Star only pulsates with half


The star HD74423 is deformed and shows brightness fluctuations on only one side. (Image: Gabriel Pérez Díaz / IAC)

Astronomers know many stars that change their brightness in a regular rhythm. But now researchers have discovered a very unusual example of these variable stars: the star HD74423 only pulsates half. While one hemisphere remains calm, the other turns alternately lighter and darker. The reason for this is a close companion – a red dwarf, whose attraction not only deforms the star in an ovoid shape, but also causes the half-sided changes in brightness. As the astronomers report, HD74423 is the first known representative of a whole new class of variable stars.

Our sun shines – apart from short-term bursts of radiation – with a relatively constant brightness. But there are stars that change their brightness more or less regularly due to external influences or internal processes. One of the reasons for this can be the tidal forces caused by a close companion, who deforms the star depending on its position in orbit and “kneads” it, thus changing its brightness. However, internal processes such as regular oscillations of the radius or the temperature can also cause such changes in luminosity. But all the variable stars known so far had one thing in common: the fluctuations in their luminosity always encompass the entire star and are visible from all sides.

Unusual light curve

However, astronomers around Gerald Handler from the Nicolaus Copernicus Center for Astronomy in Warsaw have discovered a star that contradicts this common pattern. The object baptized HD74423 is around 1,500 light-years from Earth and is around 1.7 times more massive than our sun. The star is one of many that NASA’s TESS (Transiting Exoplanet Survey Satellite) observes in search of exoplanets. It registers light fluctuations from around 200,000 sun-like stars. When volunteers scoured the telescope’s data as part of a Citizen Science program, they came across HD74423 and found that its light curve looked unusual. So you get in touch with the astronomers. “The light curve shows minima with changing depth, while the maxima do not change,” the researchers describe the phenomenon. “The amplitude of the minima changes by a factor of ten.”

It is known that the star HD74423 has a red dwarf as a close companion. Both orbit each other in less than two days. “However, a covering of the pulsating star by its companion cannot explain this modulation, because it would also have to generate eclipses of at least 0.7 magnitude deep – which, however, cannot be seen,” said the researchers. Closer analyzes showed that the attraction of the red dwarf at this short distance is sufficient to deform the star slightly elliptically. Just as the tidal forces of the moon tend to pull the oceans and even the earth’s crust towards you, the HD74423 gas envelope bulges towards the red dwarf. “The exquisite data from TESS allowed us to observe the variations in brightness due to this gravitational deformation as well as the pulsations of the star,” explains Handler.

Brightness only fluctuates on one side

But the deformation of the star alone could not explain the strange fluctuation pattern of star brightness. But another observation: The phase pattern and the frequency of the fluctuations indicated that the intensity of the pulsation depends on which side the star was facing the telescope. “As the two components of the binary star orbit, we see different parts of the variable star,” explains co-author David Jones from the Canary Islands Astrophysical Institute. “Sometimes we see the side that faces the companion and sometimes we see the outer side.” And the rhythm of this change of perspective also fluctuates the brightness of the star. From this, the astronomers conclude that the brightness of HD74423 apparently does not pulsate across its entire surface, but only on a hemisphere.

“We have known since the 1980s that there could theoretically be such stars,” says co-author Don Kurtz of the University of Central Lancashire. But so far, astronomers have never been able to find a representative of such half-sided pulsating stars – until now. HD74423 is the first representative of a completely new class of mutually variable stars. “The pulsation mode of HD74423 is still unique, but there must be a whole class of such stars with which the axis of the brightness fluctuations is aligned with the axis of the tidal forces,” the astronomers state. “This discovery is therefore the motivation to look for more stars of this kind.” At the same time, there is still an open question to answer: “It is still unclear whether the brightness fluctuations lie on the side facing the companion or on the side facing away from him” the explorers. This should also clarify future observations.

Source: Gerald Handler (Nicolaus Copernicus Astronomical Center, Warsaw) et al., Nature Astronomy, doi: 10.1038 / s41550-020-1035-1

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