Protoplanetary disk, definition and study. A-Z index of Cognitio.

A Protoplanetary disk is a discoidal structure of gas and dust in orbit around a star or, more often, a star. The Protoplanetary disks reach sizes ranging from a few tens of astronomical units (AU) up to 1000 AU, with temperatures ranging between tens of Kelvin in the outermost areas of the disc orbit up to a thousand Kelvin in the parts more internal and superficial of the same. Protoplanetary disks are the place of formation of planetary systems. The stars are typically formed from a molecular cloud, which consists mainly of molecular hydrogen. When one of these clouds reaches a critical mass, density and size, it begins to collapse under the action of its own gravity.

As the cloud contracts, the preservation of the angular momentum causes the random movements present in the cloud to become a coherent rotation; the centrifugal force generated by rotation causes the cloud to take on the appearance of a disk.

The initial collapse lasts approximately 100 000 years, after which the surface of the star reaches a temperature similar to that of the main sequence stars (dwarf stars) of the same mass and becomes visible. In the early stages of settling, the star undergoes frequent and vigorous growth episodes, labeled as the FU Orionis phase. When the rate of material growth from the disk on the star begins to decrease, the high energy radiation of the star is the cause of the most efficient heating processes in the immediately internal layers and in the atmosphere of the disk.

Atmosphere, definition and study. A-Z index of Cognitio.

Atmosphere

The absorption of this radiation, in particular UV and X-rays, causes the evaporation of the disk, which will be completely dissipated within a few million years. The oldest known Protoplanetary disk has an estimated age of 25 million years. Some Protoplanetary disks have been observed around young stars in our galaxy.

Galaxy, definition and study. A-Z index of Cognitio.

Galaxy

Recent observations made by the Hubble Space Telescope have shown several proplyds around stars in formation in the Orion Nebula.

Astronomers have discovered vast discs of matter, which could also be Protoplanetary discs around the stars Vega, Alphecca and Fomalhaut, all three very close to our Sun.

Sun, definition and study. A-Z index of Cognitio.

Sun

Water is the only known substance that has been found in both solid (ice) and gaseous state in large quantities in planetary discs. On the basis of their relative proportions, the physical characteristics of a nebula and the stage of the formation process of the planets can be established.

The typical Protoplanetary disk is characterized by the predominance of water in the gaseous state in the center and by ice in the area between the center and the circumference of the disk. 

Strange undulations have been observed in a disk of gas and dust around a young star, which does not seem to be related to the formation of new planets.

Strange undulations have been observed in a disk of gas and dust around a young star, which does not seem to be related to the formation of new planets.

Strange undulations have been observed in a disk of gas and dust around a young star, which does not seem to be related to the formation of new planets.

Circumstellation disc images around the star AU Microscopes. The black circles in the center show the region in which the bright light of the central star was “locked” to highlight the disc: the position of the star is not representative. The bar at the top corresponds to 60 astronomical units (60 times the Sun-Earth distance), and the brightness of the external areas has been artificially increased to better show the structure of the disk.

Earth origins, definition and study. A-Z index of Cognitio.

Earth origins

AU Microscopii is a young red dwarf star of spectral type M, visible in the southern skies, about 32 light years from us: almost eight times the distance that separates the Sun from Proxima Centauri, the nearest star. This small star has a mass of just under one third of that of the Sun, an age estimated at only 12 million years and is surrounded by a vast Protoplanetary disk. Its proximity and the inclination of the disc, which from the Earth is seen cut, have made AU Mic an interesting target for the search for possible signs of planetary formation. Now, thanks to the images and spectroscopic data obtained with the Hubble Space Telescope and the Very Large Telescope, “strange undulations” have been discovered inside the disk of gas and dust that surrounds this young neighboring star. The wave motion that has been detected appears to be different from any other characteristic observed up to now or predicted by theories for a disk. The observations, which were made with the SPHERE instrument (Spectro-Polarimetric High-contrast Exoplanet Research) applied to the VLT, were directed to the search for densities or deformed structures in the disk, as evidence of the possible presence of planets during training or who are going through their very early evolutionary phases.

However, the observations showed something unexpected, he explains. The images of SPHERE show a set of unexplained structures in the disk, which have an arc or wave shape, different from anything that has ever been observed until now. In fact, in the images, five wavy arches are clearly visible at different distances from the star, a structure similar to that of the ripples that are formed when a stone is thrown into the water. After having detected them, the team analyzed the images of AU Mic obtained in 2010 and 2011 with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope, discovering that these circumstellar disk morphologies were already present, but that they changed and that move outward very quickly. Based on measurements taken over a period of about four years, it has been discovered that these arches are moving away from the star at a speed of up to 10 km / s. The ripples farthest from the star seem to move faster than the nearest ones and at least three of them at such a speed that they could also escape the gravitational attraction of the star. The rapid movement of these strange structures leads to the exclusion of the possibility that they may originate from planets forming inside the disk, which with their presence disturb the matter in the disk orbiting the star. Some other uncommon phenomenon must have accelerated the ripples to make them move so fast.

Protoplanetary disk, definition and study. A-Z index of Cognitio.

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Protoplanetary disk, definition and study. A-Z index of Cognitio.

Protoplanetary disk