Astroboffin Kurtz ends 40-year quest to find a predicted one-sided vibrating star that was never seen – until now

I love the smell of plasma in the morning. It smells like ...victory

tidally_locked_binary
Artist's impression of the strange binary star system containing HD74423 (left) and its red dwarf star companion (right). Image credit: Gabriel Pérez Díaz (IAC)

Astronomers have discovered for the first time a bizarre star floating in space some 1,500 light years from Earth that seems to only pulsate on one side.

Stars, being giant balls of plasma, are constantly in vibrating motion, with the layers of ionised matter expanding and contracting rhythmically under gravity. At first glance, the object known as HD74423 doesn’t appear too different from the Sun, but something odd is going on.

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Hiding on the other side of the star is a red dwarf. The two stars circle each other in a tidally locked orbit: in other words, one hemisphere of HD74423 constantly faces one side of the red dwarf star.

The red dwarf star exerts a stronger gravitational pull on one side of HD74423 and distorts the star’s oscillations. Although the researchers know the binary system is tidally locked with one another, they’re not quite sure what is dampening HD74423’s pulsations.

"The pulsations get trapped in that tidal bulge, and we’re working on the details of why that happens," Simon Murphy, co-author of the study published in Nature Astronomy and an astronomer at the Australian National University, told The Register.

"We’re just observing this phenomenon for the first time — Jim Fuller, a young professor at Caltech, is working on the theory and developing a mathematical description of what we’re seeing."

Stars that pulsate asymmetrically like HD74423 have been theorised since the 1980’s, but they were never discovered until now. “I've been looking for a star like this for nearly 40 years and now we have finally found one," said Donald Kurtz, an astronomy professor at the University of Central Lancashire in the UK.

Kurtz and his colleagues can’t take all the credit, however. The odd binary system was originally flagged by amateur scientists digging through public data recorded by NASA’s Transiting Exoplanet Survey Satellite (TESS).

In this case, instead of an exoplanet as a neighbouring object to the star HD74423, it’s a red dwarf star. The older companion star lies so close to HD74423 that it completes an orbit once every 1.6 days.

The strength of the HD74423’s vibrations depends on what angle the astronomers are looking at the star. "As the binary stars orbit each other we see different parts of the pulsating star," said David Jones, co-author of the study, an astronomer at the Instituto de Astrofisica de Canarias, the Canary Islands of Spain. "Sometimes we see the side that points towards the companion star, and sometimes we see the outer face."

The researchers credited TESS for being able to finally spot a star that only pulsates on one side. "You need sensitive telescopes to detect these systems, and you need to observe a lot of systems to find one. We suspect that the conditions need to be ‘just right’ for this phenomenon to occur — the stars need to be the right distance apart, and the hotter star needs to be within a narrow range of masses — but we’re still working on the theory," Murphy told El Reg.

"The breakthrough is occurring now because of TESS. TESS is collecting data for more than 200,000 stars, and is in the process of observing most of the night sky during its two-year mission. So we have better data for more stars than we’ve ever had before. We therefore expect to find more of these systems in future TESS data, even if they are rare." ®

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