New type of pulsating star discovered

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Astronomers, including a UCLan professor, find single-sided pulsator; citizen scientists provide the decisive clue

Stars that pulsate have been known in astronomy for a long time. The 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. Now an international team, including researchers from the University of Central Lancashire (UCLan), has for the first time discovered a star that oscillates largely over one hemisphere.

The scientists have already identified the cause of the unusual single-sided pulsation: the star is located in a binary star system; its close companion distorts the oscillations with its gravitational pull, as the scientists write in the latest issue of Nature Astronomy.

The clue that led to its discovery came from citizen scientists.

The orbital period of the binary, at less than two days, is so short that the two stars in the system are being distorted into a tear-drop shape by the gravitational pull of the companion. 

Professor Gerald Handler, researcher at the Nicolaus Copernicus Astronomical Center (Poland) and lead author of the paper, explains: “The exquisite data from the TESS satellite meant that we could observe variations in brightness due to both the gravitational distortion of the star as well as the pulsations.”

To their surprise the team observed that the strength of the pulsations depended very strongly on the aspect angle under which the star was observed, and the corresponding orientation of the star within the binary. Thus, the pulsation strength varies with the same period as that of the binary.

The exquisite data from the TESS satellite meant that we could observe variations in brightness due to both the gravitational distortion of the star as well as the pulsations.

An animation of a star that oscillates largely over one hemisphere.

Dr David Jones, researcher at the Instituto de Astrofisica de Canarias and co-author of the study, said: “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.”

This is how the astronomers could be certain that the pulsations were only found on one side of the star, with the tiny fluctuations in brightness always appearing in their observations when the same hemisphere of the star was pointed towards the telescope.

The idea that stars whose pulsations are affected by a close companion must exist was theoretically predicted already in the 1940s. Also, the notion that the pulsation axis of a star can be moved through tidal forces, was conjectured for over 30 years. The proof via observational data was however missing until now.

UCLan researcher Professor Don Kurtz, co-author of the study, commented: “Since the 1980s, we've believed that systems like this could exist, but it is only now that we have finally found one!”

The initial discovery of the unusual behaviour of the star was actually made by citizen scientists. These amateur astronomy sleuths painstakingly inspect the enormous amounts of data that TESS regularly supplies, as they search for new and interesting phenomena.

They then alert their professional astronomer colleagues in case they notice something unusual. While this is the first such star to be found where only one side is pulsating, the authors believe that there must be more such stars. 

Professor Saul Rappaport, researcher at the Massachusetts Institute of Technology (USA), co-author of the study and contact person of the successful citizen scientists, added: “Beyond its pulsations, there doesn't seem to be anything special about this system, so we expect to find many more hidden in the TESS data!”

Rachel Atkinson | 09 March 2020