is thought to be rejuvenated by taking material from a nearby binary star. Star clusters are good places to study this because they have many binary stars, some of which form vampire stars.

These rejuvenated stars evolve differently from single stars like the Sun. However, finding the material they’ve absorbed and spotting their companion star has been challenging.

Star clusters are ideal laboratories for understanding the life and death of single and binary stars. M67, in the Cancer constellation, is one such interesting star cluster.

This newly discovered vampire star- called WOCS 9005- 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 team discovered this using the UltraViolet Imaging Telescope data onboard AstroSat, India’s first dedicated space observatory. Using spectroscopy, they studied the vampire star’s surface composition.

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), which used the Two-Degree Field fibre positioner with the HERMES spectrograph at the Anglo-Australian Telescope.

Harshit Pal, the paper’s lead author, said, “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.”

These heavy elements are rare and are mostly found in stars called asymptotic giant branch (AGB) stars. In these stars, a slow process called s-process uses neutrons to create heavy elements from lighter ones. This process makes about half of the atomic nuclei heavier than iron. Before AGB stars die and become white dwarfs, they shed their outer layers rich in these heavy elements. However, AGB stars are more massive and evolved than WOCS 9005, which is puzzling.

Prof. Annapurni Subramaniam, co-author of the paper and Director IIA, said, “The presence of heavy elements in the spectrum pointed to a polluted atmosphere of the vampire star and an external source of pollution. The external source is likely to be its binary companion, which must have made the heavy elements when it passed through its AGB phase and later became a white dwarf star.”

“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. 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.”

Dr. Bala Sudhakara Reddy, a co-author of the paper, said, “The presence of significant barium in this vampire star makes it the first barium blue straggler star discovered in the cluster M67. 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.”

The vampire star was known to have a tiny, unseen companion that is half the mass of the Sun. Scientists used the Ultra-Violet Imaging Telescope (UVIT) on the AstroSat satellite to take pictures of the vampire star and measure its UV brightness.

As its temperature is similar to the Sun’s, it was unexpectedly bright in the UV. Scientists detected considerable UV brightness for this star, which, on analysis, proved that it indeed originated from its hot and small companion.

Based on theoretical calculations and validations, scientists confirmed 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.”

Journal Reference:

1. Harshit Pal, Annapurni Subramaniam, Arumalla B. S. Reddy and Vikrant Jadhav. Discovery of a Barium Blue Straggler Star in M67 and “Sighting” of Its White Dwarf Companion. The Astrophysical Journal Letters. DOI 10.3847/2041-8213/ad6316