A white dwarf could use its magnetic field to cannibalize

A white dwarf could use its magnetic field to cannibalize the remains of an exoplanet

Stars less than 8 solar masses are not destined to die as SNSN-II supernova after the gravitational collapse of their cores once the thermonuclear fuel is exhausted and releases the photon explosion. This is necessary to balance and counteract the pressure of a star's gravity. The theory of stellar structure and evolution, already well developed in the 1930s to 1950s, tells us that these stars, and this will be the case with our SunSun, will become white dwarfs (but they could give SN Ia if they would be part of it). a binary system binary system). In a volume about the size of the Earth there is then a mass that is approximately equal to that of our sun.

We also know that stars with masses comparable to the Sun go through the red giant stage before turning into white dwarfs. They will swell and eject some of their mass through violent stellar winds, meaning that after this stage, generally only white dwarfs will remain, weighing 0.5 to 0.7 times the mass of the Sun. . If our sun becomes a red giant, it will span at least the orbits of Mercury, Mercury and VenusVenus, and perhaps Earth's as well.

A quantum diamond core that creates a magnetic field

All this is known, but since we have found that the formation of a procession of planets around a star seems to be the rule, we can ask ourselves whether we can not use white dwarfs to obtain new information about the fate of planets and in particular preservation of the solar systemsolar system when the sun itself has become a white dwarf, the interior of which cools and crystallized matter is formed, more precisely what we call a Wigner crystal (in a sense a quantum avatar of the diamond).

There is reason to believe that the |0dced86d4e3fedcadee38cb27015cc74|-crystallized oxygen core in a white dwarf can generate the magnetic field in the same way that the Earth's molten iron core generates its magnetic field, that is, by dynamo effect. Young white dwarfs generally appear to have no magnetic field, although one appears to appear as their core cools and crystallizes.

A dynamo mechanism occurs when a rotating object, such as a white dwarf or the Earth, contains an electrically conductive molten liquid. In the case of the Earth, the dynamo's energy is provided in part by its liquid metal core in the crystallization process.

Excerpt from the documentary From the Big Bang to the Living (ECP Productions, 2010), Jean-Pierre Luminet talks about the evolution of sun-like stars, their transformation into red giants and then into white dwarfs. © Jean-Pierre Luminet

Star atmospheres polluted by iron and silicon

For years we have been finding cosmochemical spectral traces of certain elements on the surface of several white dwarfs in the Milky Way, the presence of which can perhaps only be interpreted by the recent pollution of the star's atmosphere by the arrival of a small rocky celestial body, a remnant of telluric planets that were once at the end of their lives were engulfed by the swelling of their host stars and collided and were destabilized by the phenomenon.

There is reason to believe that some survive to some extent. They were called chthonic planets and we also know of two of these exoplanets that survived the red giant phase of the star KIC 05807616, which was near the constellations Cygnus and Lyra about 3,900 years ago. Light years away from Earth. They are smaller than our planet and complete their orbit in less than 12 hours. We also discovered the case of a gas giant exoplanet orbiting a white dwarf, suggesting that the solar system's giants could ultimately suffer a similar fate.

Let's take a closer look at what it's all about.

In the 1940s, French astrophysicist Evry Schatzman further developed the theory of stellar structure and evolution by proposing that white dwarfs contain a lot of carbon and oxygen, but their atmospheres are largely dominated by hydrogen, helium, and helium. In fact, white dwarfs are very compact objects and their surface gravity is very high. They are no longer the seat of convective movements, as in the case of the Sun, but of a phenomenon of “gravitational sorting,” so that the light elements are on the surface of the star while the heavy elements dive inward.

However, observations have not really confirmed these predictions. Particularly unusual cases were discovered: the atmospheres sometimes contained too many very specific heavy elements, namely iron and silicon. The best explanation for these anomalies are asteroids, asteroids and even rocky planets that would have recently collided with white dwarfs while the heavy elements have not yet sedimented into the compact stars. This also gives us information about the composition of the rock bodies that polluted the white dwarfs. PeridotitePeridotite, the main rock of the upper mantle, is a collection of ferromagnesian silicate silicates and therefore contains iron and silicon.

Using ESO's Very Large Telescope (VLT), astronomers have discovered a metallic “scar” on the surface of a dead star. This video summarizes the discovery. To get a reasonably accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then click on “Subtitles” and finally “Auto-translate”. Select “French”. ©ESO

The UniverseThe Universe has just surprised us again in terms of the pollution of white dwarfs by the remains of rocky bodies, in this case metallic metals, according to a publication today in The Astrophysical Journal Letters, revealing the work of a team of astrophysicists who used the Very Large TelescopeVery Large Telescope (VLT) of the European Southern Observatory in Chile, more precisely the instrument called FORS2, which not only allows spectroscopic measurements to study chemical composition, but also allows determining the presence and intensity of a magnetic field and measuring its impact on the polarization of the emitted light.

In an ESOESO press release announcing the discovery of white dwarf WD 0816-310, an Earth-sized remnant of a star similar to our Sun but slightly larger, said Stefano Bagnulo, an astronomer at the Armagh Observatory and Planetarium in Northern Ireland ( United Kingdom). ) and lead author of the study, explains: “It is well known that some white dwarfs – the slowly cooling remnants of stars like our Sun – cannibalize parts of their planetary systems.” We have now found that the star's magnetic field plays a key role in this process plays, which leads to a scar on the surface of the white dwarf.”

John Landstreet, co-author and professor at Western University in Canada – who is also associated with the Armagh Observatory and Planetarium – again specifies in the ESO press release: “It is surprising to note that the material at the Armagh Observatory and Planetarium is not was mixed evenly.” surface of the star, as the theory predicted. Rather, this scar is a concentrated slab of planetary material held in place by the same magnetic field that guided the molten fragments. Nothing comparable has ever been observed before.”

For his part, Jay Farihi, professor at University College London (United Kingdom) and another co-author of the study, adds: “We have shown that these metals come from a planetary fragment that is as large or even larger than Vesta.” 500 kilometers in diameter and is the second largest asteroid in the solar system. »

Data from the FORS2 instrument for WD 0816-310 showed that the intensity of the spectral signature of the metals present changed with the rotation of the star, which is understandable if the metals are concentrated in a specific area of ​​the white dwarf's stellar surface, rather than uniformly to be spread over it. The changes in the spectroscopic measurements were also synchronized with measurements of variations in the white dwarf's magnetic field, suggesting that metals were concentrated at one of its magnetic poles.

Remember that a white dwarf's surface temperature averages between 8,000 K and 40,000 K, which is hotter than the Sun's surface temperature (6,000 K). The metals brought by a rocky body are therefore in an ionized state and their atoms are then guided by the white dwarf's magnetic field to the magnetic poles, where they accumulate. This is the scenario proposed by astrophysicists to explain the observations on WD 0816-310. It is reminiscent of the formation of auroras on Earth and Jupiter, says the ESO press release.

Did you know ?

Despite their low luminosity, astronomers discovered white dwarfs in the 18th century. They didn't know how exotic these stars were at the time, but right at the beginning of the 20th century, to the astonishment of astrophysicists at the time, a value in the order of one ton per cubic centimeter was derived from observations of stars like Sirius.

However, British physicist Ralph Fowler quickly understood that the brand new quantum statistical mechanics discovered by his colleague Paul Dirac in the late 1920s (who at the same time theoretically predicted the existence of antimatter) described a gas that degenerates electrons, as the technical term goes by physicists, could explain the existence of these stars. This gas could exert a pressure great enough to withstand the pressure caused by the gravity of a star as dense as white dwarfs.

The very young astrophysicist Subrahmanyan Chandrasekhar (20 years old at the time) picked up Fowler's work and came up with the idea of ​​introducing the implications of special relativity and laid the foundation for the stellar structure of these strange objects. This led him to a now-famous conclusion: there cannot be a star with a mass greater than about 1.4 solar masses once it has evolved into a white dwarf after exhausting its nuclear fuel. This is the famous Chandrasekhar border.