Jano: a white dwarf star with two faces
The astronomer Rafael Bachiller reveals to us in this series the most spectacular phenomena of the Cosmos. Pulsating research topics, astronomical adventures and scientific news about the Universe analyzed in depth.
An exceptional white dwarf star has been discovered that presents two faces: the brighter hemisphere is dominated by hydrogen and the other by helium.
stellar corpses
White dwarfs are the corpses that result when small stars, similar in type to our Sun, die.. Indeed, when a solar-type star exhausts its nuclear energy, it collapses under the effect of its own gravity, but the mass is not enough to form a black hole, thus forming a highly compact object, the size of the Earth, but containing a mass comparable to that of the Sun. Of course, our own Sun will also inexorably become a white dwarf; but for this it takes a long time: about 5,000 million years.
The gravity on the surface of these white dwarfs is 100,000 times greater than on the Earth's surface and this causes a segregation in the chemical composition: the heavier atoms (such as iron) sink inwards, while the lighter ones ( such as those of hydrogen and helium) remain in the superficial region.
Normally, hydrogen forms a shallower shell than helium.. But, sometimes, there are convection phenomena that alter this disposition. For example, shortly after the catastrophic collapse that, in the middle of a great glow, gives rise to the white dwarf with a temperature of about 100,000 degrees, the star begins to cool down and the convection in the helium layer causes it to mix with that of hydrogen. Eventually, the helium remains on the surface for a long time as the star continues to get colder and colder.
15 minutes
These ideas have been corroborated by the discovery of a few white dwarf stars with different percentages of helium on their surfaces, possibly because they are in intermediate stages between the transition from hydrogen to helium mode.. But now a new star has just been discovered which is surprising because it seems like a very extreme case.
Ilaria Caiazzo, a young Italian astronomer working at Caltech, California, was part of a group of researchers who, using the Zwicky Transient Facility (ZTF) telescope at the Mount Palomar Observatory near San Diego, California, were looking for peculiar variable white dwarf stars. During this search, the star ZTF J2033 appeared, which is located about 1,300 light-years away in the constellation Cygnus and rotates dizzyingly every 15 minutes.
The most surprising properties of the white dwarf became apparent in follow-up observations that Caiazzo made using several more powerful ground-based and space telescopes than the ZTF: As the star rotated, its brightness varied along with its chemical composition.. When the brighter hemisphere faces the Earth, only hydrogen is detected and when, after half a turn, the other (darker) hemisphere offers us, only helium is detected.
The researchers found no indication of a companion star or the existence of pulsations that could explain such peculiar behavior.. Therefore, they concluded, the variations must be rooted in the stellar surface.. It is as if the star had two different faces: one bright with hydrogen and the other darker with an abundance of helium.. That is why they have nicknamed her Janus, after the Roman divinity that has two faces (the same god that gives its name to the month January).
Jano white dwarf recreation Magnetism
To try to explain the peculiar structure of this star, the authors of the study turn to magnetic fields. And it is known that white dwarfs, being so compact and rapidly rotating, can have very intense magnetic fields, much more intense than those that prevail, for example, in the dark spots on the surface of the Sun.
Suppose there are notable differences in magnetic field strength in the two Jano hemispheres.. In one zone (bright hemisphere), the field could be strong enough to prevent convective movements, leaving helium submerged in the depths, while a weaker field in another zone (dark hemisphere) could freely allow convection. causing the hydrogen to sink under the rising mass of helium.
The stellar surface is now at a temperature of about 35,000 degrees, in full cooling transition, between the hydrogen phase and the helium phase.. As the surface temperature continues to drop, it is expected that convection will eventually win out over magnetic effects and that the star will complete its evolution completely covered in helium.
These hypotheses are perfectly reasonable because they fit well with the properties of other white dwarfs that have been found with irregular areas on their surface.. But none as extreme as Jano was known, capable of changing its appearance and its spectrum in just a quarter of an hour.
Caiazzo's article on the. entitled A rotating white dwarf shows different compositions on its opposite faces has been published just a few days ago in the prestigious journal Nature.
Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.