Researchers named this celestial body Janus - after the two-faced god from ancient Roman mythology - because of its unique parameters.
Astronomical observations of highly magnetized white dwarfs more than 1,000 light-years away from Earth have revealed an object with unique processes. One side of the white dwarf's atmosphere consists almost entirely of hydrogen, while the other side consists almost entirely of helium. For the first time in the history of astronomy, scientists have observed a white dwarf whose outer layers differ dramatically in chemical composition. The article is published in the journal Nature.
White dwarfs - compact bodies, whose mass is comparable to the mass of the Sun, and the radius - with the radius of the Earth (6371 kilometers). They are formed in the final stages of the evolution of stars whose mass does not exceed 8-10 solar masses.
Over time, these relatively light luminaries "age" and turn into red giants. Gradually in the interior of red giants runs out of fuel for thermonuclear reactions and the outer layers begin to expand, and the core - to shrink. Eventually, the outer layers of the star become a planetary nebula and the core itself becomes a white dwarf.
In fact, these are no longer stars, because in their depths do not go thermonuclear reactions. White dwarfs cool down very slowly, although they have no permanent internal heat sources. These bodies are small, which means they emit relatively little energy.
Mostly white dwarfs consist of carbon, hydrogen, oxygen, helium, magnesium, neon. In some cases (depends on the mass), heavy elements are found in their composition, but in small amounts. The lightest chemical elements, hydrogen and helium, rise, replenishing the outer layers, the atmosphere. The heavier ones sink to the core (which includes O, Ne, and Mg) under the influence of strong gravity.
When white dwarfs cool down, the temperature drops to 30 thousand kelvin, the layers of the atmosphere begin to expand, the layer of hydrogen and helium are mixed with each other, so the chemical composition of the atmosphere of these bodies is not homogeneous. At least, so thought until recently, until American astronomers from the California Institute of Technology did not discover a white dwarf, which completely changes the idea of the atmosphere of these objects.
During the observation of a group of strongly magnetized white dwarfs, which were conducted by astronomers with the help of the Zwicky Transient Facility, connected to the Samuel Oschin Telescope (Samuel Oschin Telescope) at the Palomar Observatory, scientists came across atypical behavior of one of the objects. The white dwarf, which was labeled ZTF J203349.8+322901.1, stood out from the others with a rapid change in brightness.
Scientists requested additional observations of the space body, they were interested in the chemical composition of the atmosphere of ZTF J203349.8+322901.1. High-resolution spectrometer in the observatory Keck showed that the outer layers of the object are not mixed: on one of its sides registered hydrogen, but no helium, on the other - helium without hydrogen, which is atypical for this class of bodies.
The white dwarf was named in honor of the two-faced Roman god Janus. Researchers can not yet explain the phenomenon of its atmosphere, but they have several hypotheses. One of them says that the reason why gases from different hemispheres of the white dwarf have not mixed is because of magnetic fields.
"Magnetic fields around cosmic bodies tend to be asymmetric or may be stronger on one side of the atmosphere. If the magnetic field is stronger on one side, it can interfere with the mixing of gases, consequently, in this part there will be an uneven distribution of them, a layer with the lightest element, hydrogen, will prevail," explained Ilaria Caiazzo, study leader and researcher at the California Institute of Technology.
To solve the mystery of the "two-faced" Janus, a team of American scientists need to study as many similar white dwarfs. Looking for them will be using the same Zwicky Transient Facility.
Janus may be the "brightest" representative of strongly magnetized white dwarfs. Scientists believe that such objects will help in the future to better understand the mechanisms of evolution of such bodies.