Astronomers led by Durham University in the UK used NASA's planet-hunting satellite called Transiting Exoplanet Survey Satellite (TESS) to observe a unique phenomenon of a white dwarf abruptly switching on and off for the first time.
According to Phys.org, the team observed the phenomenon on a white dwarf binary system 1,400 light-years away. It has approximately the same size as Earth but almost has the same mass as the Sun. Astronomers said that the white dwarf is accreting or feeding on its companion star.
Accreting White Dwarf TW Pictoris
In the study, titled "An Accreting White Dwarf Displaying Fast Transitional Mode Switching" published in the journal Nature Astronomy, the team wrote that the white dwarf binary system called TW Pictoris consists of a white dwarf feeding on a surrounding accretion disk fuelled by hydrogen and helium from its smaller companion star, making it brighter.
Using NASA's TESS, which is usually used for looking for exoplanets, astronomers saw abrupt falls and rises in brightness for the first time in an accreting white dwarf. The team explained that the flow of material between the white dwarf's accretion disk and companion star is constant and should not affect its luminosity on short timescales.
Researchers believe that this phenomenon is also caused by the reconfiguration of the surface magnetic field of the white dwarf. The white dwarf's brightness is high during its "on" mode as it feeds off the accretion disk. But its brightness plummets as it abruptly switches "off."
The team said that this happens when the magnetic field spins so rapidly that the fuel from the accretion disk is stopped by a centrifugal barrier in a process called magnetic gating.
It means that the spinning magnetic field acts as a gate to the accretion disk that regulates the fuel from passing, resulting in semi-regular small increases in brightness. Then it turns "on" again that increases its brightness and back to its original luminosity.
The team noted that this phenomenon has never been recorded before in any accreting white dwarf. On the other hand, the findings could help astronomers draw comparisons with similar behavior in a neutron star to help understand the process of how an accreting object feeds on the material that surrounds them as well as the significant role of magnetic fields.
White Dwarfs vs. Red Giants
According to National Geographic, smaller stars that are up to eight times the mass of the Sun become white dwarfs at the end of their evolution. White dwarfs are incredibly dense that even a teaspoon of matter from them would weigh 5.5 tons here on Earth. They typically have a radius of 0.1 times the Sun but have a similar mass.
White dwarfs were stars that have burned up all of the hydrogens that they once used as nuclear fuel. The Sun in the Solar System is also fueled by hydrogen in its core.
Meanwhile, red giants are large stars that spread out their heat and have a red-hot core. They only exist for a short time, perhaps only a billion years, compared to the ten billion years that a white dwarf could live.
Red giants are formed as the star condenses and compacts before heating further and burning the last of its hydrogen that causes its surface to expand outward. They are so hot that they turn the helium in their core into heavy elements, such as carbon, by fusing with hydrogen.
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