At the end of its life, a star as light as the SunWhite dwarf” is a compact object. A white dwarf is a celestial body with a very high density and strong surface gravity, with a diameter as small as that of the Earth, but a mass similar to that of the Sun. Due to the strength of this gravitational force, elementary separation occurs strongly on the surface of white dwarfs.
While a white dwarf’s body contains oxygen and carbon produced by nuclear fusion reactions when it was a star, it is thought to have an atmosphere made of hydrogen and helium on its surface. Light elements such as hydrogen and helium can exist in the outermost layer of a white dwarf, while heavier elements are attracted by the strong gravitational force and enter the white dwarf. Therefore, in principle, only hydrogen and helium should be detected when observing the atmosphere of a white dwarf.
In reality, however, heavy elements other than hydrogen and helium have been found in the atmospheres of hot white dwarfs. Notably, dozens of elements heavier than element 28, iron, have been discovered, and since germanium was discovered in three white dwarfs in 2005, new element discoveries have been reported one after another.
A research team led by Pierre Chayer at the Space Telescope Science Institute (STScI) has discovered a white dwarf with a surface temperature of around 50,000 degrees Celsius.HD149499B” was observed. As a result, the 55th element “cesiumit was successfully detected from the atmosphere of a white dwarf for the first time. Its abundance has been measured in “-5.45” relative to helium. This indicates that cesium exists at a rate of approximately 1/10,000th that of helium.
[▲ Figura: La presenza di elementi pesanti nelle nane bianche è di per sé un mistero, ma si ritiene che siano stati forniti da collisioni con pianeti rocciosi in orbita attorno alle nane bianche. Tuttavia, la forte gravità di una nana bianca dovrebbe esistere in superficie solo per un breve periodo di tempo, e il motivo per cui esiste da molto tempo è avvolto nel mistero. (Credito immagine: CfA/Mark A. Garlick)]
Previous observations show that the amount of elements heavier than iron detected in white dwarf atmospheres is excessive compared to stellar atmospheres. For example, the white dwarf ‘Feige 86’, where germanium has been detected from the atmosphere, contains 3 to 10,000 times more gold and platinum than the sun. This time we also observed an excessive abundance of cesium,Excessive heavy elements in the atmosphere of white dwarfs are commonI can’t help but think.
On the other hand, the white dwarf’s strong gravity causes the heavy elements from the atmosphere to sink into the main body in a short period of time, which should hide the existence of the elements in the spectral analysis. While the sources of the heavy elements themselves could be rocky planets (or their remnants) that have recently collided with white dwarfs, it seems likely that all of these white dwarfs have had very recent collisions. Therefore, for heavy elements to exist in a white dwarf’s atmosphere, the force to “levitate” the heavy elements against the white dwarf’s strong gravity would have to be required.
Currently, the leading candidate for why heavy elements are found in white dwarfs is “radial levitation (Radiative levitation). Atoms ionize in hot environments, such as in white dwarfs, and are more likely to absorb photons (subatomic particles that mediate electromagnetic forces) than when unionized (neutral). Part of the energy of the absorbed photons is converted into the kinetic energy of the atoms, which “kicks” them and allows them to exist in the atmosphere. This effect tends to be stronger for heavier elements. Also, since the photon-absorption effect due to radiative levitation is stronger at shorter wavelengths, it is consistent with what is seen in hot white dwarfs that emit many photons with short wavelengths.
While radiative levitation has been a long-standing candidate, there is still no definitive evidence or observation to support it. When radiative levitation actually occurs, extreme ultraviolet and X-rays are absorbed, so if observations are made at those wavelengths, it should appear significantly darker than the expected radiation level. Future observations will demonstrate this.
Source
Text: Riri Ayae
