Astronomers solve the mystery of “impossible” star

A few years ago, scientists discovered a white dwarf orbiting a sun-like star in the KIC 8145411 system. After a series of spectroscopic studies, it turned out that this object is very “light”. In contrast, computer simulations showed that low-mass white dwarfs cannot exist in such systems. Following a new analysis, American astronomers have finally explained this phenomenon.

In 2019, an international team of astronomers announced the discovery of an unusual star system called KIC 8145411, in which a white dwarf orbits a solar-type star every 456 days. KIC 8145411 is a rare example of a binary system in which the gravitational pull of a compact white dwarf slightly increases the brightness of a more powerful companion star relative to the observer. It was this effect that helped researchers calculate the dwarf’s mass.

In their study, astronomers said the mass of the white dwarf was three times smaller than expected from such luminaries: only one-fifth the mass of the Sun (about 0.2 solar masses). Scientists had previously observed such low-mass objects only in systems where two stars were very close to each other. That is, at a distance that would allow the larger companion to drag matter away from its neighbor, thereby accelerating its “evolution”.

A massive star “tears off” the outer layers of a small star, turning it into a low-mass white dwarf. White dwarfs with such a small mass do not arise on their own because they form from larger stars.

In KIC 8145411 the picture is completely different. A white dwarf orbits a Sun-like star at a distance greater than the distance between the Earth and the Sun (one astronomical unit). In other words, it is too far away for the larger star to “peel off” the shell of its smaller neighbor. Modern astronomical models have shown that low-mass white dwarfs cannot exist in systems of the type KIC 8145411.

A group of astronomers from the California Institute of Technology, led by Natsuko Yamaguchi, tried to unravel this phenomenon. The scientists reported their results in a paper published in the arXiv electronic preprint archive.

To study the KIC 8145411 star system, Yamaguchi and his colleagues used telescopes at the Palomar Observatory in San Diego County, California: the 200-inch Hale Telescope, the 48-inch Samuel Odium Telescope.

The researchers found that the KIC 8145411 system is definitely not a binary system, as predicted by colleagues who discovered an “impossible” white dwarf. There is another third star; a solar-type luminary orbiting two stars at a distance of about 700 AU.

Yamaguchi’s team calculated the white dwarf’s mass once again, but did so using the new star’s luminosity. Researchers determined that its mass is quite acceptable for such objects – 0.5-0.6 solar. That is, it turns out that the white dwarf in the KIC 8145411 system is not at all “impossible”, it was made by the mistake of the previous group of scientists.

The discovery of hidden star KIC 8145411 suggests that some binary star systems actually contain a third, “invisible” star. This can cause confusion in the calculations required to determine stellar masses. According to the authors of the scientific study, their colleagues probably fell into a situation where “two bright stars mistakenly perceived one.”