A new type of star has been discovered

Gabriel Pérez Díaz (IAC)

The universe continues to surprise us. For the first time, an international team of astronomers has discovered a star that pulsates on one side. Although it had certainly been predicted decades ago, it had not been seen until now. The star is about 1,500 light-years away from Earth and has approximately 1.7 times the mass of our Sun.

"Theoretically we have known that stars like this should exist since the 1980s," said astronomer Don Kurtz of Lancashire Central University (UK). "I’ve been looking for a star like this for almost 40 years and now we’ve finally found one.”

Named HD74423, it is a primary sequence star of type A younger than our Sun.

A pulsating star shaped like a tear

Their bizarre behaviour was first observed by citizen scientists who studied in detail the data from NASA’s TESS Space Telescope hunting exoplanets, looking for anomalies. As they were not quite sure what they had found, they turned to professional astronomers.

Is it unusual?

Not really. Many stars have large and small pulsations, caused by waves bouncing off the star. These waves are believed to be created by convection and the magnetic field of the star. The peculiar thing in this case is that the star only presses on one side.

"What struck me first was the fact that it was a chemically strange star," explained Simon Murphy of the Sydney Institute of Astronomy at the University of Sydney and co-author of the work published by the magazine Nature. “Stars like this are usually quite rich in metals, but this one is metal-poor, so it’s a rare type of hot star.”

Low metallicity is one of the defining characteristics of older stars but HD74423 is different. It is a type of chemically peculiar object known as the Lambda Boötis star, and its low metallicity is believed to be the result of the star extracting depleted metal gas from its nearest environment.

What’s causing it to happen?

These tidal pulsations observed in a single hemisphere are due to the presence of a stellar companion close to HD74423: a red dwarf that orbits the star every 39 hours (1.6 days) and is so close to it that its gravity has changed the shape of the star from a sphere to a tear, trapping the pulsations in a single half of the star.

"Pulsation mode in HD74423 is currently unique, but there must be a type of star that have their pulse axes aligned with their tidal axes, and this discovery is an impulse to look for more," the authors wrote.

"We hope to find many more asymmetric stars hidden in TESS data," added Saul Rappaport of the Massachusetts Institute of Technology. As always, better technology will help us bring to light everything that so far remains hidden in plain sight.

Reference: Tidally trapped pulsations in a close binary star system discovered by TESS, Nature Astronomy (2020). DOI: 10.1038/s41550-020-1035-1 , https://nature.com/articles/s41550-020-1035-1

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