Spin, spin, but not too fast!
Researchers from the University of Costa Rica, the University of Geneva (Switzerland) and the Yunnan Observatories (China) developed a new computational model that explains how massive stars acquire their rotation rates with accretion disks and jets, shedding light on a decades-old problem. Their findings are published in a letter to the editor in the journal Astronomy & Astrophysics.
Simulation of the formation of a massive star. The disc feeds material onto the star, increasing its rotational speed. The jets carry away the excess angular momentum and keep the star’s rotation below the threshold value at which it would rip apart. © André Oliva
Astronomers call a star "massive" when it has at least eight times the mass of our Sun. Massive stars emit powerful radiation and enrich their surroundings with heavy chemical elements essential for life, in a way that lower-mass stars like the Sun are not able to do. Despite decades of investigation on the formation and evolution on massive stars, studies focusing on the origin of their rotation rates are rare. That is, we know how rotating stars evolve, but not how they acquire their rotation rates in the first place.
Stars form from a cloud of gas and dust that contracts under the pulling action of gravity. During this process, its center rotates faster and faster, in a similar way than what happens when a spinning ice skater pulls their arms together. "When enough spin is reached at the center of the cloud, an accretion disk is formed, which is essentially spinning material that is queuing to be eaten by the forming star at the center of the cloud" explained Prof. André Oliva, who lead the research project.
However, for decades, calculations have shown that this picture is incomplete, because the star would quickly start rotating so fast that it would rip apart. This is known as the "angular momentum problem". At the same time, magnetic fields present in the cloud launch powerful jets which remove some material from the center of the cloud. These jets are commonly observed in nature and the research team found that they are key in explaining why the forming star doesn't rip apart.
By joining two state-of-the-art computer simulations, one that models the disk and jet and one that models the interior of the star, the team was able to show that the jet acts as a "regulator". The accretion disk spins the star up, while the jet spins it down until a balance is found. The model also proposes that the rotation rate of the star is regulated by the strength of the jet, but it doesn't depend on whether the star has any magnetic fields of its own. "We know that low-mass stars have stellar magnetic fields which play a role in braking them and not letting them spin too fast. But 90% of massive stars don't have such magnetic fields, so then the magnetic field of the jet does all the job of regulating their spin", concluded Prof. Oliva.
The team is composed of Prof. André Oliva of the Space Research Center of the University of Costa Rica and external collaborator of the University of Geneva, Dr. Facundo Moyano of the Yunnan Observatory, and Luca Sciarini, Prof. Sylvia Ekström, Dr. Patrick Eggenberger and Prof. Georges Meynet of the Department of Astronomy of the University of Geneva. For modelling the star, the team used GENEC, a stellar evolution program originally developed in Geneva with great tradition studying rotating stars.
Link to the A&A article