The formation of stars and their planetary systems is an extremely complex process.
At first order, we can nevertheless see things this way: in a vast cloud of gas and dust, sometimes called a star nursery, a small area reaches for one reason or another a critical density which leads it to collapse onto herself.
At the center of this
“
lump
”
, when the mass reaches a certain threshold, gravity becomes powerful enough to force the nuclei of the hydrogen atoms to fuse.
A balance between thermonuclear radiation (which pushes matter outwards) and gravity (which attracts it inwards) is established, forming a dense and sparkling sphere: a star is born.
During this process, the material naturally begins to rotate, under the effect of the small inhomogeneities of the lump.
It's a bit like what happens when a bathtub finishes emptying and the water rolls up to form a whirlpool.
Around the star, the residual gas and dust form a disk which rotates in the equatorial plane.
This is where the planets will form, in the few million years following the birth of the star.
The processes that take place there, a mixture of collisions and accretion, not to mention the intense radiation of the star, are by nature very unstable, which explains at least in part the extreme diversity of the planetary systems that we have observed so far.
As if this swirling abundance was not complex enough in itself, it is also influenced... by the powerful ultraviolet radiation emitted by neighboring giant stars!
Two studies published last Friday in Nature Astronomy and Thursday in Science have just confirmed this in a superb manner thanks to new observations carried out by the monumental James Webb space telescope.
Also read: An unprecedented “x-ray” reveals the lace of the universe
It is a small protoplanetary disk from the Orion cloud, quite prosaically called d203-506, which is the subject of this work.
It had been identified by the Hubble Space Telescope by observing the
"
Orion bar
"
, a fairly dense area (which forms a sort of oblique ridge in the bottom left corner in the image above) located in the heart of the eponymous cloud, and in which astronomers suspected that stars were probably being born.
This region is also illuminated by the powerful stars of the Trapezium cluster (center), supermassive giants whose ultraviolet (UV) radiation is tens of thousands of times more intense than that of our Sun.
From Earth, the d203-506 disk is seen edge-on and hides the star nestled at its heart.
The remarkable performances of the JWST nevertheless made it possible to uncover an envelope of gas which forms a cocoon around it.
“
It is in fact hydrogen gas which has been heated up to a thousand degrees by the powerful UV radiation from neighboring giant stars ,
”
explains Olivier Berné, CNRS astrophysicist at Irap, in Toulouse, specialist in these star formation regions and first author of the study published in
Science
.
The molecules are in fact so agitated that they have reached sufficient speed to escape from the gravitational attraction of their star, forming this bubble which continues to grow.
The disk is actually evaporating.
“
And we were able to measure at what speed
: each year, a mass of gas equivalent to that of our Earth escapes from the disk ,
”
specifies the researcher.
“
We think that it will be stripped of all its gas in less than 1 million years, which
a priori leaves
too little time to form gas giants like Saturn or Jupiter.
»
Ultraviolet Sculpture
In other words, supermassive stars
“
sculpt
”
the protoplanetary disks of their neighbors with their ultraviolet radiation.
“
This is the first time that we have managed to characterize and quantify this phenomenon of evaporation in a zone of planetary formation, it is really very impressive ,
”
comments Tristan Guillot, astrophysicist at the Coast Observatory. Azure.
“
Our Sun did not form in a UV environment as extreme as d203-506, but there must still have been massive stars nearby that played a role in the evolution of our protoplanetary disk and its planets .
This evaporation could notably have had an influence on the composition of Jupiter.
»
This
“
extrastellar
”
UV radiation also plays an even finer role in the chemistry of the disk.
This is the subject of the second study published in
Nature Astronomy
, which was carried out using the same observations (a certain number of authors are also common to both articles).
“
We identified OH molecules
(one hydrogen atom and one oxygen atom, editor’s note)
in two very different states of excitation ,
”
explains Marion Zannese, doctoral student at the Institute of Space Astrophysics at the University of Paris. Saclay, in Orsay, and first author of this study.
“
A first state corresponds to molecules obtained by the dissociation of water under the influence of UV radiation
;
and the second state corresponds to OH molecules which instead result from a reaction between dihydrogen and oxygen, and constitute a step towards the formation of a water molecule.
»
Incessant ballet
In other words, researchers see water being destroyed then reforming in an incessant ballet.
A primordial water cycle on the scale of a planetary system.
This process is also not trivial because it will change the heavy hydrogen content (also called deuterium, its nucleus containing two neutrons compared to only one for “
classic
”
hydrogen
) of the water initially present in the disk.
This could partly explain why water on Earth does not have the same composition as that observed in proto-stars.
“
Our protoplanetary disk was perhaps not as irradiated by neighboring stars in its outer part, but the UV radiation coming from the Sun itself could have affected the inner disk where the Earth was formed ,
”
estimates Marion Zannese.