(Paris) With 2.3 million degrees for an existence of only 841 years, the neutron star J0205 is too cold to agree with a large number of models explaining the cooling of these singular objects of the cosmic bestiary, according to a study by Spanish astronomers.
A neutron star is the relic of the explosion of a massive star at the end of its life. If this relic exceeds a certain mass, it transforms into a black hole. Below, it becomes a neutron star, like PSR J0205 6449, its full name.
A phenomenally dense object, with the equivalent of 1.4 Suns compressed into a sphere with a diameter of 20 to 30 km. And a rotation of fifteen revolutions per second generating a powerful magnetic field, accompanied by the emission of X-rays.
This cosmic spinning top holds “unique information on the properties and behavior of matter under extreme conditions of density and magnetic fields,” recalls the study published Thursday in Nature Astronomy.
Conditions that cannot be replicated in the laboratory, and for which physicists have established models, called equations of state. They make it possible to describe the processes at work at the heart of these stars, in states where the nuclei of the atoms disintegrate, and their components behave in strange ways.
By going through the catalog of the two space telescopes XMM-Newton and Chandra, dedicated to the detection of neutron stars, the team of Spanish astronomers identified three that stand out from the rest.
“On paper, their temperature is very high, but unusually cold for their young age,” summarizes for AFP Dr. Alessio Marino, co-author of the study and member of the Barcelona Institute of Space Sciences. And not just a little, since it is at least half as high as that of neutron stars of the same age.
Typically, the star is born “at a temperature of around 500 billion degrees, and after just a few minutes, it drops below 10 billion degrees,” Micaela Oertel, CNRS research director, explains to AFP. at the Strasbourg Observatory and a specialist in these compact objects. This temperature will then decrease sharply with age, after a million years.
In this case, astronomers calculated cooling curves according to age, allowing comparison with neutron stars. They determined this age by observing the residual cloud from the original explosion which saw the birth of the stars.
The other two are 7700 years old and between 2500 and 5000 years old, with temperatures of 1.9 and 4.6 million degrees, respectively. At least twice as small as those of contemporary neutron stars.
But “the cooling of the star is something that is really sensitive to its interior composition,” and in particular its proportion of neutrons to protons, according to Micaela Oertel, who was not involved in the study.
The researcher thus welcomes “extremely interesting” work, because it restricts the number of models applicable to stars of a certain mass.
In this case the study concludes that for the neutron stars considered, these models must include a rapid cooling mechanism, which is linked to the composition of the star.
The interest of this work concerns fundamental physics, to help understand in particular the strong interaction, one of the fundamental forces governing matter in the infinitely small. But also astrophysics, therefore the infinitely large.
Since, as Ms. Oertel explains, “we now know that the fusion of neutron stars is the main source of heavy elements on Earth”, such as gold or platinum.