Astronomers Have Spotted An "Impossible" White Dwarf

Artist impression of a white dwarf gravitationally lensing its companion's light. NASA/JPL-Caltech

All of our science is limited to a degree, so it often happens that we encounter "impossible" objects and events that go beyond what we know about the universe. KIC 8145411 is one of these. The discovery of the tiny white dwarf that cannot be easily explained is reported in The Astrophysical Journal Letters.

A white dwarf is the final stage of the lifecycle of medium and small stars. Red giants, dying stars that have burned all their hydrogen fuel, sometimes expel their outer layers, leaving behind a dead dense core that we call a white dwarf. Ninety-seven percent of stars in the Milky Way are destined to become one.

Typical white dwarfs are around 60 percent of the mass of the Sun. KIC 8145411 is just 20 percent. This is not in itself unusual, we have seen smaller. But for such a small white dwarf to form, it would have to have had a very small progenitor star. The problem with this is small stars are very long-lived and take much longer to evolve. The original star would have to be older than the universe to have run out of fuel and blown out its material to end up with such a white dwarf.

There is another way for white dwarfs to lose their layers and become dense cores. If they have a companion, it might steal the layers and expose the core more quickly. KIC 8145411 does have a companion, but this explanation doesn’t work for this scenario. KIC 8145411 is 195 million kilometers (119 million miles) from its Sun-sized companion, 10 times too far for effective mass transfer between the stars.

The researchers have proposed several explanations for the evolution of this white dwarf. There could have been a third star that stole the mass and then was ejected by the system. There might have been a large planet or a brown dwarf (objects too large to be planets but too small to be stars) that siphoned the gas layers, before being destroyed by the companion star. Or the white dwarf had some special properties that allowed it to stretch its envelope all the way to its companion. The team points out, however, that all these explanations are unsatisfactory.

So while we still may not know exactly how a system like this forms, astronomers expect to have a better idea when more low-mass white dwarfs like this are found. The system was discovered in data from Kepler, the now-defunct planet-hunting satellite, which astronomers are still going through. It is a self-lensing system, meaning as the white dwarf passes in front of the companion, it gravitationally bends the light. It's one of only five known self-lensing binary systems. 

There was only a 1 in 200 chance for such a system to have the right geometry to be detected by Kepler, so the researchers are confident that with newer telescopes, there are more white dwarfs like this to be discovered out there.  



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