These Bengaluru astronomers discover a new kind of stellar chemistry

Bengaluru: While India was focused on the International Space Station and a Gaganyaatri code-named Shux, researchers from Bengaluru-based Indian Institute of Astrophysics (IIA) have quietly uncovered a cosmic twist.

That, in the far away Ophiuchus Constellation, a peculiar star named A980, some 25,800 light years away, is rewriting what we know about stellar chemistry.

Gajendra Pandey, Senior Professor at IIA, an autonomous institute of the Department of Science and Technology (DST), said, it all began when his PhD student, Ajay Saini proposed to study 27 newly identified hydrogen-deficient carbon stars in high resolution spectrum for his thesis work.

According to Pandey, a hydrogen-deficient carbon star is a peculiar breed of cool star that lacks hydrogen -- the most common element in the universe.

“Until 2022, only five stars were known to be hydrogen deficient. But in 2022, 27 stars, including A980, were identified. The problem is, it was done in a low-resolution spectrum. So, we decided to look at them in high resolution as well as study their chemical composition,” said Pandey to PTI.

A closer look by Saini and Pandey, using the Hanle Echelle Spectrograph on the Himalayan Chandra Telescope in Ladakh, showed something strange.

A980’s spectrum -- essentially a stellar fingerprint -- did not match what they expected.

Instead, this mysterious star that belongs to a rare class called Extreme Helium (EHe) stars, carries a surprisingly high amount of germanium -- a metallic element never before observed in this type of star.

EHe stars, said Pandey, are almost entirely made of helium, and they are likely formed when two white dwarf stars --one carbon-oxygen rich and the other helium-rich --merged in a spectacular cosmic collision.” According to Pandey, they were surprised to see singly-ionized germanium (Ge II) lines in the A980’s optical spectrum.

“These are the first-ever detections of germanium lines in an EHe’s observed spectrum,” explained Pandey.

More surprises awaited the duo. They found that germanium is eight times more abundant in A980 than in the Sun, which is evidence for the synthesis of germanium in EHe stars.

“Germanium has never been detected in these stars before, and here it was -- eight times more abundant than in the Sun,” said Saini.

The discovery is important, as every new clue helps astronomers’ piece together the cosmic story of how matter came to be, added the researcher duo.

The detection of germanium, for instance, means that there are other plausible scenarios than the long-held theory that EHe stars originated from the merger of a carbon-oxygen white dwarf with a less massive helium white dwarf, said Pandey.

Pandey and Saini suspect that A980’s unusual chemistry could trace back to a phase in stellar evolution called the Asymptotic Giant Branch (AGB) -- a stage when stars puff up and start producing heavy elements like barium, strontium, and germanium. These stars eventually shed their outer layers, and their cores become white dwarfs.

And then there are Thorne-Żytkow Objects (TŻO) -- theoretical hybrid stars that have a neutron star at their core.

"They are also known to produce lots of germanium, using a different method called the rp-process (rapid proton capture). While A980 does not fully match the expected properties of a TŻO, the similarities are intriguing,” added Pandey.

The discovery, said Pandey, pushes the boundaries of what we thought we knew about stellar chemistry.

“It also shows how powerful high-resolution spectroscopy can be used in uncovering the hidden stories written in starlight,” he added.

A980, said Saini, has given scientists a new puzzle to solve.

“One that might deepen our understanding of how rare stars evolve and what happens when white dwarfs collide,” he added.

The research has been published in The Astrophysical Journal.