The star rotation curve reveals a flatter profile than predictions. Researchers have tried to understand a key mechanism behind it, and they may well have found the answer.
During their life, stars modify their rotation profile, without all the mechanisms at the origin being formally identified. This is what asteroseismology tries to understand, a discipline that aims to understand the different vibratory modes of stars, and their internal structure with. Because if we know most of the processes that take place at the level of the outer layers and on the surface of stars, their heart still remains mysterious, because it is almost impossible to probe.
The stars are divided into several layers, which vary according to their massemasse. Below 0.5 solar masses, a star has only a large area of convectionconvectionbut beyond, there is both a radiative zone and a convective zone.
It is the latter which makes it possible, in theory, to generate a magnetic field. When the fluid in convection is electrically charged, its movementsmovements lead by dynamo effectdynamo effect the creation of a magnetospheremagnetosphere. The two then talk to each other: the magnetic field imposes movements of mattermatter, and these feed the magnetosphere! But in the radiative zones, it is more complicated: the transfer of matter takes place by photon-matter interaction, and not by the movement of charged particles.
A mysterious slowing down of the star’s core
During the life of a star, its rotation profile must vary. Early in life, when theaccretionaccretion material is not yet complete, the rotation should be particularly fast. It then slows down, only to speed up again when the star has consumed its energy reserves.hydrogenhydrogen for the nuclear fusionnuclear fusion : the heart contracts and accelerates its rotation. But observations have revealed star rotation profiles that are much flatter than predicted by current stellar evolution models, especially at radiative zones.
Also, the cores seem to spin slower than expected. A study in Science, conducted by researchers from the CNRS, Inria and ENS-PSL, reports on a possible explanation, thanks to numerous numerical simulationsnumerical simulations. According to the researchers, these flat profiles suggest “ the existence of a powerful mechanism capable of extracting the cinematic momentcinematic moment of the stellar core and to remove the differential rotation as the star evolves”. A mechanism they associate with magnetismmagnetism stellar, and especially to the interactions between the plasma and the magnetic field.
A better mixing of chemical elements thanks to this mechanism
To make sure, the team modeled the radiative layers of a star, trying to explain why the dynamo effect occurs there. They explain in the study that “ in radiative zones, the existence of a dynamo action – and of the resulting magnetic braking – is difficult to account for in the absence of a source of turbulenceturbulence clearly identified hydrodynamics, which a priori seems to be lacking in regions with stratificationstratification stable ».
According to them, the Tayler-Spruit effect would account for the phenomenon, an effect only theoretically described in several studies. It assumes instabilities of a toroidal magnetic field involving fluid displacements. And their simulations confirm it: it is quite possible to create a dynamo effect in the radiative layers in this way. More exactly, an internal magnetic field is amplified by flows of plasma, even laminarialaminaria, until it causes turbulence. Then begins the dynamo effect, where the magnetic field is in turn amplified, causing the star to slow down.
The phenomenon illustrated also makes it possible to improve the transport of chemical elements within the star, because it “slows the rotation of stellar cores and increases the rotation of the outer layers, improving the mixing of chemical elementschemical elements », explains the study. Thus allowing to modify the durationduration life of some stars, when hydrogen is brought back to the inner layers to fuel nuclear fusion reactions. But proving their results by asteroseismological observations will prove complicated, because the shape of the internal magnetic field makes it invisible, or rather, hidden under the outer layers!
