Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), a team of professional astronomers and citizen scientists has discovered a star that oscillates largely over one hemisphere. Named HD 74423, the star has a mass of 1.7 solar masses and is located about 1,601 light-years away from Earth.
An artist’s impression of HD 74423 with its tidally locked red dwarf companion. Image credit: Gabriel Pérez Díaz, IAC.
Stars that pulsate have been known in astronomy for a long time. Our own Sun dances to its own rhythms. These rhythmic pulsations of the stellar surface occur in young and in old stars, and can have long or short periods, a wide range of strengths and different causes.
There is however one thing that all these stars had thus far in common: the oscillations were always visible on all sides of the star.
“What first caught my attention was the fact HD 74423 was a chemically peculiar star. Stars like this are usually fairly rich with metals — but this is metal poor, making it a rare type of hot star,” said Dr. Simon Murphy, an astronomer in the Sydney Institute for Astronomy at the University of Sydney.
“We’ve known theoretically that stars like this should exist since the 1980s,” added Professor Don Kurtz, an astronomer at the University of Central Lancashire and the University of Sydney.
“I’ve been looking for a star like this for nearly 40 years and now we have finally found one.”
HD 74423 (center). Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD / DSS.
The team identified the cause of the unusual single-sided pulsation of HD 74423 — the star is located in a binary star system with a red dwarf. Its close companion distorts the oscillations with its gravitational pull.
The orbital period of the binary system, at less than two days, is so short that the larger star is being distorted into a tear-drop shape by the gravitational pull of the companion.
“The exquisite data from the TESS satellite meant that we could observe variations in brightness due to the gravitational distortion of the star as well as the pulsations,” said Professor Gerald Handler, an astronomer in the Nicolaus Copernicus Astronomical Centre.
To their surprise the researchers observed that the strength of the pulsations depended on the aspect angle under which HD 74423 was observed, and the corresponding orientation of the star within the binary.
This means the pulsation strength varies with the same period as that of the binary.
“As the binary stars orbit each other we see different parts of the pulsating star. Sometimes we see the side that points towards the companion star, and sometimes we see the outer face,” said Dr. David Jones, an astronomer with the Instituto de Astrofisica de Canarias.
The discovery is reported in the journal Nature Astronomy.
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G. Handler et al. Tidally trapped pulsations in a close binary star system discovered by TESS. Nat Astron, published online March 9, 2020; doi: 10.1038/s41550-020-1035-1
