How to spin a disc around young protostars

How are stars and planets formed? Scientists are now on the verge of defining the conditions for the formation of protostellar disks. Observations of three systems in the early stages of star formation in the Perseus cloud revealed that the kinetic momentum profile in these systems lies between that expected for a solid body and pure turbulence, indicating that the influence of the heart extends further than previously thought. These results could lead to more realistic initial conditions for numerical simulations of disk formation.

The main stages in the formation of stars and planets are well understood: a dense and interstellar cloud will collapse under its own gravity; a central nucleus is formed as well as a proto-stellar disk due to the conservation of kinetic momentum; finally, after about 100,000 years, the star will become dense enough to trigger nuclear fusion at its center and begin to shine while, in the disc, planets will form. However, many questions remain unanswered about the details of this process, for example. What is the role of kinetic momentum in the formation of the disc, or how does the circumstellar disc gather most of its mass?

An international team of scientists led by the Max Planck Institute for Extraterrestrial Physics (MPE) has now observed three of the most recent proto-stellar sources of the Perseus molecular cloud. These sources are close to the edge in the plane of the sky, which makes it possible to study the distribution of the velocities of the dense cloud.

"This is the first time that we have been able to analyze gas kinematics around three circumstellar disks in the early stages of their training," says Jaime Pineda, who led the study at MPE. "All systems can be adapted with the same model, which gave us the first clue that dense clouds do not turn out like a solid body." The simplest hypothesis is a solid body rotation that describes the gas in the dense cloud with a fixed angular velocity at a given radius. The model that best describes the three systems is between those expected for rotation of the solid body and pure turbulence.

Moreover, when we compare these observations to previous numerical models, it is clear that magnetic fields play a role in the formation of these disks: "If a magnetic field is included, it ensures that the collapse It's not too fast and the gas rotation corresponds to the observed one, "says Pineda. "Our latest observations give us an upper limit for the size of the discs, which are in perfect agreement with previous studies."

In particular, the specific kinetic momentum of the incoming material is directly related to the possible maximum Kepler radius of the protostellar disk. Assuming a star mass of about 5% of the mass of our Sun, scientists estimate that the upper limit of Kepler's disk is about 60 astronomical units, about twice the size of our system planetary, which is consistent with previous estimates. This suggests that large disks (greater than 80 AU) can not be formed early in the life of a star and therefore affects the starting point of planet formation scenarios.

The next step for astronomers will be to observe such systems at different stages of evolution and in different environments to see if they influence the specific angular momentum profile. These results can then be incorporated or compared with numerical simulations to better understand the co-evolution of the star-forming dense nucleus and planetary circumstellar disc.